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External walls design: The role of periodic thermal transmittance and internal areal heat capacity
Energy and Buildings 68 (2014) 732–740
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Energy and Buildings journal homepage: www.elsevier.com/locate/enbuild
External walls design: The role of periodic thermal transmittance and internal areal heat capacity Monica Rossi a,∗ , Valeria Marta Rocco b a b
University of Camerino, School of Architecture and Design “Eduardo Vittoria”, Italy Politecnico di Torino, DIST: Interuniversity Department of Regional and Urban Studies and Planning, Italy
a r t i c l e
i n f o
Keywords: Energy demand External walls Thermal parameters Periodic thermal transmittance Internal areal heat capacity Indoor comfort Italian regulation
a b s t r a c t Recent studies have shown that considering the values of superficial mass (Ms ) and periodic thermal transmittance (Ymn ) of external walls is not sufficient to achieve energy savings, particularly in summer. For this reason experimental reference values of the internal areal heat capacity (k1 ) were introduced. This study aims to understand the interdependency between some thermal parameters (U, Ms , ϕ, Fa , Ymn , k1 ) of massive and lightweight walls with respect to their energy performance in use in office buildings in Southern Europe. The study has analyzed eight walls with Italian standard U and Ymn values. These walls have been then modified in order to reach k1 values corresponding to the reference ones. The energy demand to ensure a defined level of indoor thermal comfort has been verified with thermodynamic simulations on a virtual test-room localized in two Italian cities, characterized by different climate conditions. The research results are: to develop design change strategies for external walls to achieve the k1 reference values; to quantify the thermal annual energy demand of a virtual test-room equipped with the sample walls and then equipped with the improved walls; to compare the energetic and economic impact for the improved walls against the sample ones. © 2012 Elsevier B.V. All rights reserved.
1. Introduction In 2002 the European Directive 2002/91/EC [1] introduced the topic of reducing buildings energy demand during summertime: “Recent years have seen a rise in the number of air-conditioning systems in southern European countries. This creates considerable problems at peak load times, increasing the cost of electricity and disrupting the energy balance in those countries. Priority should be given to strategies which enhance the thermal performance of buildings during the summer period. To this end there should be further development of passive cooling techniques, primarily those that improve indoor climatic conditions and the microclimate around buildings.” Therefore in recent years the reduction of the building energy demand for cooling and the achievement of a high level of indoor thermal comfort in summertime are the subjects of many scientific researches carried out in South Europe. Several studies have demonstrated the importance of having a building envelope with a high thermal inertia – particularly for the summertime performance – both for saving energy and indoor comfort [2].
∗ Corresponding author. Tel.: +39 347 6990886; fax: +39 0734 404258. E-mail addresses: [email protected], [email protected] (M. Rossi), [email protected] (V.M. Rocco). 0378-7788/$ – see front matter © 2012 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.enbuild.2012.07.049
Furthermore, the European regulation UNI EN ISO 13786/2008 [3] has introduced other parameters – in addition to the thermal transmittance (U in W/m2 K) and the superficial mass (Ms in kg/m2 ) – for the evaluation of the thermal inertia and the summer thermal performance of building envelopes. These parameters are: -
thermal time shift, ϕ (h); thermal decrement factor, Fa (dimensionless); periodic thermal transmittance, Ymn (W/m2 K); internal areal heat capacity, k1 (kJ/m2 K).
The time shift is the period of time between the maximum amplitude of a cause and the maximum amplitude of its effect [3]. The decrement factor is the ratio of the modulus of the periodic thermal transmittance to the steady-state thermal transmittance U [3]. The periodic thermal transmittance (incorporating the concepts of thermal transmittance, time shift and decrement factor) is a complex quantity defined as the complex amplitude of the density of heat flow rate through the surface of the component adjacent to zone m, divided by the complex amplitude of the temperature in zone n when the temperature in zone m is held constant [3]. The internal areal heat capacity is the heat capacity divided by area of element and the heat capacity is the modulus of the net periodic thermal conductance divided by the angular frequency [3].
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Table 1 Thermal parameters of external walls according to the Italian regulations: climate zone E.
U Ms ϕ Fa Ymn
Legislation
Reference value
D.Lgs. 311/2006 D.Lgs. 311/2006 D.M. – 26th June 2009 D.M. – 26th June 2009 DPR 59/2009
U < 0.34 W/m2 K – climate zone E (a cold Italian zone) Ms > 230 kg/m2 (exception to zone F, the coldest one) 8 h ≥ ϕ > 6 h average – ϕ > 12 h excellent 0.40 ≤ Fa < 0.60 average – Fa < 0.15 excellent Ymn < 0.12 W/m2 K
Table 2 Reference value of k1 defined in the research carried out by Di Perna et al. [7]. Reference value of k1 k1 ≥ 50 if Ymn ≤ 0,04 k1 ≥ 70 if 0.04 ≤ Ymn ≤ 0.08 k1 ≥ 90 if 0.08 ≤ Ymn ≤ 0.1
The Italian legislations [4–6] have defined reference values for some of the mentioned parameters (Table 1) depending on the climate zone. Indeed the Italian law divides Italy into 6 different climate zones, from the coldest (zone F) to the hottest (zone A).1 In this study the limit of winter and summer performance values of the climate zone E has been considered. The climate zone E is the first zone, among the cold ones, that requires the verification of summer parameters. Recent research carried out by Di Perna et al. [7] has shown “how the limit introduced for the periodic thermal transmittance value leads to the development of envelopes which are totally different from the point of view of comfort. Walls with the same stationary and periodic thermal transmittance, but which differ from each other only in the k1 value, behave very differently from the point of view of indoor comfort.” In fact, a low Ymn value leads to a reduction of outside thermal load impact, particularly from direct sunlight irradiation on external walls, but it is not able to reduce the contribution of the internal loads. Internal heat loads are however the main cause of excessive indoor temperatures during summertime, particularly in office buildings. Indeed the k1 value describes the actual capacity to accumulate heat on the inner side of a building element and characterizes the internal thermal mass. An envelope with a high potential for heat accumulation on the inner side has a high k1 value. Italian regulations neither mention nor define any limit and/or reference values for k1 . The above mentioned study [7] defined minimum values for k1 depending on the values of Ymn (Table 2) for the first time. 2. Methodology This study aims to understand the interdependency between thermal parameters of external walls (Table 1) and to analyze the influence of these thermal parameters on the indoor comfort level in office buildings (characterized by the presence of high internal thermal loads) localized in different climate zones of Southern Europe. To achieve this goal a research organized by the following three steps has been carried out: - calculation of the thermal parameters’ (U, Ms , ϕ, Fa , Ymn , k1 ) initial status, for eight selected massive and lightweight external walls; - design changes on the eight sample walls in order to get k1 values which are comparable to the new introduced k1 reference values,
1 Italian Regulations define different building envelopes U value limits for each climate zone and one universal reference Ymn value for all climate zones. The verification of the Ymn is not necessary for the coldest climate zone (F).
still complying with the legislation limits for the other thermal parameters; - energy demand comparison between a virtual test-room equipped with the sample walls and the same test-rooms equipped with the improved walls. The energy demand has been calculated to achieve a level of indoor hydro-thermal comfort (temperature, relative humidity and air velocity) conforming to Italian regulations. 3. First step: energy performance of the selected external walls In order to evaluate the relationship between thermal transmittance (U), superficial mass (Ms ), time shift (ϕ), decrement factor (Fa ), periodic thermal transmittance (Ymn ) and internal areal heat capacity (k1 ), 4 massive and 4 lightweight external walls (Table 3 and Fig. 1) have been selected. The selected massive walls have been taken as samples because these are the most commonly used ones in Southern Europe (particularly in Italy). Lightweight external walls, instead, are not really common in Southern European countries. In this study these lightweight walls have been chosen as samples too, because they are often more sustainable than massive walls. Indeed lightweight walls are generally characterized by a reduced weight and material usage, are easy to dismantle and use natural, recycled and/or recyclable materials (for example wood). The 4 selected lightweight walls are not very commonly used in Southern Europe, but they are easily available. Composition and characteristics of walls layers are taken from literature [8–11] or from product/material technical datasheets. The sample walls are characterized by different layers, materials and thickness. They have been fully analyzed and the compliance with the Italian Regulation verified (Table 1). Table 4 shows that all selected walls do not have k1 corresponding to the reported minimum values although they meet the current European and Italian Regulation for summer thermal performance (particularly Ymn ). Therefore the walls can be modified to increase k1 values or to lower the minimum k1 in accordance to the k1 –Ymn relation as shown in Table 2, where lower Ymn imply lower k1 . The walls have been investigated to verify their hygrometric performance2 in two Italian cites characterized by different climate conditions (Milan and Catania). According to the results of water condensation simulations, all walls fall within the limits indicated by the Italian Regulations for office buildings. 4. Second step: improving the external walls By analyzing every sample wall and changing the layers characteristics and sequence to achieve an adequate k1 value (in relation
2 According to UNI EN ISO 13786:2008 and UNI EN ISO 13788:2003, the thermohygrometric performance for each external wall has been calculated by means of three analytical software packages: TermoK8® CALC developed by ANIT (Associazione Nazionale per l’Isolamento Termico e acustico) – and free available at http://www.termok8.com/ vti g1 exe.aspx?rpstry=1 ; a software created by A. Ursini Casalena and free available at www.mygreenbuildings.org and a spreadsheet created by Professor V. Corrado and in use for students of the Politecnico di Torino.
