Sustainable energy efficiency retrofits as residenial buildings move towards nearly zero energy building (NZEB) standards

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Sustainable energy efficiency retrofits as residenial buildings move towards nearly zero energy building (NZEB) standards Paul Moran , John O’Connell , Jamie Goggins PII: DOI: Reference:

S0378-7788(19)32305-9 https://doi.org/10.1016/j.enbuild.2020.109816 ENB 109816

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Energy & Buildings

Received date: Revised date: Accepted date:

25 July 2019 19 January 2020 23 January 2020

Please cite this article as: Paul Moran , John O’Connell , Jamie Goggins , Sustainable energy efficiency retrofits as residenial buildings move towards nearly zero energy building (NZEB) standards, Energy & Buildings (2020), doi: https://doi.org/10.1016/j.enbuild.2020.109816

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HIGHLIGHTS 

Deep energy efficiency retrofits are financially feasible in Ireland with grant aid



Without grant incentives, only shallow retrofits of homes are economically feasible



Environmental impact of various retrofit designs for homes built 1991-2002 assessed



Best solution differs depending on life cycle environmental indicators considered



Environmental impact of materials from 20-100% of share of building‘s life cycle

Sustainable energy efficiency retrofits as residenial buildings move towards nearly zero energy building (NZEB) standards Paul Moran1, 2, John O‘Connell1, Jamie Goggins1, 2 * 1 2

School of Engineering, National University of Ireland Galway, Ireland. MaREI Centre, Ryan Institute, National University of Ireland Galway, Ireland.

* Corresponding author: J Goggins ([email protected]) Abstract 1.9 million housing units in Ireland are required to be retrofitted for the existing Irish national housing stock to be considered nearly zero energy building (nZEB). This paper assesses optimum retrofit packages aimed at improving the building material thermal efficiencies and energy demand of gas-heated semi-detached and end-terraced houses in Ireland. The costoptimal methodology framework for calculating cost optimal levels of minimum energy performance requirements for buildings and building elements is used to determine the optimum retrofit packages from both a householder perspective and a societal perspective. The results of the cost-optimal approach are compared to the results from a weighted framework approach which incorporates life cycle environmental and cost indicators from both a householder perspective and a societal perspective. The results found that the ranking of energy efficiency retrofit designs based on multiple life cycle environmental and cost indicators differ compared to cost optimum designs based on only life cycle energy and cost indicators. The cost-optimal approach was found to be effective for identifying retrofit packages that are among the best packages even without accounting for multiple environmental indicators. However, once multiple indicators were considered, the hierarchy of the optimal retrofit packages changed. In the Irish context, the environmental impact of the Irish electricity grid was found to play a significant role in the hierarchy of the retrofit packages examined.

While the cost-optimal method should not be relied on solely for identifying the optimum retrofit package solution, it could be employed as part of the multistage assessment methodology for narrowing down design solution sample sizes. Using the remaining array of retrofit packages solutions, the optimum retrofit package could be identified using multiple indicators. Keywords: Energy Retrofit, Residential Buildings, Nearly Zero Energy Building, Life Cycle Analysis, Cost-Optimal 1

Introduction

With the world‘s energy consumption and population increasing, climate change is linked to humankind‘s heavy reliance on fossil fuels to meet energy demand and the resultant production of greenhouse gases (GHGs) [1]. The Intergovernmental Panel on Climate Change (IPCC) report strong evidence that suggests the increase in GHGs in the atmosphere is changing the world‘s climate [1–4]. Climate change is predicted to have a negative effect on the world unless proper mitigation measures are implemented [1–4]. Various international agreements have been made to help address climate change including the United Nations Framework Convention on Climate Change Treaty [5], the Kyoto Protocol [6] and the Paris Agreement [7]. To combat their energy demand and GHG production, buildings in the European Union (EU) are now being designed and constructed to tighter building standards and codes. The Energy Performance Building Directive (EPBD) introduced a new revised directive in 2013 [8] requiring the mandatory introduction of nearly zero energy buildings (nZEBs) in all member states. A nZEB is a building with a very high energy performance. The nearly zero or very low amount of energy required by a nZEB should be produced, to a very significant extent, from renewable sources, including those on-site or nearby. Starting from the end of 2020, all

new buildings or those receiving significant retrofit must show a very high energy performance [8]. Similarly, this is a requirement for all public buildings from the end of 2018 [8]. To determine building energy performance requirements for nZEBs, EU countries had to use a cost-optimal methodology framework to assess cost-optimal levels of minimum energy performance requirements for buildings/building elements/building materials [9]. This methodology framework assesses the global costs (or life cycle costs) for buildings/building elements/building materials and their respective impact on primary operational energy demand for space heating, cooling, ventilation, domestic hot water and lighting systems. Numerous studies have applied a cost-optimal approach to minimum energy performance methodology to assess cost-optimal retrofit solutions for the building stock of several countries. Studies on single family houses [10,11] and multi-family/apartment buildings [10,12–15] have been conducted on Italian [10,11], Portuguese [12,13], Swedish [15] and Turkish [14] building stock. Several sensitivity parameters effecting the cost-optimal results have been examined including discount rates [12,13,15], energy prices [12], insulation thicknesses [15], building orientation [13], building age [10] and building lifespan [10]. 1.1

Life Cycle Analysis in Building Design

Achieving more energy efficient buildings comes at an additional cost. In many instances, the design solutions involve additional material in constructing the building. Buildings and construction in the EU accounts for 50% of all extracted materials, 30% of water consumption and 33% of total generated waste [16]. Tighter building standards and codes are resulting in lower energy demand during a building‘s operation. However, this only represents one stage of a building‘s life cycle energy demand. Building life cycle analysis

(LCA) assesses the impact of building production, construction, use and end of life during the building‘s life span. Because of the importance of selecting sustainable materials and the use of LCA in building design, France, the Netherlands and Finland have committed for LCA to be mandatory in building design [17]. Germany and Austria have introduced semi-mandatory requirements for building LCA. The standards for building LCA calculation define the boundary for refurbishment/retrofit (Module B5) to include (i) the production of new building components, (ii) transportation of new building components, (iii) construction as part of the refurbishment process, (iv) waste management and (v) end of life of the replaced components [18]. The production of new building components and their impact on the operation energy use of the building are the most common boundaries assessed in LCA retrofit studies [19]. Several studies have used LCA indicators to evaluate retrofitting/refurbishing case studies [19,20,29–31,21–28]. Oregi et al. [32] examined the relevance of each life cycle stage in relation to building retrofit and found the transportation and end of life stages to be minor [32]. Famuyibo et al. [28] found that the maintenance and disassembly stages for retrofitting a residential building in Ireland to passive standards accounted for at most 3.7% and 0.9% of the building‘s environmental LCA, respectively [28]. Famuyibo et al. [28] also found that Irish semi-detached housing units retrofitted to the same energy performance levels had different life cycle global warming potential (GWP) impacts. Thiers & Peuportier [29] examined the environmental life cycle impact of retrofitting passive house and multi-family social housing building to net-zero source energy building using 12 environmental indicators. The authors discovered no heating system solution was the optimum choice for each of the 12 environmental indicators evaluated.

