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Role of airtightness in energy loss from windows: Experimental results from in-situ tests
Energy and Buildings 139 (2017) 449–455
Contents lists available at ScienceDirect
Energy and Buildings journal homepage: www.elsevier.com/locate/enbuild
Role of airtightness in energy loss from windows: Experimental results from in-situ tests Erdem Cuce a,b a b
Department of Mechanical Engineering, Faculty of Engineering, University of Bayburt, 69000 Bayburt, Turkey Central Research Laboratory, University of Bayburt, 69000 Bayburt, Turkey
a r t i c l e
i n f o
Article history: Received 22 September 2016 Received in revised form 16 November 2016 Accepted 7 January 2017 Available online 13 January 2017 Keywords: Double glazed windows Heat loss Air leakage Airtightness U-value Energy saving
a b s t r a c t The rate of air leakage related energy loss from glazed areas is unequivocal especially in older and poorly installed windows. Therefore, in this research, a comprehensive experimental investigation is done to analyse the importance of air leakage on overall heat transfer coefficient (U-value) of conventional air filled double glazed windows. The tests are conducted in a typical UK dwelling of Nottingham fitted with traditional air filled double glazed windows. One sash of the test window is sealed internally with a special transparent cover to provide excellent airtightness whereas the second window sash is left as it is to represent the ordinary case. The experiments are conducted in April 2016, and dynamic co-heating test methodology is applied to evaluate the rate of enhancement in the U-value of airtight window sash. The results indicate that the airtight window sash has a notably lower U-value compared to the ordinary window sash due to the impact of airtightness and reverse heat flux during noon time owing to the greenhouse effect between transparent cover and internal glazing. The overall U-value of ordinary window sash is found to be 2.67 W/m2 K, whereas it is 1.79 W/m2 K for airtight window sash. It is observed that about 33% of reduction in heat losses can be achieved via airtight windows. © 2017 Elsevier B.V. All rights reserved.
1. Introduction Total energy consumption in the world has a steadily increasing trend despite of intensive measures taken worldwide and consensus on exhaustion of energy resources as well as growing importance of environmental issues [1,2]. It is clear from the latest reports based on energy consumption models that buildings are of significant relevance for notably rising global energy use [3]. About 40% of total world energy use is attributed to building sector [4]. This remarkable impact of buildings on energy consumption is a consequence of poor structural features and insufficient thermal insulation characteristics of existing building elements notably windows [5]. Glazed areas are responsible for the greatest energy loss from building envelope [6]. Approximately 47% of total heat loss through a typical residential building can be attributed to the windows due to their considerably higher U-values compared to other building elements [7–9]. Table 1 illustrates the latest performance figures of the three most common fenestration products in market [10,11]. It is clear from the data that even Argon filled double glazed windows equipped with suspended films are responsible for
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a significant amount of energy loss from building envelope [12]. In this respect, cost-effective and eco-friendly solutions are required to mitigate energy losses in buildings due to windows. Heat loss through windows occurs in a number of ways as shown in Fig. 1. Air leakage can be considered one of the greatest contributors to energy loss from conventional windows, especially in older and badly installed fenestration products. Even in temperate climates, air leakage related heat losses account for about 20% of total energy loss from windows [13]. In colder climates, the significance of airtightness in overall thermal insulation performance of windows becomes much more noticeable [14]. From this point of view, achieving a reasonable level of airtightness is of vital importance not only for the overall energy efficiency of dwellings but also for the thermal comfort of occupants [15]. Poor airtightness might be responsible for about 40% of heat loss from building fabric depending on the temperature difference between indoor and outdoor [16]. There are different building standards in developed countries regarding minimum airtightness levels which need to be achieved. The level of airtightness is given as the quantity of air (in m3 ) that leaks into or out of the building per hour, divided by the unit surface area (in m2 ) of the building fabric at a differential pressure of 50 Pa. In the UK, maximum air permeability is determined to be 3 m3 /h m2 through building standards, whereas it is 6 m3 /h m2 for Netherlands, 1.5 m3 /h m2 for Canada and 1.8–3.8 m3 /h m2 for
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Table 1 Overall heat transfer coefficients (U-values) of commercial glazing products. U-value (W/m2 K)