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Table 3 Sequence of the layers of the sample external walls. For each layer the values of s (thickness), (thermal conductivity), c (specific heat), (density), (water vapor diffusion resistance factor) and ı (periodic penetration depth) are shown. Material (from inner to outer side) External wall M01 [8] Gypsum fiber board – GKF (12.5 mm) Glass mineral wool insulation Poroton brick – type 800 Lime and cement plaster External wall M02 [8] Lime and gypsum plaster Alveolar brick XPS isulation Alveolar brick Lime and cement plaster External wall M03 [8] Lime and gypsum plaster Small honeycomb brick Vapor barrier Glass mineral wool insulation Lime and cement plaster Big honeycomb brick Lime and cement plaster External wall M04 [8] Gypsum fiber board – GKF (12.5 mm) Vapor barrier XPS isulation Precast concrete wall panel Lime and cement plaster External wall L01 [9,10] Gypsum fiber board – GKF (12.5 mm) CLT timber panel Multilayer wool wood board Lime or lime and cement plaster External wall L02 [11] Gypsum fiber board – GKF (12.5 mm) CLT timber panel Vacuum insulation panels – VIP Playwood panel (Kerto) External wall L03 [9,10] Gypsum fiber board – GKF (12.5 mm) Sheep wool insulation CLT timber panel Wood–fiber insulation board Lime or lime and cement plaster External wall L04 [9,10] Gypsum fiber board – GKF (12.5 mm) Sheep wool insulation Vapor diffusion retarder OSB board Still air Cellulose insulation Wooden board Water and air barrier (and external finish)
s [m]
[W/m K]
c [J/kg K]
0.013 0.050 0.250 0.010
0.320 0.033 0.200 0.900
1100 1030 1000 910
1000 22 800 1800
21 1 10 20
0.089 0.200 0.083 0.123
0.010 0.120 0.075 0.120 0.020
0.350 0.388 0.036 0.388 0.900
1000 840 1200 840 910
1200 840 30 840 1800
10 5 120 5 20
0.090 0.123 0.166 0.123 0.123
0.010 0.080 0.000 0.070 0.015 0.120 0.020
0.350 0.250 0.170 0.033 0.900 0.500 0.900
1000 840 840 1030 910 840 910
1200 600 625 22 1800 1400 1800
10 5 52,000 1 20 10 20
0.090 0.117 0.094 0.200 0.123 0.108 0.123
0.013 0.000 0.090 0.200 0.020
0.320 0.170 0.035 1.800 0.900
1100 840 1200 1000 910
1000 625 30 2400 1800
21 52,000 120 100 20
0.089 0.094 0.164 0.144 0.123
0.013 0.100 0.090 0.030
0.320 0.130 0.047 1.000
1100 1600 1000 1130
1000 500 200 2000
21 50 3 20
0.089 0.067 0.080 0.110
0.025 0.100 0.019 0.010
0.320 0.130 0.010 0.130
1100 1600 1050 1600
1000 500 200 450
21 50 5,000,000 60
0.089 0.067 0.036 0.070
0.025 0.040 0.100 0.050 0.030
0.320 0.040 0.130 0.055 1.000
1100 1000 1600 2500 1130
1000 30 500 200 2000
21 1.4 50 6 20
0.089 0.191 0.067 0.055 0.110
0.025 0.040 0.000 0.018 0.040 0.100 0.025 0.000
0.320 0.040 0.220 0.130
1100 1000 1700 1700
1000 30 238 650
0.089 0.191 0.122 0.057
0.040 0.130 0.220
1900 2700 1700
60 450 303
21 1.4 42,857 40 1 2 50 61
to Ymn ) it is possible to identify strategies for the improvement of thermal parameters. Two possible actions to increase k1 or decrease Ymn have shown to be the most efficient: - moving the layers with the highest value of (specific weight) towards the inner side and the one with lowest (insulation
[kg/m3 ]
[−]
ı [m]
0.098 0.054 0.108
layer) towards the outer side of the wall. The position of layers is irrelevant for the value of U, it has only light influence on Fa , ϕ, Ymn , but high influence on k1 . The inner side layer density affects the wall capacity of absorbing internal thermal loads more than the density of the other layers does; - choosing for the internal lining a material with high density but also high specific heat (c in J/kg K) and a lower value of periodic
Table 4 Thermal parameters of the sample external walls vs minimum k1 from UNIVPM study, which should be achieved in relation to the value of Ymn . External walls
s [cm]
U [W/m2 K]
Ms [kg/m2 ]
Fa [−]
ϕ [h]
Ymn [W/m2 K]
k1 [kJ/m2 K]
Min. k1 value [kJ/m2 K]
M 01 M 02 M 03 M 04 L 01 L 02 L 03 L 04
32.25 34.50 31.50 32.30 23.30 15.40 24.50 24.80
0.34 0.34 0.34 0.34 0.34 0.33 0.34 0.23
232 252 293 531 141 83 146 55
0.13 0.37 0.36 0.26 0.35 0.37 0.20 0.48
12.35 9.54 9.30 8.17 9.65 7.88 11.13 7.80
0.04 0.12 0.12 0.09 0.12 0.12 0.07 0.11
15.8 49.5 38.9 16.1 37.4 41.4 27.5 29.9
50 90 90 90 90 90 70 90
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Fig. 1. Description of the sample external walls (M01–M04 massive, L01–L04 lightweight).
penetration depth (ı in m), e.g. clay plaster instead of gypsum plaster (Table 5). The periodic penetration depth ı (which depends on the density and specific heat) is the thickness of the wall being affected by a temperature increase caused by an internal thermal load. The higher the mass and thickness affected by the thermal loads, the higher is the capacity of the wall to absorb heat from inside, thus leading to a better indoor comfort. Table 6 and Fig. 2 show the layers and characteristics of the improved external walls. Based on these considerations, the selected external walls have been improved (Fig. 2) in order to obtain good k1 values in relation to the Ymn values (Table 7). The improved walls have also been investigated to verify their hygrometric performance in Milan and Catania. According to the results of the water condensation simulations all the walls satisfy the reference values of hygrometric performance for office
buildings indicated by the Italian Regulations. In addition, one optimized wall (M03 k1 ) has shown a better hygrometric performance than its original configuration (M03). In general, as a partial conclusion, the improvement of thermal performance can lead to an improvement of the hygrometric performance. This topic should be deepened in future studies about the monitoring of existing buildings hygrometric behavior.
5. Third step: thermodynamic simulations In the third step of this study the annual energy demand for heating and cooling – needed to ensure a level of indoor hydro-thermal comfort conforming to the Italian Regulations – has been verified by means of thermodynamic simulations, using the software package “Energy Plus”,3 applied to a virtual test room. Italian regulations define two different ranges of temperature, humidity and air velocity-values for winter and summer.4 In the thermodynamic simulations the following values has been taken as reference:
Table 5 Comparison between some characteristics of the different interior lining materials. Materials
[W/m K]
c [J/kg K]
[kg/m3 ]
ı [m]
Thin clay plaster Mineralized wooden board Clay plaster Gypsum fiber board Lime and gypsum plaster
0.350 0.260 0.900 0.320 0.350
2100 2100 2100 1100 1000
3000 1800 1800 1000 1200
0.039 0.052 0.081 0.089 0.090
3 EnergyPlus is a building energy simulation software developed by the U.S. Energy: Energy Efficiency & Renewable Energy. http://apps1.eere.energy. gov/buildings/energyplus/. 4 The comfort range conforming to the Italian Regulations are: in winter: temperature: 20–22 ◦ C, humidity: 30–60%, air velocity: 0.1–0.2 m/s; in summer: temperature: 24–26 ◦ C, humidity: 30–50%, air velocity: 0.05–0.1 m/s.