Other studies incorporated environmental life cycle assessment in their assessment methodology, along with the cost-optimal approach [33–42]. Almeida et al. [42] extended the methodology for comparing cost-optimality in building renovations, developed in the International Energy Agency (IEA)–Energy in Buildings and Communities (EBC) Annex 56 project [41], with a life cycle assessment by including embodied primary energy and carbon emissions in the calculations. Almeida et al. [42] found that including embodied energy and carbon emissions did not affect the ranking of renovation packages from a cost-optimal perspective. 1.2

Aim of the study

In Ireland, major retrofit works are classified as energy efficiency retrofit works where more than 25% of the surface area of the building envelope undergoes renovation [43]. To be considered a nZEB retrofit in Ireland, a residential building must achieve a minimum energy performance of 125 kWh/m2/yr. [43]. However, if an energy performance of 125 kWh/m2/yr. is found to be cost-optimally unfeasible, a cost-optimal retrofit solution, which does not achieve an energy performance of 125 kwh/m2/yr., may be used instead [43]. There are currently ca. 2 million housing units (detached houses, semi-detached houses, terraced houses and apartment units) built within Ireland [44]. 800,059 have received an Energy Performance Certificate (EPC) known as a Building Energy Rating (BER) label in Ireland, with 730,641 (i.e. 91%) currently not achieving cost optimal energy efficiency standard of 125 kWh/m2/yr. [45]. Assuming the remaining housing with no BER label were built before the mandatory BER labelling of new buildings in the 2000s, 1.9 million housing units (i.e. ca 95% of the building stock) in Ireland are required to be retrofitted for the Irish national housing stock to be considered nZEB standard. 46,106 occupied Irish housing units are gas heated semi-detached buildings built between 1991 to 2000 [44].

This paper assesses the optimum building energy efficiency retrofit packages aimed at improving the material efficiencies and energy demand of gas-heated semi-detached and endterraced houses built in Ireland between 1991 to 2000. The optimum retrofit design packages based on evaluated environmental and economic life cycle indicators are compared to the optimum design strategies based on the cost-optimal methodology framework which is used for determining the nZEB retrofit energy performance building regulations [9]. From the reviewed retrofit studies [19,20,29–31,21–28], it was found that energy and GWP (also referred to as greenhouse gas emissions and climate change) are the most common environmental indicators in LCA studies. Eutrophication potential (EP), acidification potential (AP) and ozone depletion potential (ODP) are also often used. The analysis in this paper uses energy, GWP, ODP, AP, EP and photochemical ozone creation potential (POCP) as environmental indicators which are recommended by the recently published EU guidelines on sustainability indicators for office and residential buildings [46]. 2

Methodology

In the analysis presented, the production of new building components (Module A1-A3) and their impact on the building operational energy use (Module B6) are assessed. Section 2.1 presents the method for determining the environmental embodied impact of producing the various retrofit components, while the method for assessing the building operational environmental impact from the retrofit components is detailed in Section 2.2. The approach to assessing the construction cost and operational cost of the retrofit designs is presented in Section 2.3 and Section 2.4, respectively. The method in which the evaluated environmental and economic life cycle indicators are used in the cost-optimal methodology framework and a weighted framework approach

(Sustainable Index Factor) to compare the retrofit designs is given in Section 2.5 and Section 2.6, respectively. 2.1

Material Production Stage (Module A1-A3)

The embodied environmental impact caused by extracting, processing and manufacturing of materials can be expressed mathematically as by any of the equations 1 to 4 depending on the data available: ∑

(1)

∑

(2)

∑

(3)

∑

(4)

where Em,n is the embodied environmental impact of indicator m for case study n. The embodied intensity for material i of indicator m can be given in terms per kg ( EKGm,i), per m3 (EVm,i), per m2 (EAm,i) or per m (ELm,i). ρi is the density (kg/m3), Vi is the volume (m3), Ai is the area (m2) and Li is length (m) of material i. Environmental intensities of materials/products are sourced from environmental product declarations (EPDs) or Ecoinvent (v3.4) [47]. EPDs are sourced from the multiple available online databases [48–50]. The Irish EPD platform is the default online EPD database used in this study [48]. If an EPD is unavailable on the Irish EPD platform, other online databases are used [49,50]. If EPDs are unable to be sourced for a material/product, environmental intensities are sourced from Ecoinvent (v3.4) [47].

This analysis uses energy (EN), GWP, ODP, AP, EP and POCP as environmental indicators. The EU guidelines on sustainability indictors for office and residential buildings also recommend using abiotic depletion potential of elements (ADPE) and abiotic depletion potential fossil fuels (ADPF) [46]. However, as part of this paper, how the environmental impact of the Irish electricity grid changes as the electricity grid decarbonises in the future is considered. ADPE and ADPF are measures of the scarcity of a substance, which depends on the amount of resources and extraction rate [51]. As it is not possible to predict how the amount of resources and extraction rates change due to grid decarbonisation, ADPE and ADPF are not included in this analysis. 2.2

Use Stage: Building Operation (Module B6)

Annual operational primary energy and the energy savings from the examined retrofit packages are estimated using Dwelling Energy Assessment Procedure (DEAP) [52]. DEAP is the standard assessment procedure for calculating and rating the energy performance of Irish housing units. DEAP accounts for the energy required for (i) lighting, (ii) ventilation, (iii) space heating and (iv) domestic water heating. The operational environmental impact from the case study buildings can be expressed mathematically as: ∑

[∑

]

(5)

where OIm,j,x (indicator m/MJdel) is the conversion factor of fuel type j for environmental indicator m in year x of the building lifespan y and DEj (MJdel/m2/yr.) is the delivered energy per heated floor area per year of fuel type j. Using the total values of Em,n and Om,n, the life cycle environmental impact of indicator m for case study n is evaluated. The environmental impacts per MJdel of electricity, gas and coal are taken from Ecoinvent (v3.4) [47]. The environmental impact of the building‘s operational energy use is determined

using Equation 5, where the estimated operational secondary energy use is estimated from DEAP and the fuel environmental impacts from Ecoinvent (v3.4) [47]. Decarbonising electricity generation in Ireland is seen as one of the key measures to transitioning to a low carbon, climate resilient and environmentally sustainable economy by 2050 [53]. The Sustainable Energy Authority of Ireland (SEAI) have projected Ireland‘s energy demand across multiple sectors including electricity generation until 2035 [54]. Using the data [55], the conversion efficiency and primary energy factor for each of the energy sources used in electricity generation are calculated. Ecoinvent (v3.4) [47] provides the energy, GWP, ODP, AP, EP, and POCP intensities per MJdel of electricity generated in Ireland during 2017. Ecoinvent (v3.4) [47] also provides the energy, GWP, ODP, AP, EP and POCP intensities per MJdel of electricity generated for fossil fuel and renewable energy sources used in the generation of electricity in Ireland. The data from Ecoinvent (v3.4) on the environmental impact of fossil fuel and renewable energy source electricity generation is based on power plants in Ireland in 2012 which has been extrapolated to 2017. An example of the GWP per MJ of different energy sources used for electricity generation in Ireland is given in Table 1. Table 1: GWP per MJ of fossil fuels and renewable energy sources used in electricity generation in Ireland (Source: Ecoinvent (v3.4) [47]). Energy Source Coal Oil Gas Peat Wind <1MW Wind >3MW Wind 1-3MW Hydro-Pumped Storage Hydro-Run of River

GWP/MJdel 0.301 0.255 0.105 0.285 0.003 0.006 0.004 0.234 0.001

To estimate the annual GWP intensity of the electricity grid up 2035, the annual secondary energy contribution of fossil fuels and renewable energy sources for electricity generation sourced from SEAI are multiplied by the energy source intensities of Ecoinvent (v3.4) [47] to calculate the total primary GWP emissions for electricity generation. As shown in Table 2 for the calculated primary energy factor of peat, the conversion efficiency is found to change over time. Any change in conversion efficiency for the energy sources is assumed to occur for the GWP MJdel values given in Table 1. Table 2: Calculated primary energy factor of peat for electricity generation in Ireland. MJprim/MJdel Energy Source 2014 2015 2016 2017 2018 2019 2020 2021 2022 2023 2024 2025 Peat 2.70 2.63 2.63 2.66 2.66 2.66 2.63 2.63 2.63 2.63 2.63 2.63

Using the calculated total primary GWP emissions from electricity generation divided by the total secondary energy for electricity generation, annual GWP/MJdel of electricity generated intensities for the Irish electricity grid up to 2035 are estimated. This process is repeated for the energy, ODP, AP, EP and POCP indicators to estimate the change in the impact of each indicator as the grid becomes decarbonised. The calculated average annual growth rates of the indicators per MJdel in electricity generation from 2014-2035 and from 2028-2035 are shown in Table 3. As can be seen in Table 3, the environmental impact of the Irish electricity grid is expected to reduce across all environmental indicators between 2017 and 2035. Demand for electricity in Ireland is expected to grow up to 2035. Even though renewable energy is expected to contribute to the growth in electricity demand, gas is expected to be the main energy source for electricity generation in Ireland by 2035. Peat‘s contribution to electricity generation is expected to drastically reduce from 2020 and is predicted to be phased out of electricity generation by 2026. Coal‘s contribution to electricity generation has grown the last number of

years but is expected to start reducing from 2024 onwards before having a marginal contribution from 2028 onwards. Table 3: Average annual growth rates of the indicators impact per MJ of electricity generated from 2014-2035 and 2028-2035.