Pilkington [11]
Ref. [9]
Air filled double glazed window Air filled double glazed window with low-e Argon filled double glazed window with low-e
2.70 2.00 1.80
2.53 2.10 1.90
Germany [17]. The amount of air leakage in a typical window is affected by design and quality of construction, wind speed and direction. Micro gaps around window sashes play an important role in air infiltration. In addition, air can easily leak directly to the outside or into the cavity through micro gaps between the wall reveals and window frame. Despite the pronounced impacts of airtightness on overall thermal insulation performance of conventional windows, there are very few quantitative works conducted in literature aiming to determine the rate of energy loss due to air infiltration trough glazed areas. Hilliaho et al. [18] carry out a sensitivity analysis and evaluate the energy saving potential of glazed balconies in northern climatic conditions through five key performance factors covering airtightness. Their results reveal that airtight windows can provide about 15% energy saving owing to the remarkable enhancement in overall U-value of the windows. Sadauskiene et al. [19] investigate the role of airtightness in overall building energy performance through a case study conducted in Lithuania. The impact of airtightness is evaluated for 27 single-family detached houses built during 2007–2011. The building samples are split into three categories (A–C class) according to the building energy performance rates. The results reveal that A class buildings, which have an air change rate below 0.6, require notably lower heating demand compared to B and C class buildings. The heating demand of an A class building with an air change rate of 0.55 is found to be 130.50 kWh/m2 · year, whereas it is 255.41 kWh/m2 · year for a C class building with an air change rate of 2.99. Alfano et al. [20] evaluate the airtightness in Mediterranean buildings by using fan pressurization method. The impact of air leakage on energy use and thermal comfort of occupants is analysed for different architectural typologies. The tests conducted on wooden frame windows indicate that the retrofitting of conventional windows with appropriate sealing rubbers reduce the air infiltration by 25% resulting in about 50% lower heating demand on dwellings located in Southern Italy. Kalamees [21] performs a field work on 32 lightweight single-family detached houses in Estonia and evaluate them in terms of airtightness performance. The air leakage from each house is determined via a standardized blower door method. It is reported in the research that the overall U-value of windows in Estonian buildings cannot exceed 2.1 W/m2 K according to the Estonian building standard. However,
Fig. 1. The ways of heat loss through conventional windows.
it is found that only 41% of the test houses can fulfil this requirement because of insufficient airtightness, which results in notable rises in heating demand due to the extreme weather conditions in winter. Sfakianaki et al. [22] carry out airtightness and infiltration measurements in 20 residential buildings of Athens. Two different airtightness measurement methods (the tracer gas decay method and the blower door method) are utilised in the tests. It is observed from the measurements that airtightness level is highly affected by the total frame length. In this respect, design, material and construction features of window frames are of significant relevance to total energy loss from building envelope. Sinnott and Dyer [23] conduct some field measurements on old and new buildings in Ireland to assess the airtightness performance. Air leakage tests indicate that the air permeability of pre-1975, 1980’s and 2008 dwellings is found to be 7.5, 9.4 and 10.4 m3 /h m2 , respectively. The most airtight dwelling is observed to be from 1961s with 5.1 m3 /h m2 , which means new dwellings cannot automatically be presumed more airtight than older buildings. It is reported in the research that a proper retrofitting can have a positive impact on airtightness in residential buildings, and airtightness can be enhanced by 35%. It is clear in previous literature that airtightness is a significant performance parameter of windows, and overall energy efficiency and indoor comfort of any dwelling is greatly affected by the level of airtightness achieved in glazing products. There are several ways of enhancing the level of airtightness in windows, and one of them is to retrofit the existing windows internally with special transparent covers. Comprehensive literature survey indicates that there is no attempt done so far to analyse the impact of such transparent covers on airtightness of existing conventional windows through in-situ tests. Therefore in this research, a comprehensive experimental analysis is done to demonstrate the significance of air leakage on the U-value of conventional air filled double glazed windows. Insitu tests are conducted in a traditional UK dwelling, and ordinary and airtight window sashes are simultaneously tested for a reliable and realistic approach.