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Table 6 Sequence of layers of the improved external walls. For each layer the values of s (thickness), (thermal conductivity), c (specific heat), (density), (water vapor diffusion resistance factor) and ı (periodic penetration depth) are shown. Material (from inner to outer side) External wall M01 k1 Clay plaster Poroton brick – type 800 Glass mineral wool insulation Lime and cement plaster External wall M02 k1 Thin clay plaster Alveolar brick Alveolar brick XPS isulation Lime and cement plaster External wall M03 k1 Lime and gypsum plaster Big honeycomb brick Big honeycomb brick Glass mineral wool insulation Lime and cement plaster External wall M04 k1 Clay plaster Precast concrete wall panel Vapor barrier XPS isulation Precast concrete wall panel Lime and cement plaster External wall L01 k1 Mineralized wooden board CLT timber panel Multilayer wool wood board Lime or lime and cement plaster External wall L02 k1 Mineralized wooden board CLT timber panel Vacuum insulation panels – VIP Playwood panel (Kerto) External wall L03 k1 Mineralized wooden board Sheep wool insulation CLT timber panel Wood–fiber insulation board Lime or lime and cement plaster External wall L04 k1 Mineralized wooden board Sheep wool insulation Vapor diffusion retarder OSB board Cellulose insulation Wooden board Wood–fiber insulation board Lime or lime and cement plaster
s [m]
[W/m K]
c [J/kg K]
[kg/m3 ]
0.010 0.250 0.050 0.015
0.900 0.200 0.033 0.900
2100 1000 1030 910
1800 800 22 1800
10 10 1 20
0.081 0.083 0.200 0.123
0.015 0.120 0.120 0.100 0.020
0.350 0.388 0.388 0.034 0.900
2100 840 840 1200 910
3000 840 840 30 1800
10 5 5 120 20
0.039 0.123 0.123 0.161 0.123
0.015 0.100 0.100 0.110 0.015
0.350 0.500 0.500 0.033 0.900
1000 840 840 1030 910
1200 1400 1400 22 1800
10 10 10 1 20
0.090 0.108 0.108 0.200 0.123
0.010 0.100 0.000 0.090 0.100 0.025
0.900 1.800 0.170 0.035 1.800 0.900
2100 1000 840 1200 1000 910
1800 2400 625 30 2400 1800
10 100 52,000 120 100 20
0.081 0.144 0.094 0.164 0.144 0.123
0.030 0.100 0.150 0.030
0.260 0.130 0.047 1.000
2100 1600 1000 1130
1250 500 200 2000
5 50 3 20
0.052 0.067 0.080 0.110
0.025 0.100 0.050 0.010
0.260 0.130 0.010 0.130
2100 1600 1050 1600
1250 500 200 450
5 50 5,000,000 60
0.052 0.067 0.036 0.070
0.030 0.040 0.100 0.070 0.030
0.260 0.040 0.130 0.055 1.000
2100 1000 1600 2500 1130
1250 30 500 200 2000
5 1.4 50 6 20
0.052 0.191 0.067 0.055 0.110
0.030 0.040 0.000 0.018 0.140 0.025 0.015 0.015
0.260 0.040 0.220 0.130 0.040 0.130 0.055 1.000
2100 1000 1700 1700 1900 2700 2500 1130
1250 30 238 650 60 450 200 2000
5 1.4 42,857 40 2 50 6 20
0.052 0.191 0.122 0.057 0.098 0.054 0.055 0.110
- In winter. Temperature: 20 ◦ C, humidity: 45%, air velocity: 0.15 m/s. - In summer. Temperature: 26 ◦ C, humidity: 40%, air velocity: 0.075 m/s. The virtual test room has a surface of 22.5 m2 (5.00 m × 4.50 m h = 3.00 m) and has been simulated as an office space for 2 people with 380 W of internal thermal loads. The envelope of
[−]
ı [m]
the test-room is adiabatic except for the south oriented wall being under investigation. The south oriented wall has a window (3.00 m × 1.35 m) without a solar shading system. There is not a mechanical ventilation system and the natural ventilation rate is 0.6 h−1 . The virtual test-room has been simulated for one year (a step every 10 min) in two different Italian cities (Fig. 3) characterized by different climatic conditions: tempered continental for Milan and
Table 7 Thermal parameters of the improved external wall and minimum k1 -values that should be achieved in relation to Ymn . External walls M01 k1 M02 k1 M03 k1 M04 k1 L01 k1 L02 k1 L03 k1 L04 k1
S [cm] 32.50 37.50 34.00 32.50 31.00 18.50 27.00 28.30
U [W/m2 K]
Ms [kg/m2 ]
Fa [−]
ϕ [h]
Ymn [W/m2 K]
k1 [kJ/m2 K]
Min. k1 value [kJ/m2 K]
0.34 0.27 0.25 0.34 0.23 0.16 0.30 0.19
246 286 328 546 178 96 163 103
0.106 0.153 0.171 0.170 0.168 0.235 0.122 0.219
13.40 11.58 9.71 10.54 14.45 11.39 14.30 13.34
0.04 0.04 0.04 0.06 0.04 0.04 0.04 0.04
52.0 67.9 51.6 85.4 55.0 53.4 53.5 54.5
>50 >50 >50 >70 >50 >50 >50 >50
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Fig. 2. Description of the improved external walls.
Mediterranean for Catania. Table 8 summarizes the methodology of the thermodynamic simulations. The energy demand for heating (with a gas heating system) and for cooling (with an electricity powered cooling system) was
expressed in terms of kWh, D and kg of CO2 . In this way it is possible to evaluate both energetic and economic saving for each simulation. Table 9 shows the method of calculating costs and produced CO2 .
Fig. 3. The two test locations, 3D view, section and floor plan of the virtual test-room.
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Table 8 Simulation methodology: input and output. Dimensional data of the test-room Test-room Test-wall Locations Milan Catania Internal heat loads 2 people + 2 computers + 1 lamp Ventilation Natural ventilation rate = 0.6 h−1 Heating system Fuel = gas Heating period (conforming to the Italian regulations) Cooling system Energy source = electric power Cooling period (no Italian Regulations about the cooling period) Schedules of internal heat loads, heating and cooling Output of the simulations Heating, cooling and total energy demand [kWh, D, kg of CO2 ]
Office space 5.00 × 4.50 m and h = 3.00 m. Walls, floor and roof are adiabatic, except the test-wall. South oriented, window = 3.00 m × 1.35 m, no solar shading system Latitude Longitude Altitude a.s.l. Max temp. Min temp. 45◦ 27 E 9◦ 11 N 122 m 29 ◦ C −2 ◦ C ◦ ◦ ◦ 37 30 E 10 05 N 7m 33.6 C 5 ◦C 180 W + 150 W + 50 W Mechanical ventilation = none Temperature set point of heating device < 20 ◦ C Milan: October 15th–April 15th Catania: December 1st–March 15th Temperature set point of cooling device > 26 ◦ C Milan–Catania May 1st–September 30th 9:00 a.m.–1:00 p.m. and 3:00–7:00 p.m. (Monday–Friday except holidays) Month and year
Table 9 Method of calculating costs and produced CO2 . Efficiency of the heating system Price of natural gas (first quarter 2011 – www.autorita.renergia.it) CO2 emission per kWh produced 1 kWh for heating with gas EER: energy efficiency ratio Price of electric energy (first quarter 2011 – www.autorita.renergia.it) CO2 emission per kWh produced 1 kWh for heating
6. Results This study has confirmed that values of U (thermal transmittance) and Ymn (periodic thermal transmittance) of external walls conforming to the Italian Regulation are not enough to obtain adequate values of k1 (internal areal heat capacity). The analysis of a sample of eight massive and lightweight external walls has demonstrated how by reaching certain values of k1 – corresponding to the reference study by Di Perna et al. [7] – can effectively reduce the energy demand in ensuring an indoor thermal comfort condition conforming to Italian Regulation, 20 ◦ C in winter and 26 ◦ C in summer. The analysis of the walls and of the proposed improvements has shown that a simple design can make external walls better in terms of k1 (in relation to the value of Ymn ). In summary:
- if the value of Ymn is already fairly low and the wall is massive (e.g. 0.04 W/m2 K for the M01 and 0.09 for the M04), it is possible to obtain the desired k1 value by modifying the inner layer of the wall (choosing materials with high and c and a low ı) without changing the thickness or the type of the insulating material; - if the value of Ymn is quite high (0.11–0.12 W/m2 K), it is necessary to increase the thermal insulation in order to reduce the value of Ymn (which also depends on U) and therefore decrease the minimum value of k1 . In massive walls it is usually enough to modify the inner layer; on the other hand the improving of lightweight walls, characterized by many layers, is more complex and it is necessary to know the physical properties of each layer to be able to determine its correct position and thickness. The design strategy for lightweight walls (Ms < 230 kg/m2 ) is to reduce the Ymn value below 0.04 W/m2 K in order to have 50 kJ/m2 K as limit for k1 .