Environmental Indicator EN GWP ODP AP EP POCP

Average Annual Growth Rate (%) 2017-2035 2028-2035 -1.07 -0.11 -1.21 -0.10 -0.07 -0.07 -1.26 0.07 -0.94 0.02 -0.63 0.02

GWP, ODP, AP, EP, POCP are all related to the emission of substances to the earth‘s atmosphere. This analysis does not account for any additional processes that may be installed to mitigate the emission of substances to the earth‘s atmosphere in the future. Beyond 2035, the average annual growth rates of the indicators‘ impact per MJ of electricity generated from 2028-2035 are used to estimate the change in the impact of the indicator (Table 3). Peat and coal are predicted to have been phased out or their contribution marginalised in electricity generation by 2028. AP, EP and POCP are expected to grow marginally from 2028-2035 as the contribution of oil to the electricity generation fuel mix is projected to increase marginally in this period. In the case studies which export electricity to the grid, the GWP savings are assumed to have the same GWP intensity of the electricity grid for that given year. Other studies have suggested using the same GWP intensity as the electricity grid initially, before plateauing due to the GWP intensity of the marginal power plant supplier. Eventually the GWP intensity of the electricity supply would start declining again due to the increasing decarbonisation of the electricity fuel supply [56]. This would have minimal impact on the total GWP results of the case studies as the GWP intensity would reach the same intensity at some point in the future

in these scenarios. Therefore, the exported electricity is assumed to have the same GWP intensity as the electricity grid. The same assumption is made with regards to all the electricity generation environmental indicators. 2.3

Net Construction Costs

The external wall expanded polystyrene (EPS) insulation and cavity wall beaded EPS insulation material and labour costs are sourced from an insulation supplier [57]. The Tabula project assessed basic and advanced retrofit steps to improve the energy efficiency of Irish residential buildings [58]. The costs of material and labour for the roof insulation energy efficiency retrofit measures are taken from the Tabula project. For the price of rock wool insulation, the material costs are sourced from an insulation price guide [59]. For the cost of labour for installing the rock wool insulation, the cost of labour for installing the mineral wool insulation is used. The cost of labour for installing the mineral wool insulation is assumed to be the difference in the material cost of mineral wool insulation from the insulation price guide [59] and the cost of material and labour from the Tabula project [58]. For the window and door costs, Tabula only provides a cost/m2 for different window and door U-values. However, a price for windows and doors with a U-value of 0.7 W/m2K is not included in the Tabula document [58]. In addition, the cost of a window and door depends on multiple factors including frame design, frame size, and material. As such, costs for the manufacturing and installation of windows and doors are instead sourced from Munster Joinery [60]. Costs for the materials and labour of upgrading the heating system and the installation of renewable technology are sourced from an architect firm [61], a building contractor [62], a heat pump supplier [63], a solar PV supplier [64] and the Tabula project [58].

2.4

Operational Economic Costs

Annual operational secondary energy and the energy savings from the examined retrofit packages are estimated using DEAP [52]. A life span of 30 years is assumed in this analysis. 2015 energy prices for electricity, gas and coal [65] are used with the operational energy demand estimated from DEAP to determine the operational economic costs and savings for the retrofit solutions. As required in the cost-optimal methodology [8], the operational costs account for future energy price scenarios. The price of electricity, gas and coal in Ireland is sourced every year dating back to 1990 [65]. The Irish energy market experienced price increases and decreases over the past three decades with the increases outweighing the decreases. Residential electricity prices grew by an annual average growth rate of 3.0% between 1990 and 2018. Residential gas and coal prices grew by annual average growth rates of 2.7% and 3.3%, respectively. It is assumed that Irish fuel energy prices will experience the same annual growth rates into the future in this analysis. 2.5

Cost-Optimal Methodology Framework

The cost-optimal methodology framework for calculating cost optimal levels of minimum energy performance requirements for buildings and building elements [8,9] is used in this analysis to determine the cost-optimal solution from both a householder perspective and a societal perspective. However, the embodied energy of the materials invested into a building is accounted for in the current study despite not being required as part of the cost-optimal methodology framework set out by the EPBD. The life cycle cost (LCC) part of the costoptimal methodology framework for case study n can be expressed mathematically as: ∑ [∑

]

(6)

where NCC is the net construction cost or initial investment costs (€), OECj,x is the operational economic cost during year x for heating system j, Ccj,x is the carbon cost during year x for heating system j based on the sum of the annual GWP emissions, Vfj,y is the residual value of heating system j at the end of the building lifespan y and Rdx which is the discount factor for year x based on discount rate r which is determined using:

(

)

(7)

where x is the number of years from the starting period and r means the real discount rate. The LCC can be used from a customer/householder perspective and from a macroeconomic/societal perspective. From a customer/householder perspective, all grant subsidies and applicable taxes including Value Added Tax (VAT) and charges should be taken into account, but the cost of carbon should be excluded [9]. When examining the LCC of a retrofit package from a macroeconomic/societal perspective, all grant subsidies and applicable taxes including VAT and charges should be excluded and a new cost category cost of carbon should be included [9]. Ireland is facing fines from the EU if it does not meet energy efficiency and carbon reduction targets by 2020 and beyond to 2030 [54]. Instead of paying fines, the Irish government are investing in building energy efficiency retrofit measures to achieve energy and carbon reduction targets by offering grants to householders. Multiple grant programmes are offered by SEAI to householders to encourage investment in energy efficiency retrofit measures [66], as for example in Table 4 and Table 5, respectively. In this analysis, grant subsidies are taken into account from both the householder and societal perspective, as the government will have to pay fines if energy efficiency and carbon reduction targets are not met.

Table 4: Retrofit energy efficiency grants available to householders in Ireland from SEAI [66]. SEAI Better Energy Grants Thermal Fabric Grant Attic €400 Cavity Wall €400 Semi-Detached Internal Dry lining €2200 Semi-Detached External Insulation €4500 Heating System Grant Air to Water Heat Pump €3500 Heating Controls €700 Renewable Technologies Grant Solar Thermal €1200 PV (per kWp installed) €700

Table 5: Maximum available funding levels available to homes as part of the Better Energy Communities Scheme [67]. Home Type Private Energy Poor Private Non-Energy Poor Local Authority Homes Housing Association Homes Deep Retrofit (BER A3)

Funding levels for each component Up to 80% Up to 35% Up to 35% Up to 50% (Maximum 25% of homes may be fuel poor) Additional 15%

The cost per tonne of carbon produced by a building is assumed to be that used in the study carried out to determine the cost-optimal nZEB energy performance standards for residential buildings in Ireland [68]. As the analysis in the study for determining the nZEB energy performance standards is for 30 years, the cost per tonne of carbon in the final year of their analysis is assumed to remain constant for the remaining 30 years of this analysis. This cost per tonne of carbon is also applied in the householder perspective scenario, as Irish energy customers pay a carbon tax depending on their energy usage. A discount rate of 4% is applied for both the householder and societal scenarios. The costs associated with maintaining/repairing a building and recycling/selling/disposing of materials/systems are not considered in the life cycle cost analysis, as they are excluded from the life cycle environmental analysis. The standing charges for energy companies are excluded from the analysis in both the householder and societal scenarios.