2. Methodology Air and Argon filled double glazed windows are the most common fenestration products in market as a consequence of their well-documented manufacturing process and considerably better thermal insulation feature compared to conventional single glazing [24]. However, their current U-value range is not promising due to some design, material and construction related performance parameters, and the airtightness is of vital relevance. Therefore in this experimental research, air filled double glazed windows are taken into consideration to investigate the impact of airtightness on the overall thermal insulation performance of conventional windows. A typical two-storey dwelling located in Nottingham, UK is selected for the in-situ tests as shown in Fig. 2. The test window of the dwelling consists of two sashes in which one sash is internally retrofitted with a 1 mm thick special transparent cover for airtightness and the neighbour sash is left as it is to represent the pre-retrofit case. The aforesaid special transparent cover is a polyethylene based sheet specifically developed for the internal sealing of fenestration products. This polyethylene cover has 72% visible light transmittance, which is very promising for efficient daylighting. Thermal conductivity of the material is given to be 0.33 W/mK, whereas the specific heat capacity is 1.9 kJ/kgK. The special transparent cover also provides a flexible operating temperature range from −60 to 90 ◦ C. A dynamic co-heating test methodology is performed to the test house in April, and the thermal insulation characteristics of conventional air filled double glazed windows are determined at pre and post retrofit case. The
E. Cuce / Energy and Buildings 139 (2017) 449–455
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Fig. 2. Traditional two-storey dwelling in the UK for the in-situ tests.
role of airtightness in the overall U-value of the test window is evaluated through a systematic experimental approach. Schematic of the airtight and ordinary window sashes with measurement details is given in Fig. 3. 3. Experimental work The tests are conducted in April over three complete days under the climatic conditions of Nottingham, UK. A typical two-storey UK dwelling of Nottingham having conventional air filled double glazed windows is utilised for the airtightness tests. Nottingham has temperate and erratic weather conditions representing the characteristic climatic features of continental regions. The heating season is still active in April in which the co-heating tests are conducted. According to the Nottingham Weather Centre at Watnall [25], the average high and low temperature in April over the last three decades is reported to be 12.5 and 4.3 ◦ C, respectively. The measurements presented in this research also accord with the average climatic data of Nottingham. The test window has an entire glazing thickness of 24 mm (6 mm pane + 12 mm air gap + 6 mm pane) and its frame is made of polyvinyl chloride (PVC). One sash of the test window is sealed internally with a 1 mm thick special transparent cover to enable required airtightness while the second window sash is left as it is to characterise the ordinary case. The experiments are conducted in April 2016, and dynamic co-heating test approach is performed to evaluate the rate of enhancement in overall thermal insulation performance of airtight window sash. Dynamic co-heating test methodology is based on providing sensible temperature difference between indoor and outdoor air and calculating the overall heat transfer coefficient of the window holistically by considering day and night time data rather than doing analysis with the data from conditioned environments [4,26]. In this respect, sensible temperature difference between indoor and outdoor air is provided by occupants when the heating need arises. The occupants utilise thermostatic wall mounted panel heaters for heating purposes, and they continue to live in the test house during the experiments. The measurement sensors utilised in the experiments are shown in Fig. 4. HFS-4 thin film heat flux sensors, which are specially fabricated for glazing tests and standard K type thermocouples are implemented to the window sashes centrally. Thermocouples are protected from direct sunlight by covering them with reflective and thermal insulation tapes. HFS-4 thin film heat flux sensors already have a K type thermocouple to be able to measure the internal glazing temperature as well as heat flux
Fig. 3. Schematic of the airtight and ordinary window sashes with measurement details.