90% 0.076 D/kWh 0.20 kgCO2 /kWh 0.08 D = 0.20 kgCO2 3.3 W/W 0.16 D/kWh 0.58 kgCO2 /kWh 0.05D = 0.16 kgCO
Furthermore the results of the thermodynamic simulation in the two Italian cities (Figs. 4 and 5) have demonstrated that the same improved walls give different results in different climate conditions. In Catania the cooling energy savings have been higher than in Milan; in Milan the heating energy savings have been higher than in Catania (Table 10 and Fig. 6). This result has confirmed the necessity to design the building envelope according to the climate conditions. In addition, this study has shown that there is not a direct proportion between costs for improving the walls and the consequent energy saving (kWh/m2 year, D, kg of CO2 ) in the analysis of these test-rooms (Table 10). The price analysis only considers the material cost, not the labor cost. The cost of the wall’s improvement have been taken from the
Fig. 4. Energy demand for heating and cooling of the test-rooms equipped with the original and improved external walls: Milan.
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Table 10 Improved walls versus original walls (M01 k1 – M01): materials cost for improving the thermal performance and savings of energy demand, fuel cost and CO2 emission after the improving. Negative cost figures are savings. External wall
M01 k1 M02 k1 M03 k1 M04 k1 L01 k1 L02 k1 L03 k1 L04 k1
Cost for wall improving [D]
11.39 D −3.95 D −7.04 D 11.39 D 234.93 D 576.36 D 24.85 D 34.45 D
Milan
Catania
Energy saving [kWh/year]
Fuel cost savings [D]
kg of CO2 saving
Energy demand [kWh/year]
Fuel cost savings [D]
kg of CO2 saving
Heating
Cooling
Total
Total
Total
Heating
Cooling
Total
Total
Total
−9.01 −9.04 −33.89 −4.63 −19.93 −37.65 −16.74 −39.74
−2.60 −1.43 −1.55 −6.25 0.49 7.33 −1.59 1.82
−11.61 −10.47 −35.44 −10.88 −19.43 −30.31 −18.33 −37.92
−0.74 D −0.69 D −2.39 D −0.62 D −1.34 D −2.22 D −1.22 D −2.63 D
−2.46 −2.26 −7.80 −2.13 −4.34 −7.07 −4.00 −8.51
−1.30 −1.39 −3.15 −2.61 −1.58 −1.80 −1.70 −0.90
−2.36 −2.52 −4.87 −5.01 −0.79 −11.21 −2.97 6.91
−3.65 −3.91 −8.02 −7.62 −2.37 −12.27 −4.67 6.01
−0.21 D −0.21 D −0.45 D −0.42 D −0.15 D −0.67 D −0.26 D 0.27 D
−0.71 −0.75 −1.55 −1.47 −0.49 −2.38 −0.90 1.02
price list of Piemonte has a special section “Bio-architecture” and contains the prices of all the material used in this research. M02 and M03 walls allow the lowest cost variation of building materials (Table 10). The saving is due to the type of plaster taken into consideration. The price of the clay plaster is lower than the gypsum plaster. An increment of 2–4 cm of insulation material does not lead to a relevant increase of cost of the improved walls (in order to satisfy the k1 value). The most expensive changes to improve the walls thermal performance are related to the use of the VIP panels (L02) and to the increase of the insulation materials thickness (L01) by 6 cm at least.
Fig. 5. Energy demand for heating and cooling of the test-rooms equipped with original and improved external walls: Catania.
7. Conclusions The energetic analysis and the thermodynamic simulations of this study have demonstrated that: - The current Italian Regulations are not always sufficient to obtain energy efficient external walls, because there are not reference values for the internal areal heat capacity (k1 ). - The reference k1 -value (depending only on the Ymn and not on the climate conditions, as defined by recent researches) is not always sufficient to obtain energy efficient external walls. - Simple design choices – for example modifying the type of plaster – can generate a high reduction of the energy demand. On the cost–benefit side, this study led to the following conclusions: - Massive and lightweight external walls may show a good k1 value in relation to the Ymn by means of easy (but not always cheap) changes. - Improving the k1 does not always lead to a big reduction of heating and cooling cost. - It is easier, cheaper and more effective, to improve the thermal performance of external walls with a low value of Ymn (e.g., M01, M04 and L03).
Fig. 6. Energy saving potential for the improved walls in comparison with the sample walls (a negative value means an energy saving, a positive value an energy demand increase).
public works price list of one of the Italian Regions.5 Because the price lists of Lombardia (where Milan is located) and Sicily (where Catania is located) do not contain all of the used materials, the public works price list of Piemonte [12] has been used. Indeed the
5
In Italy every region has an annual official public works price list.
In conclusion this study has demonstrated that in Southern European climate conditions (for example in Italy) it is necessary to introduce a legislation limiting the k1 value and not only the Ymn value. Particularly k1 and Ymn values conforming to the above mentioned recent research [7] determine an energy saving (for heating and particularly for cooling) in the sample massive walls but are not sufficient to obtain a reduction of the energy demand for cooling in the lightweight walls (Fig. 6). Furthermore the climate conditions assume a very important role in the building energy demand. Indeed the same external wall with the same k1 and Ymn values has
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different energy performance on geographic location (for example Milan and Catania). Accordingly the k1 reference value should be depending not only on the Ymn value but on the climate conditions and on the weight (kg/m3 ) of the external wall too. However, the true extent of the benefits coming from external walls improvements has to be carefully evaluated in terms of feasibility and real building business case.
[5]
[6]
[7]
References [1] Directive 2002/91/EC of the European Parliament and of the Council of 16 December 2002 on the energy performance of buildings. [2] N. Aste, A. Angelotti, M. Buzzetti, The influence of external walls thermal inertia on the energy performance of well insulated buildings, Energy and Buildings 41 (2009) 1181–1187. [3] UNI EN ISO 13786. Thermal performance of building components. Dynamic thermal characteristics. Calculation methods, 2008. [4] DECRETO LEGISLATIVO 29 dicembre 2006, n.311, Disposizioni correttive ed integrative al decreto legislativo 19 agosto 2005, n. 192, recante attuazione
[8] [9] [10] [11]
[12]
della direttiva 2002/91/CE, relativa al rendimento energetico nell’edilizia (GU n. 26 del 1-2-2007-Suppl. Ordinario n. 26). DECRETO MINISTERIALE 26/6/2009 – Ministero dello Sviluppo Economico Linee guida nazionali per la certificazione energetica degli edifici (G.U. n. 158 del 10/7/2009). DECRETO DEL PRESIDENTE DELLA REPUBBLICA 2 aprile 2009, n. 59, Regolamento di attuazione dell’articolo 4, comma 1, lettere a) e b), del decreto legislativo 19 agosto 2005, n. 192, concernente attuazione della direttiva 2002/91/CE sul rendimento energetico in edilizia. (09G0068) (GU n. 132 del 10-6-2009). C. Di Perna, F. Stazi, A. Ursini Casalena, M. D’Orazio, Influence of the internal inertia of the building envelope on summertime comfort in buildings with high internal heat loads, Energy and Buildings 43 (2011) 200–206. M. Lavagna, Sostenibilità e risparmio energetico. Soluzioni tecniche per edifici eco-efficienti, Libreria CLUP, Milano, 2005.. C. Benedetti, Costruire in Legno, Bozen-Bolzano University Press, Bolzano, 2009. M. Rossi, Innovative fac¸ades: lightweight and thin systems with high inertia for the thermal comfort. Application in office buildings in Southern Europe, Journal of Green Building 6–2 (2011) 1–15. Prezzario per i lavori e le opere pubbliche della Regione Lombardia, dei Tipografia del genio civile, edizione, 2011.