2.6

Sustainability Index Factor (SIF)

This study examines the impact of the residential retrofit packages using six life cycle environmental indicators and one life cycle cost indicator. To be able to examine the tradeoffs between multiple indicators from a householder and societal perspective, this analysis uses the Sustainable Index Factor (SIF) previously used to assess the sustainability of new build nZEB residential buildings [69], which can be expressed mathematically for case study

n as:

(8) where a, b and c are weighting factors for each of the respective categories; k is (a,b,c);

ECOn is the economic impact of case study n; SOCn is the social impact of case study n; and ENVn is the environmental impact of case study n and can be expressed mathematically as:

∑

*(

∑

(

∑

*( (

∑

*( (

∑

∑

)

+

(9)

)

+

(10)

+

(11)

)

)

) )

where ecom,n is the impact of economic indicator m for case study n, socm,n is the impact of social indicator m for case study n, envm,n is the impact of environmental indicator m for case study n, wm is the weighting applied for each indicator depending on the categories importance, q is the number of indicators evaluated in each of the economic, social and

environmental categories and p is the number of case studies evaluated. The sum of the weightings ((wm)) applied to indicators in each category must add up to one. As can be seen from Equation 8, each of the three categories for the SIF can be given a different weighting depending on the importance each of the three categories. Furthermore, the indicators in each of the categories can be given a different weighting depending on their importance. LCC is used as the economic indicator in the SIF. The LCC calculated from the householder‘s perspective as part of the cost-optimal methodology is used in the SIF householder‘s perspective scenario. The LCC calculated from a societal perspective as part of the cost-optimal methodology is used in the SIF societal perspective scenario. From a householder perspective, a householder is likely to place the most importance on the energy demand of the house, as this is what their energy bills are primarily based on. In the householder scenario, energy is given a weighting of one and the other environmental indicators are ignored. The environmental category is given the same weighting as the economic category. From a societal perspective, many building retrofit environmental policy targets are related to energy and/or carbon. In this study, both energy and GWP are given a weighting of 0.25. The remaining four environmental indicators are given an equal weighting of 0.125. The environmental category is given the same weighting as the economic category. Table 6: Category and indicator weightings for the householder and societal scenarios assessed using the SIF. Category Economic Environmental Householder 1 1 Societal 1 1 Indicator LCC EN GWP ODP AP EP Householder 1 1 0 0 0 0 Societal 1 0.25 0.25 0.125 0.125 0.125 Abbreviations AP: Acidification Potential LCC: Life Cycle Cost

POCP 0 0.125

Social 0 0 N/A 0 0

EN: Energy EP: Eutrophication Potential GWP: Global Warming Potential

3

ODP: Ozone Depletion Potential POCP: Photochemical Ozone Creation Potential

Case Study Building and Retrofit Packages

The cost-optimal methodology and SIF framework are both used to assess the optimum retrofit solutions for retrofitting a theoretical typical 3-bed semi-detached residential house in Ireland. The theoretical case study building employed has been used in other studies [69,70]. To examine the cost-optimal methodology for retrofitting cavity wall construction in Ireland against the results of the SIF, basic and advanced thermal fabric and heating systems retrofit energy efficiency solutions were examined. The theoretical case study building was assumed to have thermal fabric efficiencies which meet the 1991 and 1997 Irish building element thermal fabric efficiency standards pre-retrofit [71,72]. The U-values of the building elements before the retrofit and target U-values for basic and advanced thermal fabric retrofit packages are shown in Table 7. The retrofit process was split into steps, as summarised in Table 8. Each step builds upon the previous. The steps were laid out to allow the retrofit to be completed over an extended period. The order of retrofit steps was similar to the Irish Tabula retrofit guide [73]. Table 7: U-values (W/m2K) of existing building elements and target U-values for basic and advanced retrofit packages.

Building Element Roof Windows Doors Wall Floor

PreRetrofit 0.36 2.8 3.1 0.46 0.43

Post-Retrofit Basic Advanced 0.10 0.10 1.5 0.7 1.5 0.7 0.33 0.15 Existing Existing

Table 8: Retrofit steps applied to building. Step 0 1 2 3 4 5

Action Existing Building Roof Insulation Window & Door Replacement Wall Insulation Heating System Renewable Energy Technology

The impact of each step on the energy demand in the case study building was calculated using the DEAP software. Retrofitting of floors is often omitted due to the financial cost of removing and replacing the existing floors [35,74,75]. Therefore, energy saving measures for the floor of the house were not considered in this analysis. The final step involved the addition of renewable energy technology to achieve the new nZEB building regulations for new residential buildings in Ireland. The regulations require new buildings to achieve a maximum energy performance coefficient (EPC) of 0.3 and a maximum carbon performance coefficient (CPC) of 0.35 [43]. The basic and advanced thermal fabric and heating system solutions employed are summarised in Table 9. Typical fabric insulation products such as expanded polystyrene (EPS), polyethylene and fibreglass have varying environmental impacts [23]. ‗Greener‘ measures for the external wall and attic insulation were also assessed in the analysis. A matrix of the retrofit packages assessed is shown in

Table 10. 35 different retrofit packages across five case study building are assessed. A more detailed description of the existing building elements and heating system and the energy efficiency retrofit measures for each case study building is given in Error! Reference source not found. Table 11. Table 9: Basic and advanced thermal fabric and heating system retrofit measures. Fabric Heating System Roof Windows & Door Wall Heating System* Renewable

Basic Basic JO JO DG DG

ADV ADV

JO DG

JO DG

Basic JO TG

JO TG

JO TG

ADV JO TG

JO TG

JO TG

JO TG

JO TG

CW CW CW CW CW/ CW/ CW/ CW/ CW/ CW/ CW/ CW/ EWI IWI EWI IWI EWI IWI EWI IWI GB HP GB HP GB GB HP HP GB GB HP HP SC/ PV

PV

Abbreviations ADV: Advanced Retrofit Measure CW: Pumped Cavity Wall Insulation DG: uPVC double glazed windows and uPVC doors EWI: External Wall Insulation GB: Gas boiler HP: Air Source Heat Pump IWI: Internal Wall Insulation

SC/ PV

SC/ PV

PV

PV

JO: Insulation on Joists of Roof PV: Multi-Crystalline Photovoltaic SC: Evacuated Tube Solar Collector TG: uPVC Triple Glazed Windows and uPVC Doors *Includes Hot Water Tank and Heating Controls

Table 10: Matrix of retrofit packages applied to the theoretical case study building. Refer to Table 8, Table 9 and Table 11 for the retrofit steps and measures applied. Retrofit Step Retrofit Package A B C D E

S0

S1

S2

S3

S4(a) S4(b)

EXT BAS

BAS

BAS

BASASHP BASASHP BASASHP BASASHP BASASHP

BASGas EXT BAS ADV- ADV- BASPVC EWI Gas EXT BAS ADV- ADV- BASALU IWI Gas EXT BAS-G ADV- ADV- BASPVC EWI-G Gas EXT BAS-G ADV- ADV- BASALU IWI Gas

Abbreviations ADV: Advanced Retrofit Measure ALU: Aluclad Window and Door Measure ASHP: Air Source Heat Pump BAS: Basic Retrofit Measure EWI-External Wall Insulation

S5(a)

S5(b)

ADVGas ADVGas ADVGas ADVGas ADVGas

ADVASHP ADVASHP ADVASHP ADVASHP ADVASHP

EXT: Existing Building G: Green Insulation Measure IWT-Internal Wall Insulation PVC: PVC Window and Door Measure