data. However, an additional K type thermocouple is fixed next to the each heat flux sensor as illustrated in Fig. 4 for accuracy verification of the measurements. External glazing temperatures are also measured via K type thermocouples, and glazing temperature difference provided by airtight and ordinary window sash is measured time-dependently. Indoor and outdoor air temperatures are measured simultaneously for an easier analysis and interpretation of the results. The special transparent cover used in experiments not only provides airtightness but also enables a greenhouse effect between the internal glazing and indoor environment. In the sunny days of winter season, this greenhouse effect provides a reverse heat flux especially during the noon time, and thus the passive heat gain contributes to enhancing the overall thermal insulation performance of glazing products. The heat flux data is divided by the glazing temperature difference, and the dynamic U-value of airtight and ordinary window sash is determined [4,26]. Hence, the impact of airtightness on overall energy performance of conventional air filled windows is evaluated through a dynamic co-heating test methodology. DT85 Datataker data-logger is utilised for timedependent data triggering. The time step for data acquisition is selected to be 10 s. The accuracy of the measurements in the experimental work is checked through the uncertainty analysis. Through the sensitivity values of the K type thermocouples and the HFS-4 thin film heat flux sensors, total uncertainty for the U-value measurement is found to be less than 1%, which is totally acceptable for such a research. Moreover, the experimental U-value of air filled double glazed
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Fig. 4. Airtight and ordinary window sashes with measurement sensors.
window sash is verified through the previous works in literature and recent performance reports of commercial companies [11]. 4. Results and discussions The demonstration of the results starts with the variation of internal glazing temperature of each window sash with time as shown in Fig. 5. The data achieved from the T type thermocouples of thin film heat flux sensors indicate that special transparent cover is capable of providing a noticeable temperature drop in internal glazing temperature. The reduction in internal glazing temperature up to 4 ◦ C is achieved in the night time as expected and desired. On the other hand, a reverse influence occurs during the day time as a consequence of greenhouse effect, and internal glazing temperature of airtight window sash becomes considerably higher than that of ordinary window sash especially in the noon time. As previously notified, the accuracy of temperature measurements from heat flux sensors is checked through additional thermocouples, and there is an excellent agreement between the results of primary and secondary measurements as illustrated in Fig. 6. On the contrary to the internal glazing temperatures, there is an insignificant difference between the external glazing temperatures
Fig. 5. Internal glazing temperatures of ordinary and airtight window sashes.
of airtight and ordinary window sash. The greatest temperature difference is observed to be about 1 ◦ C as shown in Fig. 7. Similar tendency of external glazing temperatures can be attributed to the much more remarkable convective effects of outdoor air in comparison with that of indoor air. Negligible deviations are noticed in the daytime measurements of external glazing temperature. Following the determination of internal and external glazing temperatures, glazing temperature difference of airtight and ordinary window sash is calculated as illustrated in Fig. 8. As it is well-documented in literature that the rate of heat loss across any building element is driven by the temperature difference [27–31], and it is unequivocal from the night time results that ordinary window sash has notably higher glazing temperature difference than airtight window sash. The greatest night time glazing temperature difference is observed on 22nd April 2016 with 2.6 ◦ C, which is remarkable. During the day time tests, it is promisingly found that the transparent cover provides sensible rise in internal glazing temperature of airtight window sash owing to the greenhouse effect enabled between the internal glazing and the indoor environment. This greenhouse influence results in reverse heat flux inside the airtight window sash, and the thermal energy content of the warm air in
Fig. 6. Accuracy confirmation of internal glazing temperatures of ordinary and airtight window sashes through secondary measurements.
E. Cuce / Energy and Buildings 139 (2017) 449–455
Fig. 7. External glazing temperatures of ordinary and airtight window sashes.