Contents lists available at ScienceDirect
Energy and Buildings journal homepage: www.elsevier.com/locate/enbuild
External walls design: The role of periodic thermal transmittance and internal areal heat capacity Monica Rossi a,∗ , Valeria Marta Rocco b a b
University of Camerino, School of Architecture and Design “Eduardo Vittoria”, Italy Politecnico di Torino, DIST: Interuniversity Department of Regional and Urban Studies and Planning, Italy
a r t i c l e
i n f o
Keywords: Energy demand External walls Thermal parameters Periodic thermal transmittance Internal areal heat capacity Indoor comfort Italian regulation
a b s t r a c t Recent studies have shown that considering the values of superficial mass (Ms ) and periodic thermal transmittance (Ymn ) of external walls is not sufficient to achieve energy savings, particularly in summer. For this reason experimental reference values of the internal areal heat capacity (k1 ) were introduced. This study aims to understand the interdependency between some thermal parameters (U, Ms , ϕ, Fa , Ymn , k1 ) of massive and lightweight walls with respect to their energy performance in use in office buildings in Southern Europe. The study has analyzed eight walls with Italian standard U and Ymn values. These walls have been then modified in order to reach k1 values corresponding to the reference ones. The energy demand to ensure a defined level of indoor thermal comfort has been verified with thermodynamic simulations on a virtual test-room localized in two Italian cities, characterized by different climate conditions. The research results are: to develop design change strategies for external walls to achieve the k1 reference values; to quantify the thermal annual energy demand of a virtual test-room equipped with the sample walls and then equipped with the improved walls; to compare the energetic and economic impact for the improved walls against the sample ones. © 2012 Elsevier B.V. All rights reserved.
1. Introduction In 2002 the European Directive 2002/91/EC [1] introduced the topic of reducing buildings energy demand during summertime: “Recent years have seen a rise in the number of air-conditioning systems in southern European countries. This creates considerable problems at peak load times, increasing the cost of electricity and disrupting the energy balance in those countries. Priority should be given to strategies which enhance the thermal performance of buildings during the summer period. To this end there should be further development of passive cooling techniques, primarily those that improve indoor climatic conditions and the microclimate around buildings.” Therefore in recent years the reduction of the building energy demand for cooling and the achievement of a high level of indoor thermal comfort in summertime are the subjects of many scientific researches carried out in South Europe. Several studies have demonstrated the importance of having a building envelope with a high thermal inertia – particularly for the summertime performance – both for saving energy and indoor comfort [2].
∗ Corresponding author. Tel.: +39 347 6990886; fax: +39 0734 404258. E-mail addresses: [email protected], [email protected] (M. Rossi), [email protected] (V.M. Rocco). 0378-7788/$ – see front matter © 2012 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.enbuild.2012.07.049
Furthermore, the European regulation UNI EN ISO 13786/2008 [3] has introduced other parameters – in addition to the thermal transmittance (U in W/m2 K) and the superficial mass (Ms in kg/m2 ) – for the evaluation of the thermal inertia and the summer thermal performance of building envelopes. These parameters are: -
thermal time shift, ϕ (h); thermal decrement factor, Fa (dimensionless); periodic thermal transmittance, Ymn (W/m2 K); internal areal heat capacity, k1 (kJ/m2 K).
The time shift is the period of time between the maximum amplitude of a cause and the maximum amplitude of its effect [3]. The decrement factor is the ratio of the modulus of the periodic thermal transmittance to the steady-state thermal transmittance U [3]. The periodic thermal transmittance (incorporating the concepts of thermal transmittance, time shift and decrement factor) is a complex quantity defined as the complex amplitude of the density of heat flow rate through the surface of the component adjacent to zone m, divided by the complex amplitude of the temperature in zone n when the temperature in zone m is held constant [3]. The internal areal heat capacity is the heat capacity divided by area of element and the heat capacity is the modulus of the net periodic thermal conductance divided by the angular frequency [3].
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Table 1 Thermal parameters of external walls according to the Italian regulations: climate zone E.
U Ms ϕ Fa Ymn
Legislation
Reference value
D.Lgs. 311/2006 D.Lgs. 311/2006 D.M. – 26th June 2009 D.M. – 26th June 2009 DPR 59/2009
U < 0.34 W/m2 K – climate zone E (a cold Italian zone) Ms > 230 kg/m2 (exception to zone F, the coldest one) 8 h ≥ ϕ > 6 h average – ϕ > 12 h excellent 0.40 ≤ Fa < 0.60 average – Fa < 0.15 excellent Ymn < 0.12 W/m2 K
Table 2 Reference value of k1 defined in the research carried out by Di Perna et al. [7]. Reference value of k1 k1 ≥ 50 if Ymn ≤ 0,04 k1 ≥ 70 if 0.04 ≤ Ymn ≤ 0.08 k1 ≥ 90 if 0.08 ≤ Ymn ≤ 0.1
The Italian legislations [4–6] have defined reference values for some of the mentioned parameters (Table 1) depending on the climate zone. Indeed the Italian law divides Italy into 6 different climate zones, from the coldest (zone F) to the hottest (zone A).1 In this study the limit of winter and summer performance values of the climate zone E has been considered. The climate zone E is the first zone, among the cold ones, that requires the verification of summer parameters. Recent research carried out by Di Perna et al. [7] has shown “how the limit introduced for the periodic thermal transmittance value leads to the development of envelopes which are totally different from the point of view of comfort. Walls with the same stationary and periodic thermal transmittance, but which differ from each other only in the k1 value, behave very differently from the point of view of indoor comfort.” In fact, a low Ymn value leads to a reduction of outside thermal load impact, particularly from direct sunlight irradiation on external walls, but it is not able to reduce the contribution of the internal loads. Internal heat loads are however the main cause of excessive indoor temperatures during summertime, particularly in office buildings. Indeed the k1 value describes the actual capacity to accumulate heat on the inner side of a building element and characterizes the internal thermal mass. An envelope with a high potential for heat accumulation on the inner side has a high k1 value. Italian regulations neither mention nor define any limit and/or reference values for k1 . The above mentioned study [7] defined minimum values for k1 depending on the values of Ymn (Table 2) for the first time. 2. Methodology This study aims to understand the interdependency between thermal parameters of external walls (Table 1) and to analyze the influence of these thermal parameters on the indoor comfort level in office buildings (characterized by the presence of high internal thermal loads) localized in different climate zones of Southern Europe. To achieve this goal a research organized by the following three steps has been carried out: - calculation of the thermal parameters’ (U, Ms , ϕ, Fa , Ymn , k1 ) initial status, for eight selected massive and lightweight external walls; - design changes on the eight sample walls in order to get k1 values which are comparable to the new introduced k1 reference values,
1 Italian Regulations define different building envelopes U value limits for each climate zone and one universal reference Ymn value for all climate zones. The verification of the Ymn is not necessary for the coldest climate zone (F).