Table 11: Detailed description of the retrofit solutions and their associated U-values (W/m2K) for each of the five case study buildings. Description S1: Roof Insulation Existing: Domestic 30° pitched tiled roof, 2mm breathable felt, batons (35x35mm), rafters (44x175mm), joists (44x225mm), 100mm fibreglass insulation (λ=0.04 W/mK) and 13mm plasterboard ceiling. Retrofit Measures: 300mm mineral wool insulation (λ=0.04 W/mK) applied in between and above joists of roof 300mm rockwool insulation (λ=0.04 W/mK) applied in between and above joists of roof S2: Window and Door Replacement Existing: Double glazed uPVC windows (U-value: 3 W/m2k) and uPVC doors (Uvalue: 3.1 W/m2k) Retrofit Measures: Double glazed uPVC windows & uPVC doors Triple glazed passive uPVC windows & uPVC doors Triple glazed passive Aluclad windows & Aluclad doors S3: Wall Insulation Existing: Cavity block wall,19mm external plaster, 100mm concrete block, 35mm Air Cavity, 65mm cavity rigid board insulation (λ=0.04 W/mK), 100mm concrete block, 12.5mm scratch coat, 5mm gypsum coat and 3 layers of water-based paint

Retrofit UPackage Value 0.356

A-C

0.103

D, E

0.103

A B, D C, E

1.5 0.7 0.7 0.459

either side.

Retrofit Measures: 35mm polystyrene bead cavity fill insulation (λ=0.035 W/mK) 9mm external render (λ=0.57 W/mK), 170mm external EPS insulation (λ=0.037 W/mK) and 35mm polystyrene bead cavity fill insulation (λ=0.035 W/mK) 35mm polystyrene bead cavity fill insulation (λ=0.035 W/mK), 90mm internal wall phenolic insulation (λ=0.021 W/mK), 12.5mm plasterboard (λ=0.19 W/mK) and 3mm gypsum coat (λ=0.05W/mK) 9mm external render (λ=0.57 W/mK), 180mm external wood fibre insulation (λ=0.04 W/mK) and 35mm polystyrene bead cavity fill insulation (λ=0.035 W/mK) S4: Heating System Existing: Gas fired boiler with an efficiency of 77%. Solid fuel open fire with an efficiency of 30%. Natural ventilation used throughout. Retrofit Measures – Option (a): Main Space and Water Heating – Gas boiler with efficiency of 92.1% and hot water tank, Secondary Space Heating- Electric fire heater with 100% efficiency and chimney sealed Retrofit Measures – Option (b): Main Space and Water Heating – Air to water (ATW) heat pump with efficiency of 397% and hot water tank, Secondary Space Heating- Electric fire heater with 100% efficiency and chimney sealed S5: Renewable Energy Technology Retrofit Measures – Option (a): 3.2m2 evacuated solar tube collector (0.727 zero loss collector efficiency) & 20.4m2 of photovoltaic panels (3.6kWp) 3.2m2 evacuated solar tube collector & 13.6m2 of photovoltaic panels (2.4kWp) Retrofit Measures – Option (b): 15.3m2 of photovoltaic panels (2.7kWp) 10.2m2 of photovoltaic panels. (1.8kWp)

A B

0.343 0.150

C, E

0.153

D

0.151

A-E

A-E

A B-E A B-E

The existing original theoretical case study building was assumed to have an air permeability of 10m3/(hr.m2). Houses that were classified to have (i) received grant support on the SEAI National BER Research Tool database [76], (ii) had an air-tightness test complete and (iii) achieved a BER rating of B1 or B2 achieved an average air permeability of 3.9m3/(hr.m2). Houses that achieved a BER rating of B3 or C1 achieved an average air permeability of 8.6m3/(hr.m2). In this analysis after completing step 3 for the basic retrofit of the thermal fabric (Retrofit Package A), the building achieved a C1 BER rating and was assumed to have an air permeability of 7m3/(hr.m2). After completing step 3 for the more advanced upgrade of the

thermal fabric (Retrofit Packages B-E), the building achieved a B1 BER rating and was assumed to have an air permeability of 5m3/(hr.m2 ). 4 4.1

Results Cost-Optimal

Table 12 shows the energy demand of the theoretical case study building without any retrofit measures (0) and the applied retrofit packages (1-5(b)). Figure 1 shows the cost-optimal results of the theoretical case study building without any retrofit measures (0) and the applied retrofit packages (1-5(b)) from a householder perspective. The operational cost savings achieved from the energy efficiency retrofit measures did not outweigh the initial investment costs for most of the retrofit packages for the semi-detached house. Apart from applying mineral wool or rock wool insulation to the joists of the attic, all other retrofit packages were found to have larger LCCs than the original building. Table 12: Energy demand (kWh/m2) of the theoretical case study building without any retrofit measures (0) and the applied retrofit packages (1-5(b)).

Retrofit Package A B C D E

S0 189 189 189 189 189

S1 180 180 180 180 180

S2 164 151 151 151 151

Retrofit Step S3 S4(a) 152 126 122 101 122 101 122 101 122 101

S4(b) 97 79 78 78 78

S5(a) 42 41 41 41 41

S5(b) 45 43 43 43 43

Current SEAI grants available to householders, given in Table 4, were accounted for in the results of Figure 1 (b). Due to the grants, the LCC gap between the existing original case study building and the retrofit packages reduced. However, only two additional retrofit packages (Retrofit Package A-S3 and A-S4(a)) result in lower LCC compared to the original building without any retrofit measures.

VAT for construction materials and labour is currently at 13.5%. When both grants available were included and the VAT for materials and labour excluded, two additional retrofit packages (Retrofit Package A-S5(a)/(b)) reduced the LCC of the existing building (Figure 1 (c)). The two additional retrofit packages were deep retrofits with basic thermal fabric measures and advanced heating system measures.

For houses involved in the SEAI Better Energy Communities Scheme, grants covering up to 80% of the energy efficiency retrofit costs are available (Table 5). Assuming there was a grant available covering 50% of the retrofit costs, 22 of the 35 retrofit packages reduced the LCC of the building (Figure 1 (d)). A 50% grant is available to housing association homes and private households in fuel poor homes. However, private households in non-fuel poor homes and local authority homes can avail of a 50% grant if they undergo a deep retrofit and achieve a BER of A3. Retrofit Package A-S5(a) had the lowest LCC of the 35 retrofit packages. 10 of the 35 retrofit packages analysed achieved an A2 BER rating (A-E S5(a)/S5(b)). The 10 retrofit packages include advanced heating system measures in addition to either basic or advanced retrofit packages analysed achieved an A2 rating (A-E S5(a)/S5(b)). The 10 retrofit packages include advanced heating system measures in addition to either basic or advanced thermal fabric measures. Six of these 10 retrofit packages lowered the LCC of the existing building when 50% of the costs were assumed to be covered through a grant. The four retrofit packages designed to achieve an A2 rating that did not lower the LCC of the existing building, when 50% of the costs were assumed to be covered through a grant, were based on using external insulation. Furthermore, the internal insulation-based packages that achieved an A2 rating while lowering the LCC of the original semi-detached building did not account for the cost of removing and reinstalling kitchen cabinets etc. to add the internal insulation to the walls.

Figure 1: Cost-optimal results of the theoretical case study building and the applied retrofit packages from a householder perspective (a) excluding grants available, (b) including grants available to householders to retrofit home, (c) excluding VAT for materials and labour and including grants available to householders to retrofit home and (d) assuming a grant covering 50% of the costs is available.