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Fig. 10. Heat flux from airtight window sash.
the enclosure transmits into the indoor environment yielding to notable enhancements in overall thermal insulation performance of the airtight window sash. The results of heat flux measurements are shown in Figs. 9 and 10 for ordinary and airtight window
sash, respectively. The impacts of transparent cover on reverse heat flux are unequivocal from the time-dependent data. Following the determination of heat flux values, dynamic U-value of each window sash is calculated. Fig. 11 indicates the overall dynamic U-value of ordinary window sash. A regression analysis is done to determine the average U-value for an easier understanding and interpretation of the results. It is observed from the calculations that the average Uvalue of the ordinary window sash is 2.67 W/m2 K, which is in good accordance with the previous literature. Pilkington [11] presents a theoretical U-value of 2.80 W/m2 K for the same frame and glazing features. Cuce [1] also finds an experimental U-value of 2.53 W/m2 K for a similar window sample. At the post retrofit case, the U-value is considerably enhanced as a consequence of airtightness. The special transparent cover provides excellent airtightness as well as beneficial greenhouse influence resulting to an average U-value of 1.79 W/m2 K as illustrated in Fig. 12. In other words, it is observed that about 33% of reduction in heat losses can be achieved via airtight windows under reasonable differential temperatures given in Fig. 13. It can be understood from the characteristic results of the insitu tests that airtightness is of vital importance to mitigate energy losses from building envelope. Remarkable enhancements in the average U-value of conventional air filled double glazed windows can be achieved through transparent covers, which are ideal for
Fig. 9. Heat flux from ordinary window sash.
Fig. 11. Dynamic U-value of ordinary window sash.
Fig. 8. Glazing temperature difference of ordinary and airtight window sashes.
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Fig. 12. Dynamic U-value of airtight window sash.
Fig. 14. Visual quality of airtight and ordinary window sash.
Fig. 13. Dynamic indoor and outdoor air temperatures during testing.
internal retrofitting of glazed areas for winter season. The cost of such transparent covers is quite low, and the visual quality after retrofit is satisfactory even in case of condensation as shown in Fig. 14. Although it is not in the scope of this research, that would be useful to give additional information to the readers about the separate airtightness tests conducted on window sashes investigated in this paper. It is well-documented in literature that air permeability of a window refers to the amount of air that travels through the window system in its closed position. Air permeability testing relies on the quality of the systems sealing, engineering and manufacturing to ensure that all opening segments seal together properly in order stop as much air travel through the window as possible. There are several ways of measuring airtightness such as tracer gas decay method and Blower Door method. In this research, Blower Door method is utilised to check the airtightness rates of window sashes with and without special transparent cover. The name of the method comes from the fact that in the common utilization of the technology there is a fan fixed in a door [30]. Blower Door data predicts airflows at a variety of pressures and notably at a 50 Pa pressure difference. The advantage of this method is that the results are less affected by environmental conditions [31]. As the research presented in this paper is a part of a large-scale research project, several comprehensive airtightness analyses are done for different transparent covers and window products. The results from
the comprehensive airtightness tests are illustrated in Table 2. The measurements are done in accordance with EN ISO 13829 standard. Air leakage rates from separate tests for window sashes with and without special transparent cover clearly indicate that air leakage and thus air leakage related energy losses from glazed areas can be completely prevented in buildings by utilising appropriate transparent covers. The effectiveness of sealing is also verified via thermal anemometers like illustrated in Fig. 15. It is observed that there is no air penetration through the frame or glazing after retrofit as the special transparent cover entirely surrounds the window. It needs to be noted that the special transparent cover utilised within the scope of this research for airtightness purpose does not only prevent air leakage from the entire window but also provides greenhouse effect inside the glazing. The additional air gap between the indoor air and the internal glazing enables greenhouse effect in the daytime of sunny days resulting in sensible temperature rise in enclosed air. This temperature rise enables passive heat gain into the indoor air through the glazing, and this impact contributes to the overall heat transfer coefficient of the windows. The average U-value of air filled double glazed window sash is found to be 2.67 W/m2 K, while it is 1.79 W/m2 K for the airtight window sash. Table 2 Air leakage rates at different air pressure differences. Before retrofit Air leakage rate (Lt/s/m2 )
50 Pa
75 Pa
100 Pa
Test 1 Test 2 Test 3
1.288 1.285 1.290
1.521 1.518 1.524
1.775 1.772 1.778
After retrofit Air leakage rate (Lt/s/m2 )
50 Pa
75 Pa
100 Pa
Test 1 Test 2 Test 3
0.014 0.011 0.012
0.032 0.027 0.030
0.055 0.047 0.051
E. Cuce / Energy and Buildings 139 (2017) 449–455
Fig. 15. Standard thermal anemometers to check the air leakage rates.