still complying with the legislation limits for the other thermal parameters; - energy demand comparison between a virtual test-room equipped with the sample walls and the same test-rooms equipped with the improved walls. The energy demand has been calculated to achieve a level of indoor hydro-thermal comfort (temperature, relative humidity and air velocity) conforming to Italian regulations. 3. First step: energy performance of the selected external walls In order to evaluate the relationship between thermal transmittance (U), superficial mass (Ms ), time shift (ϕ), decrement factor (Fa ), periodic thermal transmittance (Ymn ) and internal areal heat capacity (k1 ), 4 massive and 4 lightweight external walls (Table 3 and Fig. 1) have been selected. The selected massive walls have been taken as samples because these are the most commonly used ones in Southern Europe (particularly in Italy). Lightweight external walls, instead, are not really common in Southern European countries. In this study these lightweight walls have been chosen as samples too, because they are often more sustainable than massive walls. Indeed lightweight walls are generally characterized by a reduced weight and material usage, are easy to dismantle and use natural, recycled and/or recyclable materials (for example wood). The 4 selected lightweight walls are not very commonly used in Southern Europe, but they are easily available. Composition and characteristics of walls layers are taken from literature [8–11] or from product/material technical datasheets. The sample walls are characterized by different layers, materials and thickness. They have been fully analyzed and the compliance with the Italian Regulation verified (Table 1). Table 4 shows that all selected walls do not have k1 corresponding to the reported minimum values although they meet the current European and Italian Regulation for summer thermal performance (particularly Ymn ). Therefore the walls can be modified to increase k1 values or to lower the minimum k1 in accordance to the k1 –Ymn relation as shown in Table 2, where lower Ymn imply lower k1 . The walls have been investigated to verify their hygrometric performance2 in two Italian cites characterized by different climate conditions (Milan and Catania). According to the results of water condensation simulations, all walls fall within the limits indicated by the Italian Regulations for office buildings. 4. Second step: improving the external walls By analyzing every sample wall and changing the layers characteristics and sequence to achieve an adequate k1 value (in relation
2 According to UNI EN ISO 13786:2008 and UNI EN ISO 13788:2003, the thermohygrometric performance for each external wall has been calculated by means of three analytical software packages: TermoK8® CALC developed by ANIT (Associazione Nazionale per l’Isolamento Termico e acustico) – and free available at http://www.termok8.com/ vti g1 exe.aspx?rpstry=1 ; a software created by A. Ursini Casalena and free available at www.mygreenbuildings.org and a spreadsheet created by Professor V. Corrado and in use for students of the Politecnico di Torino.
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Table 3 Sequence of the layers of the sample external walls. For each layer the values of s (thickness), (thermal conductivity), c (specific heat), (density), (water vapor diffusion resistance factor) and ı (periodic penetration depth) are shown. Material (from inner to outer side) External wall M01 [8] Gypsum fiber board – GKF (12.5 mm) Glass mineral wool insulation Poroton brick – type 800 Lime and cement plaster External wall M02 [8] Lime and gypsum plaster Alveolar brick XPS isulation Alveolar brick Lime and cement plaster External wall M03 [8] Lime and gypsum plaster Small honeycomb brick Vapor barrier Glass mineral wool insulation Lime and cement plaster Big honeycomb brick Lime and cement plaster External wall M04 [8] Gypsum fiber board – GKF (12.5 mm) Vapor barrier XPS isulation Precast concrete wall panel Lime and cement plaster External wall L01 [9,10] Gypsum fiber board – GKF (12.5 mm) CLT timber panel Multilayer wool wood board Lime or lime and cement plaster External wall L02 [11] Gypsum fiber board – GKF (12.5 mm) CLT timber panel Vacuum insulation panels – VIP Playwood panel (Kerto) External wall L03 [9,10] Gypsum fiber board – GKF (12.5 mm) Sheep wool insulation CLT timber panel Wood–fiber insulation board Lime or lime and cement plaster External wall L04 [9,10] Gypsum fiber board – GKF (12.5 mm) Sheep wool insulation Vapor diffusion retarder OSB board Still air Cellulose insulation Wooden board Water and air barrier (and external finish)
s [m]
[W/m K]
c [J/kg K]
0.013 0.050 0.250 0.010
0.320 0.033 0.200 0.900
1100 1030 1000 910
1000 22 800 1800
21 1 10 20
0.089 0.200 0.083 0.123
0.010 0.120 0.075 0.120 0.020
0.350 0.388 0.036 0.388 0.900
1000 840 1200 840 910
1200 840 30 840 1800
10 5 120 5 20
0.090 0.123 0.166 0.123 0.123
0.010 0.080 0.000 0.070 0.015 0.120 0.020
0.350 0.250 0.170 0.033 0.900 0.500 0.900
1000 840 840 1030 910 840 910
1200 600 625 22 1800 1400 1800
10 5 52,000 1 20 10 20
0.090 0.117 0.094 0.200 0.123 0.108 0.123
0.013 0.000 0.090 0.200 0.020
0.320 0.170 0.035 1.800 0.900
1100 840 1200 1000 910
1000 625 30 2400 1800
21 52,000 120 100 20
0.089 0.094 0.164 0.144 0.123
0.013 0.100 0.090 0.030
0.320 0.130 0.047 1.000
1100 1600 1000 1130
1000 500 200 2000
21 50 3 20
0.089 0.067 0.080 0.110
0.025 0.100 0.019 0.010
0.320 0.130 0.010 0.130
1100 1600 1050 1600
1000 500 200 450
21 50 5,000,000 60
0.089 0.067 0.036 0.070
0.025 0.040 0.100 0.050 0.030
0.320 0.040 0.130 0.055 1.000
1100 1000 1600 2500 1130
1000 30 500 200 2000
21 1.4 50 6 20
0.089 0.191 0.067 0.055 0.110
0.025 0.040 0.000 0.018 0.040 0.100 0.025 0.000
0.320 0.040 0.220 0.130
1100 1000 1700 1700
1000 30 238 650
0.089 0.191 0.122 0.057
0.040 0.130 0.220
1900 2700 1700
60 450 303
21 1.4 42,857 40 1 2 50 61
to Ymn ) it is possible to identify strategies for the improvement of thermal parameters. Two possible actions to increase k1 or decrease Ymn have shown to be the most efficient: - moving the layers with the highest value of (specific weight) towards the inner side and the one with lowest (insulation
[kg/m3 ]
[−]
ı [m]
0.098 0.054 0.108
layer) towards the outer side of the wall. The position of layers is irrelevant for the value of U, it has only light influence on Fa , ϕ, Ymn , but high influence on k1 . The inner side layer density affects the wall capacity of absorbing internal thermal loads more than the density of the other layers does; - choosing for the internal lining a material with high density but also high specific heat (c in J/kg K) and a lower value of periodic
Table 4 Thermal parameters of the sample external walls vs minimum k1 from UNIVPM study, which should be achieved in relation to the value of Ymn . External walls
s [cm]
U [W/m2 K]
Ms [kg/m2 ]
Fa [−]
ϕ [h]
Ymn [W/m2 K]
k1 [kJ/m2 K]
Min. k1 value [kJ/m2 K]
M 01 M 02 M 03 M 04 L 01 L 02 L 03 L 04
32.25 34.50 31.50 32.30 23.30 15.40 24.50 24.80
0.34 0.34 0.34 0.34 0.34 0.33 0.34 0.23
232 252 293 531 141 83 146 55
0.13 0.37 0.36 0.26 0.35 0.37 0.20 0.48
12.35 9.54 9.30 8.17 9.65 7.88 11.13 7.80
0.04 0.12 0.12 0.09 0.12 0.12 0.07 0.11
15.8 49.5 38.9 16.1 37.4 41.4 27.5 29.9
50 90 90 90 90 90 70 90
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Fig. 1. Description of the sample external walls (M01–M04 massive, L01–L04 lightweight).
penetration depth (ı in m), e.g. clay plaster instead of gypsum plaster (Table 5). The periodic penetration depth ı (which depends on the density and specific heat) is the thickness of the wall being affected by a temperature increase caused by an internal thermal load. The higher the mass and thickness affected by the thermal loads, the higher is the capacity of the wall to absorb heat from inside, thus leading to a better indoor comfort. Table 6 and Fig. 2 show the layers and characteristics of the improved external walls. Based on these considerations, the selected external walls have been improved (Fig. 2) in order to obtain good k1 values in relation to the Ymn values (Table 7). The improved walls have also been investigated to verify their hygrometric performance in Milan and Catania. According to the results of the water condensation simulations all the walls satisfy the reference values of hygrometric performance for office
buildings indicated by the Italian Regulations. In addition, one optimized wall (M03 k1 ) has shown a better hygrometric performance than its original configuration (M03). In general, as a partial conclusion, the improvement of thermal performance can lead to an improvement of the hygrometric performance. This topic should be deepened in future studies about the monitoring of existing buildings hygrometric behavior.
5. Third step: thermodynamic simulations In the third step of this study the annual energy demand for heating and cooling – needed to ensure a level of indoor hydro-thermal comfort conforming to the Italian Regulations – has been verified by means of thermodynamic simulations, using the software package “Energy Plus”,3 applied to a virtual test room. Italian regulations define two different ranges of temperature, humidity and air velocity-values for winter and summer.4 In the thermodynamic simulations the following values has been taken as reference:
Table 5 Comparison between some characteristics of the different interior lining materials. Materials
[W/m K]
c [J/kg K]
[kg/m3 ]
ı [m]
Thin clay plaster Mineralized wooden board Clay plaster Gypsum fiber board Lime and gypsum plaster
0.350 0.260 0.900 0.320 0.350
2100 2100 2100 1100 1000
3000 1800 1800 1000 1200
0.039 0.052 0.081 0.089 0.090
3 EnergyPlus is a building energy simulation software developed by the U.S. Energy: Energy Efficiency & Renewable Energy. http://apps1.eere.energy. gov/buildings/energyplus/. 4 The comfort range conforming to the Italian Regulations are: in winter: temperature: 20–22 ◦ C, humidity: 30–60%, air velocity: 0.1–0.2 m/s; in summer: temperature: 24–26 ◦ C, humidity: 30–50%, air velocity: 0.05–0.1 m/s.