The cost-optimal results of the theoretical building and the applied retrofit packages from a societal perspective showed that five shallow retrofit packages which installed mineral wool and rock wool insulation on the joists of the attic reduced the LCC of the theoretical case study building. Once the grants available to householders were considered, eight of the 35 retrofit packages had lower LCC compared to the existing theoretical building. 22 of the retrofit packages reduced the LCC of the existing building when assuming 50% of the costs are covered by a grant. Retrofit Package A-S5(a) had the lowest LCC from a societal perspective. 4.1.1 Sensitivity Analysis The results of the previous section were based on the pre- and post-retrofit energy demand estimated by the DEAP software. DEAP is based on quasi-steady state formulae for estimating the energy demand of residential buildings. However, studies have shown there to be a difference between the actual energy consumption of a building in comparison to the estimated energy demand using quasi-steady state formulae. The energy performance gap (EPG) is a metric used to assess the actual energy consumption as a proportion of theoretical energy demand [77]. Using information provided in published studies on building retrofits which estimated energy demand using steady state formulae [78–85], the EPGs of the studies ranged from -44% to 64% pre-retrofit and -66% to 55% post-retrofit. A sensitivity analysis was carried out for a range of EPGs pre- and post-retrofit for the retrofit packages examined in the previous section. Table 13 gives the impact a range of EPGs preand post-retrofit has on the number of retrofit packages which reduced the LCCs of the theoretical case study building without any retrofit measures. An EPG with a negative percentage means that the case study building has a lower energy demand in comparison to

the theoretical estimates. For example, an EPG of -50% pre-retrofit means that the case study building uses half of the estimated energy demand pre-retrofit. The estimated energy demand results presented in the previous section are applicable when there is no EPG pre- or post-retrofit. In this instance, five retrofit packages would lead to a lower LCC (Table 13). The results given in Table 13 do not account for any grant aid available. As can be seen from the results of Table 13, if the EPG pre-retrofit was less than the EPG post-retrofit, no retrofit package lead to a reduction in LCC. For example, this occurred if the EPG pre-retrofit was -10% and the EPG post-retrofit was 0%. If the EPG preretrofit was equal to the EPG post-retrofit and the EPG pre-retrofit was greater than 0%, then a greater number of retrofit packages (more than 5) reduced the LCC of the original case study building. For example, this occurred if the EPG pre-retrofit was 10% and the EPG postretrofit was 10%. Applying mineral wool insulation to the joists of the attic are found to be the most risk-free retrofit packages for achieving a decrease in LCC, as it is still cost effective even if minimum energy savings occur (i.e. if the EPG pre-retrofit is -50% and the EPG post-retrofit is -50%). Where both the EPG pre-retrofit and EPG post-retrofit is 50%, the deep retrofit packages with basic thermal fabric measures and advanced heating system measures are found to be the cost optimal packages (A S5(a)/S5(b)). Table 13: The impact a range of EPGs pre- and post-retrofit has on the number of retrofit packages which reduced the LCCs of the theoretical case study building without any retrofit measures (grant aid not accounted for).

EPG Post-Retrofit (%) 50 25 10 0 -10

50 12 18 20 21 23

EPG Pre-Retrofit (%) 25 10 0 -10 -25 0 0 0 0 0 9 0 0 0 0 12 6 0 0 0 15 10 5 0 0 17 12 10 5 0

-50 0 0 0 0 0

-25 25 18 -50 27 21

14 12 10 18 16 13

3 12

0 3

Note: Bold indicates that the life cycle cost estimates using DEAP results in zero energy performance gap and five retrofit measures result in reduced LCC compared to original building.

Even when assuming householders can avail of a 50% grant (Table 14), applying mineral wool insulation to the joists of the attic was found to be the most risk-free retrofit package for achieving a decrease in LCC. If the post-retrofit EPG was greater than the pre-retrofit EPG, Retrofit Packages A-E S1, A-S2-S4, B-S2 and D-S2 were more probable to decrease the LCC. Table 14: The impact a range of EPGs pre- and post-retrofit has on the number of retrofit packages which reduced the life cycle costs of the theoretical case study building without any retrofit measures (assuming a grant covering 50% of the costs is available).

EPG Post-Retrofit (%) 50 25 10 0 -10 -25 -50

4.2

50 33 35 35 35 35 35 35

EPG Pre-Retrofit (%) 25 10 0 -10 -25 13 6 2 1 0 31 11 5 2 0 33 26 11 2 0 35 32 22 3 1 35 32 26 19 1 35 35 31 25 12 35 35 35 30 21

-50 0 0 0 0 0 0 5

Life Cycle Cost and Life Cycle Environmental

Figure 2 shows the LCC and life cycle environmental impact for the various steps of Retrofit Package A. The LCC results shown in Figure 2 do not account for any grant available or VAT tax break. The original theoretical building reduced its life cycle EN and life cycle GWP following each retrofit step. However, the LCC and other LC environmental indicators

did not follow this trend as the building becomes more energy efficient. The same was found for Retrofit Packages B, C, D and E. Switching from a gas fuel heating system to an electric heating system (S4(a) to S4(b)) significantly reduced the life cycle EN and life cycle GWP. However, the embodied impact of the ASHP heating system and the environmental impact of the Irish electricity grid resulted in increases in the life cycle ODP, life cycle AP, life cycle EP and life cycle POCP. Moving to the nZEB new build standards for the gas fuelled buildings was achieved using ETSC and MCPV. Despite the operational ODP savings from the renewable technology, the initial environmental investment in producing the renewable technology increased the LC impact of ODP when moving from retrofit steps S4(a) to S5(a).

Figure 2: Embodied (E) and operational (O) LCC and life cycle environmental impact for the various steps of Retrofit Package A. Environmentally friendly materials come at an additional cost. For insulation on the joists of the roof, mineral wool insulation was used as a retrofit measure in retrofit packages A-C and rock wool insulation was used as a retrofit measure in retrofit packages D-E, respectively. While rock wool was more environmentally friendly in each of the assessed environmental indicators, mineral wool was financially the less expensive option. For example, rock wool had an embodied impact 28% and 38% that of mineral wool in terms of energy and GWP. However, mineral wool is €10/m2 (i.e. 51%) less expensive.

EPS external insulation, used in retrofit package B, was less expensive than wood fibre external insulation in terms of economic cost, which was used in retrofit package D. From an environmental standpoint, EPS was also the better option than wood fibre in terms of its embodied energy, AP and EP impact. However, wood fibre board had less of a GWP, ODP and POCP environmental impact. Aluclad triple glazed windows (employed in retrofit packages C and E) were a more expensive option compared to uPVC triple glazed windows (employed in retrofit package B and D), but were more environmentally friendly in terms of GWP and POCP. As the buildings became more energy efficient, the embodied impact of the retrofit measures became the main contributor to the LCC, life cycle ODP, life cycle AP, life cycle EP and life cycle POCP (Table 15). In some cases, the embodied impact of the materials represented the whole life cycle indicator impact. The LCC results shown in Table 15 do not account for any grant available or VAT tax break.

Table 16 shows the retrofit package with the minimum and maximum LC impact for each of the economic and environmental indicators. As can be seen from the results in Table 16, no one retrofit package is the optimum solution for every indicator evaluated. The LCC results shown in Table 16 do not account for any grant available or VAT tax break. Table 15: Maximum embodied share of a building’s life cycle for each of the economic and environmental indicators. Case Study/Indicator A B C

Cost (%) 89 91 89

GWP (%) 32 36 32

EN (%) 23 28 26

ODP (%) 89 87 87

AP (%) 100 100 100

EP (%) 100 100 100

POCP (%) 100 95 91

D E

91 90

26 30

36 24

86 87

100 100

100 100

90 90

Table 16: Retrofit package with the minimum and maximum LC impact for each of the economic and environmental indicators. Cost €

GWP kgCO2eq

Min A-C S1 D S5(b) Max D S5(a) S0

4.3

EN

ODP

AP

EP

POCP 3-

MJ

kg CFC11 eq

kg SO2 eq

kg (PO4) eq

kg C2H4 eq

E S5(b) S0

D S4(a) C S4(b)

A S5(a) C S4(b)

B S3 C S4(b)

A S5(a) S0

Sustainable Index Factor (SIF)

The SIF was used to assess the optimum retrofit package for the theoretical case study building both from a householder‘s perspective and societal perspective based on the weightings discussed in Section 2.6. Shown in Table 17 are the impacts of the retrofit packages from a householder‘s perspective considering LCC and life cycle EN.

Table 18 shows the impacts of the retrofit packages from a householder‘s perspective but including available grants. The SIF results of Table 19 account for available grants and exclude VAT for materials and labour and Table 20 assumes that 50% of the retrofit cost is covered through a grant. Implementing the basic thermal fabric retrofit measures for all the building elements and introducing a gas boiler or heat pump with renewable energy technology were found to be the optimum packages of energy efficiency measures in Table 17 to Table 20. The ASHP retrofit packages (S4(b) and S5(b)) outperformed most of their gas boiler retrofit package counterpart (S4(a) and S5(a)) apart from package C and when a grant of 50% is available. Regardless of

the retrofit package implemented on the case study building, all outperformed the existing building. Table 17: SIF impacts of the retrofit packages from a householder’s perspective.

Retrofit Package A B C D E

S0

S1

S2

1.160 1.160 1.160 1.160 1.160

1.112 1.112 1.113 1.119 1.119

1.082 1.021 1.063 1.028 1.068

Retrofit Step S3 S4(a) 1.019 1.070 0.992 1.134 0.993

0.906 0.987 0.909 1.051 0.910

S4(b) S5(a)

S5(b)

0.883 1.005 1.036 1.059 0.917

0.677 0.863 0.784 0.926 0.785

0.753 0.949 0.870 1.013 0.871

Table 18: SIF impacts of the retrofit packages from a householder’s perspective accounting for available grants.

Retrofit Package A B C D E

S0

S1

S2

1.205 1.205 1.205 1.205 1.205

1.151 1.151 1.152 1.158 1.158

1.125 1.062 1.108 1.070 1.115

Retrofit Step S3 S4(a) 1.055 1.070 1.010 1.138 1.012

0.934 0.980 0.920 1.048 0.922

S4(b) S5(a)

S5(b)

0.875 0.963 1.018 1.020 0.894

0.629 0.801 0.741 0.869 0.743

0.686 0.861 0.802 0.930 0.804

Table 19: SIF impacts of the retrofit packages from a householder’s perspective accounting for available grants and excluding VAT for materials and labour.

Retrofit Package A B C D E

S0

S1

S2

1.224 1.224 1.224 1.224 1.224

1.168 1.168 1.169 1.175 1.175

1.143 1.080 1.128 1.087 1.134

Retrofit Step S3 S4(a) 1.071 1.065 1.017 1.160 1.019

0.945 0.969 0.922 1.065 0.924

S4(b) S5(a)

S5(b)

0.872 0.938 1.008 1.023 0.881

0.608 0.765 0.717 0.860 0.719

0.653 0.812 0.765 0.908 0.766

Table 20: SIF impacts of the retrofit packages from a householder’s perspective assuming a grant of 50% is available.

Retrofit Package A B C

S0

S1

S2

Retrofit Step S3 S4(a)

S4(b) S5(a)

S5(b)

1.291 1.234 1.174 1.100 0.962 0.890 0.627 0.604 1.291 1.234 1.099 1.062 0.955 0.926 0.764 0.727 1.291 1.234 1.129 1.008 0.902 0.998 0.710 0.673

D E

1.291 1.238 1.103 1.114 1.007 0.966 0.815 0.778 1.291 1.238 1.132 1.007 0.900 0.859 0.708 0.671

Table 21 shows the impacts of the retrofit packages from a societal perspective including all indicators. Unlike the results of Table 17 to Table 20, many of the retrofit packages involving switching to an electric ASHP were found to be worse than the existing building. Four of the top five retrofit packages included replacing the existing gas boiler with a more efficient gas boiler. Unlike Table 17 to Table 19, the gas boiler retrofit packages (S4(a) and S5(a)) often outperformed their ASHP retrofit package counterpart (S4(b) and S5(b)) in Table 21. Once grants were accounted for in the analysis, all the gas boiler retrofit packages (S4(a) and S5(a)) often outperformed their ASHP retrofit package counterpart (S4(b) and S5(b)) (Table 22 and Table 23). With grants, more of the retrofit packages including renewable technology became more viable options. Assuming a grant covering up to 50% of the costs, Retrofit Package A5(a) was the best option both from a householder‘s perspective and societal perspective. Table 21: SIF impacts of the retrofit packages from a societal perspective.

Retrofit Package A B C D E

S0

S1

S2

1.067 1.067 1.067 1.067 1.067

1.022 1.022 1.023 1.027 1.027

1.009 0.950 1.003 0.955 1.007

Retrofit Step S3 S4(a) S4(b) 0.949 1.025 0.948 1.073 0.947

0.803 0.935 0.858 0.983 0.857

0.990 1.139 1.172 1.176 1.050

S5(a) 0.805 1.047 0.970 1.095 0.969

S5(b) 0.827 1.028 0.951 1.076 0.950

Table 22: SIF impacts of the retrofit packages from a societal perspective accounting for grants available.

Retrofit Package A B C D E

S0

S1

S2

1.116 1.116 1.116 1.116 1.116

1.064 1.064 1.065 1.070 1.070

1.056 0.996 1.054 1.002 1.059

Retrofit Step S3 S4(a) 0.988 1.023 0.969 1.080 0.969

0.834 0.926 0.871 0.983 0.871

S4(b) S5(a)

S5(b)

0.981 1.090 1.152 1.135 1.023

0.773 0.958 0.903 1.015 0.903

0.730 0.949 0.894 1.005 0.894

Table 23: SIF impacts of the retrofit packages from a societal perspective assuming 50% of the costs are covered by a grant.

Retrofit Package A B C D E

S0

S1

1.198 1.198 1.198 1.198 1.198

1.144 1.144 1.145 1.147 1.147

S2 1.100 1.027 1.067 1.030 1.069

Retrofit Step S3 S4(a) 1.029 1.018 0.963 1.045 0.960

0.859 0.905 0.850 0.932 0.847

S4(b)

S5(a)

S5(b)

0.997 1.062 1.135 1.077 0.992

0.684 0.868 0.813 0.895 0.810

0.756 0.896 0.841 0.923 0.838

4.3.1 Sensitivity Analysis The results of the previous section were based on the pre- and post-retrofit energy demand estimated by the DEAP software. A sensitivity analysis was carried out for a range of EPGs pre- and post-retrofit for the retrofit packages examined in the previous section. Table 24 gives the impact a range of EPGs pre- and post-retrofit has on the number of retrofit packages which reduced the SIF impact of the theoretical case study building from a societal perspective. The estimated energy demand results of the previous section occur when there is no EPG pre- or post-retrofit. In this instance, 31 retrofit packages would lead to a reduced SIF impact (Table 24). If minimum and maximum energy savings occurred for all the retrofit packages, Retrofit Package A-S4(a) remained the optimum package if no grants are accounted for, similar to the results of Table 21. Following Retrofit Package A-S4(a), the shallow retrofit packages applying insulation to the joists of the attic were found to be the next best retrofit options. If both the EPG pre- and post-retrofit is 50%, Retrofit Packages A-S5(a) and A-S5(b) are the best options with Retrofit Package A-S4(a) being the 6th best option. In this instance, the gas boiler retrofit packages (S4(a) and S5(a)) outperformed each of their ASHP retrofit package counterparts (S4(b) and S5(b)). Where minimum and maximum energy savings occur, the

ASHP retrofit packages (S5(b)) outperformed each of their gas boiler retrofit package counterparts (S5(a)). Table 24: The impact a range of EPGs pre- and post-retrofit has on the number of retrofit packages which reduce the SIF impact of the theoretical case study building from a societal perspective.