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Table 6 Sequence of layers of the improved external walls. For each layer the values of s (thickness), (thermal conductivity), c (specific heat), (density), (water vapor diffusion resistance factor) and ı (periodic penetration depth) are shown. Material (from inner to outer side) External wall M01 k1 Clay plaster Poroton brick – type 800 Glass mineral wool insulation Lime and cement plaster External wall M02 k1 Thin clay plaster Alveolar brick Alveolar brick XPS isulation Lime and cement plaster External wall M03 k1 Lime and gypsum plaster Big honeycomb brick Big honeycomb brick Glass mineral wool insulation Lime and cement plaster External wall M04 k1 Clay plaster Precast concrete wall panel Vapor barrier XPS isulation Precast concrete wall panel Lime and cement plaster External wall L01 k1 Mineralized wooden board CLT timber panel Multilayer wool wood board Lime or lime and cement plaster External wall L02 k1 Mineralized wooden board CLT timber panel Vacuum insulation panels – VIP Playwood panel (Kerto) External wall L03 k1 Mineralized wooden board Sheep wool insulation CLT timber panel Wood–fiber insulation board Lime or lime and cement plaster External wall L04 k1 Mineralized wooden board Sheep wool insulation Vapor diffusion retarder OSB board Cellulose insulation Wooden board Wood–fiber insulation board Lime or lime and cement plaster
s [m]
[W/m K]
c [J/kg K]
[kg/m3 ]
0.010 0.250 0.050 0.015
0.900 0.200 0.033 0.900
2100 1000 1030 910
1800 800 22 1800
10 10 1 20
0.081 0.083 0.200 0.123
0.015 0.120 0.120 0.100 0.020
0.350 0.388 0.388 0.034 0.900
2100 840 840 1200 910
3000 840 840 30 1800
10 5 5 120 20
0.039 0.123 0.123 0.161 0.123
0.015 0.100 0.100 0.110 0.015
0.350 0.500 0.500 0.033 0.900
1000 840 840 1030 910
1200 1400 1400 22 1800
10 10 10 1 20
0.090 0.108 0.108 0.200 0.123
0.010 0.100 0.000 0.090 0.100 0.025
0.900 1.800 0.170 0.035 1.800 0.900
2100 1000 840 1200 1000 910
1800 2400 625 30 2400 1800
10 100 52,000 120 100 20
0.081 0.144 0.094 0.164 0.144 0.123
0.030 0.100 0.150 0.030
0.260 0.130 0.047 1.000
2100 1600 1000 1130
1250 500 200 2000
5 50 3 20
0.052 0.067 0.080 0.110
0.025 0.100 0.050 0.010
0.260 0.130 0.010 0.130
2100 1600 1050 1600
1250 500 200 450
5 50 5,000,000 60
0.052 0.067 0.036 0.070
0.030 0.040 0.100 0.070 0.030
0.260 0.040 0.130 0.055 1.000
2100 1000 1600 2500 1130
1250 30 500 200 2000
5 1.4 50 6 20
0.052 0.191 0.067 0.055 0.110
0.030 0.040 0.000 0.018 0.140 0.025 0.015 0.015
0.260 0.040 0.220 0.130 0.040 0.130 0.055 1.000
2100 1000 1700 1700 1900 2700 2500 1130
1250 30 238 650 60 450 200 2000
5 1.4 42,857 40 2 50 6 20
0.052 0.191 0.122 0.057 0.098 0.054 0.055 0.110
- In winter. Temperature: 20 ◦ C, humidity: 45%, air velocity: 0.15 m/s. - In summer. Temperature: 26 ◦ C, humidity: 40%, air velocity: 0.075 m/s. The virtual test room has a surface of 22.5 m2 (5.00 m × 4.50 m h = 3.00 m) and has been simulated as an office space for 2 people with 380 W of internal thermal loads. The envelope of
[−]
ı [m]
the test-room is adiabatic except for the south oriented wall being under investigation. The south oriented wall has a window (3.00 m × 1.35 m) without a solar shading system. There is not a mechanical ventilation system and the natural ventilation rate is 0.6 h−1 . The virtual test-room has been simulated for one year (a step every 10 min) in two different Italian cities (Fig. 3) characterized by different climatic conditions: tempered continental for Milan and
Table 7 Thermal parameters of the improved external wall and minimum k1 -values that should be achieved in relation to Ymn . External walls M01 k1 M02 k1 M03 k1 M04 k1 L01 k1 L02 k1 L03 k1 L04 k1
S [cm] 32.50 37.50 34.00 32.50 31.00 18.50 27.00 28.30
U [W/m2 K]
Ms [kg/m2 ]
Fa [−]
ϕ [h]
Ymn [W/m2 K]
k1 [kJ/m2 K]
Min. k1 value [kJ/m2 K]
0.34 0.27 0.25 0.34 0.23 0.16 0.30 0.19
246 286 328 546 178 96 163 103
0.106 0.153 0.171 0.170 0.168 0.235 0.122 0.219
13.40 11.58 9.71 10.54 14.45 11.39 14.30 13.34
0.04 0.04 0.04 0.06 0.04 0.04 0.04 0.04
52.0 67.9 51.6 85.4 55.0 53.4 53.5 54.5
>50 >50 >50 >70 >50 >50 >50 >50
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737
Fig. 2. Description of the improved external walls.
Mediterranean for Catania. Table 8 summarizes the methodology of the thermodynamic simulations. The energy demand for heating (with a gas heating system) and for cooling (with an electricity powered cooling system) was
expressed in terms of kWh, D and kg of CO2 . In this way it is possible to evaluate both energetic and economic saving for each simulation. Table 9 shows the method of calculating costs and produced CO2 .
Fig. 3. The two test locations, 3D view, section and floor plan of the virtual test-room.
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Table 8 Simulation methodology: input and output. Dimensional data of the test-room Test-room Test-wall Locations Milan Catania Internal heat loads 2 people + 2 computers + 1 lamp Ventilation Natural ventilation rate = 0.6 h−1 Heating system Fuel = gas Heating period (conforming to the Italian regulations) Cooling system Energy source = electric power Cooling period (no Italian Regulations about the cooling period) Schedules of internal heat loads, heating and cooling Output of the simulations Heating, cooling and total energy demand [kWh, D, kg of CO2 ]
Office space 5.00 × 4.50 m and h = 3.00 m. Walls, floor and roof are adiabatic, except the test-wall. South oriented, window = 3.00 m × 1.35 m, no solar shading system Latitude Longitude Altitude a.s.l. Max temp. Min temp. 45◦ 27 E 9◦ 11 N 122 m 29 ◦ C −2 ◦ C ◦ ◦ ◦ 37 30 E 10 05 N 7m 33.6 C 5 ◦C 180 W + 150 W + 50 W Mechanical ventilation = none Temperature set point of heating device < 20 ◦ C Milan: October 15th–April 15th Catania: December 1st–March 15th Temperature set point of cooling device > 26 ◦ C Milan–Catania May 1st–September 30th 9:00 a.m.–1:00 p.m. and 3:00–7:00 p.m. (Monday–Friday except holidays) Month and year
Table 9 Method of calculating costs and produced CO2 . Efficiency of the heating system Price of natural gas (first quarter 2011 – www.autorita.renergia.it) CO2 emission per kWh produced 1 kWh for heating with gas EER: energy efficiency ratio Price of electric energy (first quarter 2011 – www.autorita.renergia.it) CO2 emission per kWh produced 1 kWh for heating
6. Results This study has confirmed that values of U (thermal transmittance) and Ymn (periodic thermal transmittance) of external walls conforming to the Italian Regulation are not enough to obtain adequate values of k1 (internal areal heat capacity). The analysis of a sample of eight massive and lightweight external walls has demonstrated how by reaching certain values of k1 – corresponding to the reference study by Di Perna et al. [7] – can effectively reduce the energy demand in ensuring an indoor thermal comfort condition conforming to Italian Regulation, 20 ◦ C in winter and 26 ◦ C in summer. The analysis of the walls and of the proposed improvements has shown that a simple design can make external walls better in terms of k1 (in relation to the value of Ymn ). In summary:
- if the value of Ymn is already fairly low and the wall is massive (e.g. 0.04 W/m2 K for the M01 and 0.09 for the M04), it is possible to obtain the desired k1 value by modifying the inner layer of the wall (choosing materials with high and c and a low ı) without changing the thickness or the type of the insulating material; - if the value of Ymn is quite high (0.11–0.12 W/m2 K), it is necessary to increase the thermal insulation in order to reduce the value of Ymn (which also depends on U) and therefore decrease the minimum value of k1 . In massive walls it is usually enough to modify the inner layer; on the other hand the improving of lightweight walls, characterized by many layers, is more complex and it is necessary to know the physical properties of each layer to be able to determine its correct position and thickness. The design strategy for lightweight walls (Ms < 230 kg/m2 ) is to reduce the Ymn value below 0.04 W/m2 K in order to have 50 kJ/m2 K as limit for k1 .
90% 0.076 D/kWh 0.20 kgCO2 /kWh 0.08 D = 0.20 kgCO2 3.3 W/W 0.16 D/kWh 0.58 kgCO2 /kWh 0.05D = 0.16 kgCO
Furthermore the results of the thermodynamic simulation in the two Italian cities (Figs. 4 and 5) have demonstrated that the same improved walls give different results in different climate conditions. In Catania the cooling energy savings have been higher than in Milan; in Milan the heating energy savings have been higher than in Catania (Table 10 and Fig. 6). This result has confirmed the necessity to design the building envelope according to the climate conditions. In addition, this study has shown that there is not a direct proportion between costs for improving the walls and the consequent energy saving (kWh/m2 year, D, kg of CO2 ) in the analysis of these test-rooms (Table 10). The price analysis only considers the material cost, not the labor cost. The cost of the wall’s improvement have been taken from the
Fig. 4. Energy demand for heating and cooling of the test-rooms equipped with the original and improved external walls: Milan.
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Table 10 Improved walls versus original walls (M01 k1 – M01): materials cost for improving the thermal performance and savings of energy demand, fuel cost and CO2 emission after the improving. Negative cost figures are savings. External wall
M01 k1 M02 k1 M03 k1 M04 k1 L01 k1 L02 k1 L03 k1 L04 k1
Cost for wall improving [D]
11.39 D −3.95 D −7.04 D 11.39 D 234.93 D 576.36 D 24.85 D 34.45 D
Milan
Catania
Energy saving [kWh/year]
Fuel cost savings [D]
kg of CO2 saving
Energy demand [kWh/year]
Fuel cost savings [D]
kg of CO2 saving
Heating
Cooling
Total
Total
Total
Heating
Cooling
Total
Total
Total
−9.01 −9.04 −33.89 −4.63 −19.93 −37.65 −16.74 −39.74
−2.60 −1.43 −1.55 −6.25 0.49 7.33 −1.59 1.82
−11.61 −10.47 −35.44 −10.88 −19.43 −30.31 −18.33 −37.92
−0.74 D −0.69 D −2.39 D −0.62 D −1.34 D −2.22 D −1.22 D −2.63 D
−2.46 −2.26 −7.80 −2.13 −4.34 −7.07 −4.00 −8.51
−1.30 −1.39 −3.15 −2.61 −1.58 −1.80 −1.70 −0.90
−2.36 −2.52 −4.87 −5.01 −0.79 −11.21 −2.97 6.91
−3.65 −3.91 −8.02 −7.62 −2.37 −12.27 −4.67 6.01
−0.21 D −0.21 D −0.45 D −0.42 D −0.15 D −0.67 D −0.26 D 0.27 D
−0.71 −0.75 −1.55 −1.47 −0.49 −2.38 −0.90 1.02
price list of Piemonte has a special section “Bio-architecture” and contains the prices of all the material used in this research. M02 and M03 walls allow the lowest cost variation of building materials (Table 10). The saving is due to the type of plaster taken into consideration. The price of the clay plaster is lower than the gypsum plaster. An increment of 2–4 cm of insulation material does not lead to a relevant increase of cost of the improved walls (in order to satisfy the k1 value). The most expensive changes to improve the walls thermal performance are related to the use of the VIP panels (L02) and to the increase of the insulation materials thickness (L01) by 6 cm at least.
Fig. 5. Energy demand for heating and cooling of the test-rooms equipped with original and improved external walls: Catania.
7. Conclusions The energetic analysis and the thermodynamic simulations of this study have demonstrated that: - The current Italian Regulations are not always sufficient to obtain energy efficient external walls, because there are not reference values for the internal areal heat capacity (k1 ). - The reference k1 -value (depending only on the Ymn and not on the climate conditions, as defined by recent researches) is not always sufficient to obtain energy efficient external walls. - Simple design choices – for example modifying the type of plaster – can generate a high reduction of the energy demand. On the cost–benefit side, this study led to the following conclusions: - Massive and lightweight external walls may show a good k1 value in relation to the Ymn by means of easy (but not always cheap) changes. - Improving the k1 does not always lead to a big reduction of heating and cooling cost. - It is easier, cheaper and more effective, to improve the thermal performance of external walls with a low value of Ymn (e.g., M01, M04 and L03).
Fig. 6. Energy saving potential for the improved walls in comparison with the sample walls (a negative value means an energy saving, a positive value an energy demand increase).
public works price list of one of the Italian Regions.5 Because the price lists of Lombardia (where Milan is located) and Sicily (where Catania is located) do not contain all of the used materials, the public works price list of Piemonte [12] has been used. Indeed the
5
In Italy every region has an annual official public works price list.
In conclusion this study has demonstrated that in Southern European climate conditions (for example in Italy) it is necessary to introduce a legislation limiting the k1 value and not only the Ymn value. Particularly k1 and Ymn values conforming to the above mentioned recent research [7] determine an energy saving (for heating and particularly for cooling) in the sample massive walls but are not sufficient to obtain a reduction of the energy demand for cooling in the lightweight walls (Fig. 6). Furthermore the climate conditions assume a very important role in the building energy demand. Indeed the same external wall with the same k1 and Ymn values has
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different energy performance on geographic location (for example Milan and Catania). Accordingly the k1 reference value should be depending not only on the Ymn value but on the climate conditions and on the weight (kg/m3 ) of the external wall too. However, the true extent of the benefits coming from external walls improvements has to be carefully evaluated in terms of feasibility and real building business case.
[5]
[6]
[7]
References [1] Directive 2002/91/EC of the European Parliament and of the Council of 16 December 2002 on the energy performance of buildings. [2] N. Aste, A. Angelotti, M. Buzzetti, The influence of external walls thermal inertia on the energy performance of well insulated buildings, Energy and Buildings 41 (2009) 1181–1187. [3] UNI EN ISO 13786. Thermal performance of building components. Dynamic thermal characteristics. Calculation methods, 2008. [4] DECRETO LEGISLATIVO 29 dicembre 2006, n.311, Disposizioni correttive ed integrative al decreto legislativo 19 agosto 2005, n. 192, recante attuazione
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della direttiva 2002/91/CE, relativa al rendimento energetico nell’edilizia (GU n. 26 del 1-2-2007-Suppl. Ordinario n. 26). DECRETO MINISTERIALE 26/6/2009 – Ministero dello Sviluppo Economico Linee guida nazionali per la certificazione energetica degli edifici (G.U. n. 158 del 10/7/2009). DECRETO DEL PRESIDENTE DELLA REPUBBLICA 2 aprile 2009, n. 59, Regolamento di attuazione dell’articolo 4, comma 1, lettere a) e b), del decreto legislativo 19 agosto 2005, n. 192, concernente attuazione della direttiva 2002/91/CE sul rendimento energetico in edilizia. (09G0068) (GU n. 132 del 10-6-2009). C. Di Perna, F. Stazi, A. Ursini Casalena, M. D’Orazio, Influence of the internal inertia of the building envelope on summertime comfort in buildings with high internal heat loads, Energy and Buildings 43 (2011) 200–206. M. Lavagna, Sostenibilità e risparmio energetico. Soluzioni tecniche per edifici eco-efficienti, Libreria CLUP, Milano, 2005.