EPG-POST (%) 50 25 10 0 -10 -25 -50

5

50 35 35 35 35 35 35 35

25 17 34 35 35 35 35 35

EPG-PRE (%) 10 0 -10 9 2 1 21 9 2 32 17 5 34 31 14 35 32 26 35 35 30 35 35 35

-25 0 0 0 1 5 20 28

-50 0 0 0 0 0 0 9

Discussion and Conclusions

The objective of the theoretical case study presented in this study was not to provide an optimal retrofit design solution for all semi-detached houses in Ireland, as the number of case studies examined was not sufficient. The objective was to examine whether the optimum energy efficiency retrofit design differs based on multiple indicators rather than relying on only energy consumption and financial cost. 35 different energy efficiency retrofit packages for a theoretical gas-heated cavity wall semi-detached house were examined using both the cost-optimal methodology and the SIF using multiple indicators from a householder and societal perspective. Using the cost-optimal approach, it was found that without the use of tax breaks and/or grants, only shallow retrofits (attic insulation) were cost-effective for an energy efficiency retrofit. This may be due to the retrofit packages examined as the difference in the shallow and advanced building element U-values was large, and window retrofits are also not recommended in terms of cost payback for houses constructed from the 1980‘s onwards [58]. However, once government aided grants were considered, deeper energy efficiency retrofit

packages became cost-optimal. This highlighted the need for government aid in encouraging deep energy efficiency retrofit of the residential housing stock. The cost-optimal methodology and the SIF householder‘s perspective scenario both found that implementing the basic thermal fabric retrofit measures for all the building elements and introducing a gas boiler or heat pump with renewable energy technology to be the optimum packages (Retrofit Package A-S5(a)/(b)) once grants were considered. Most of the retrofit packages including renewable technology that moved the case study building towards nZEB new build energy standards were found to be the optimum packages once grants were accounted for. When examined from a societal perspective considering all the evaluated indicators in the SIF, it was found that a gas boiler heating system upgrade following upgrading the building elements was among the lowest SIF impacts. For designing energy efficiency retrofits, the results have shown the importance of considering life cycle environmental indicators and including more indicators other energy and cost when evaluating the sustainability of a building retrofit design. When only considering cost and energy, the ASHP retrofit packages (S4(b) and S5(b)) outperformed many of their gas retrofit package counterpart (S4(a) and S5(a)) in many instances. Conversely, once all the indicators were considered, the gas retrofit packages (S4(a) and S5(a)) outperformed their ASHP retrofit package counterparts in most instances (S4(b) and S5(b)). The life cycle EN and life cycle GWP results supported having grants available to householders to install an ASHP and/or renewable technology. However, for other environmental indicators evaluated, heat pumps and adding renewable technology were found to be worse in many instances due to the environmental embodied impact of the systems and the environmental impact of the electricity grid. For retrofit steps S-4(b), S-5(a)

or S-5(b), the introduction of an ASHP heating system and/or renewable technology resulted in larger life cycle ODP, life cycle EP, life cycle AP and life cycle POCP impacts. In some energy efficiency retrofit packages, the embodied environmental impact from the introduction of an ASHP and/or renewable technology accounted for the total life cycle ODP, life cycle EP, life cycle AP and life cycle POCP impact. The environmental impacts can be mitigated by developing more environmentally friendly production processes or recycling the technology at the end of their life span, which was not considered in the analysis. Due to the ASHP having an efficiency of approximately four times that of the gas boiler, an ASHP was better environmentally in terms of operational EN, GWP, ODP and POCP. However, for AP and EP, a gas boiler was found to be less impactful. The analysis found that if Ireland is to continue moving toward a more electrified economy which will increase the demand for electricity, the environmental impact of electricity generation for multiple indicators needs to be continuously monitored and mitigated to ensure Ireland achieves its goal of a sustainable economy by 2050. In the SIF analysis, a limited number of indicators were assessed, and indicator weightings assumed for the householder and societal evaluations. Further research is needed into selecting appropriate indicators, indicator weightings and an optimization methodology for assessing the sustainability of a building retrofit design. The cost-optimal approach was found to be effective for identifying retrofit packages that are among the best packages even without accounting for multiple environmental indicators. However, once multiple indicators were considered the hierarchy of the optimal retrofit packages changed. While the costoptimal method should not be relied on solely for identifying the optimum retrofit package solution, it could be employed as part of the multistage assessment methodology for narrowing down design solution sample sizes such as that employed by Tadeu [35]. Using the

remaining array of retrofit packages solutions, the optimum retrofit package could be identified using multiple indicators. While partially government funded building energy efficiency retrofit packages may be costeffective in theory, the actual energy consumption of houses may result in different optimum energy efficiency retrofit packages or retrofit packages not being cost-effective for a householder. A sensitivity analysis on the impact of pre- and post-retrofit EPGs revealed the optimum retrofit packages to be impacted on the accuracy of the quasi-steady state formulae. With EPGs common in retrofit studies [78–85], more research is required to understand the factors driving EPGs for existing and retrofitted residential buildings as energy use in buildings is affected by the interrelationships between the physical performance of the fabric, building systems, the occupants‘ understanding of the building systems, expectations and perception of comfort and occupants habitual behaviours [86]. In summary, the results of this study find that: 

Deep energy efficiency retrofits are economically feasible in Ireland with aid from government grants and tax breaks for semi-detached gas heated houses in Ireland constructed between 1991 and 2002. Without these incentives, only shallow retrofits are economically feasible.



The ranking of energy efficiency retrofit designs based on multiple life cycle environmental, and cost indicators differ compared to cost optimum designs based on only life cycle energy and cost indicators.



Savings in terms of energy and GWP support the availability of grants for householders to retrofit with an ASHP and/or renewable technology. However, the embodied environmental impact of an ASHP and renewable technology in terms of

ODP, EP, AP and POCP need to be mitigated using cleaner production processes or recycling. 

If Ireland is to continue moving to a more electrified economy which will increase the demand for electricity, the environmental impact of electricity generation for multiple indicators needs to be continuously monitored and mitigated to ensure Ireland achieves its goal of a sustainable economy by 2050.

DECLARATION OF INTEREST We wish to confirm that there are no known conflicts of interest associated with this publication and there has been no significant financial support for this work that could have influenced its outcome. We confirm that the manuscript has been read and approved by all named authors and that there are no other persons who satisfied the criteria for authorship but are not listed. We further confirm that the order of authors listed in the manuscript has been approved by all of us. We confirm that we have given due consideration to the protection of intellectual property associated with this work and that there are no impediments to publication, including the timing of publication, with respect to intellectual property. In so doing we confirm that we have followed the regulations of our institutions concerning intellectual property. We understand that the Corresponding Author is the sole contact for the Editorial process (including Editorial Manager and direct communications with the office). He/she is responsible for communicating with the other authors about progress, submissions of revisions and final approval of proofs. We confirm that we have provided a current, correct email address which is accessible by the Corresponding Author and which has been configured to accept email from ([email protected]). Authors‘ statement

Dr Jamie Goggins developed the initial conceptualization for the study, which was refined by Drs Jamie Goggins and Paul Moran. The aforementioned authors also developed the methodology. Data curation and analysis was primarily conducted by Dr Paul Moran and John O‘Connell, but with input from Dr Jamie Goggins. All three authors contributed to the writing of the article with final review and editing being conducted by Drs Paul Moran and Jamie Goggins. Drs Paul Moran and Jamie Goggins are joint lead authors on the paper. Acknowledgements The authors would like to acknowledge the support of Science Foundation Ireland through the Career Development Award programme (Grant No. 13/CDA/2200). The authors would like to acknowledge the support of the European Union‘s FP7 ERA-NET Sumforest grant programme (Grant agreement No. 606803) and the European Union‘s Horizon 2020 research and innovation programme under grant agreement No 839134. References [1]

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