33% of reduction in heat losses is obtained via airtight windows. It is of course a combined consequence of airtightness and greenhouse effect. However, the dominant enhancement is achieved through the improved airtightness rate of window. This output can be easily verified through previous literature. Alfano et al. [20] clearly notify in their research to investigate the airtightness levels of Mediterranean buildings that air leakage in windows can be reduced by about 37% through proper sealing materials. In this respect, the results presented in this study are in good accordance with the previous literature. 5. Conclusions In this experimental research, the role of airtightness in overall thermal insulation performance of conventional air filled double glazed windows is analysed through comprehensive in-situ tests conducted in a typical dwelling of Nottingham, UK. One sash of the test window is fitted with a special transparent cover for airtightness, whereas the neighbour sash is left as it is to characterise the ordinary case. Dynamic U-values of airtight and ordinary window sashes are determined over three complete days, and the potential enhancements in the overall thermal insulation performance are evaluated. It is concluded from the results that airtightness is a significant performance parameter for fenestration products. The average U-value of air filled double glazed window sash is determined to be 2.67 W/m2 K, whereas it is 1.79 W/m2 K for the airtight window sash. In other words, 33% of reduction in heat losses is achieved via airtight windows. The results reveal that conventional windows still play an important role in energy demand of buildings, and cost-effective solutions such as internal retrofitting of windows in winter season with transparent covers can greatly contribute in reducing the windows related energy losses in dwellings. In further works, the role of airtightness in overall thermal insulation performance of glazed areas will be extended to overall building energy performance through comprehensive experimental and simulation works. Acknowledgements The author gratefully acknowledges the financial support of TÜBITAK (Scientific and Technological Research Council of Turkey) through Grant BIDEB2219 2015/1. References [1] E. Cuce, Development of Innovative Window and Fabric Technologies for Low-carbon Buildings, The University of Nottingham, 2014, Ph.D. Thesis.
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[2] L. Perez-Lombard, J. Ortiz, C. Pout, A review on buildings energy consumption information, Energy Build. 40 (3) (2008) 394–398. [3] V. Bianco, F. Scarpa, L.A. Tagliafico, Analysis and future outlook of natural gas consumption in the Italian residential sector, Energy Convers. Manag. 87 (2014) 754–764. [4] E. Cuce, Toward Thermal Superinsulation Technologies in Buildings: Latest Developments in Glazing and Building Fabric, LAP Lambert Academic Publishing, Saarbrücken, Germany, 2015. [5] E. Cuce, S.B. Riffat, A state-of-the-art review on innovative glazing technologies, Renew. Sustain. Energy Rev. 41 (2015) 695–714. [6] E. Cuce, S.B. Riffat, Vacuum tube window technology for highly insulating building fabric: an experimental and numerical investigation, Vacuum 111 (2015) 83–91. [7] E. Cuce, C.H. Young, S.B. Riffat, Thermal insulation, power generation, lighting and energy saving performance of heat insulation solar glass as a curtain wall application in Taiwan: a comparative experimental study, Energy Convers. Manag. 96 (2015) 31–38. [8] E. Cuce, C.H. Young, S.B. Riffat, Thermal performance investigation of heat insulation solar glass: a comparative experimental study, Energy Build. 86 (2015) 595–600. [9] E. Cuce, C.H. Young, S.B. Riffat, Performance investigation of heat insulation solar glass for low-carbon buildings, Energy Convers. Manag. 88 (2014) 834–841. [10] E. Cuce, P.M. Cuce, S.B. Riffat, Novel glazing technologies to mitigate energy consumption in low-carbon buildings: a comparative experimental investigation, Int. J. Energy Res. 40 (2016) 537–549. [11] Understanding the Government’s Data on U-values, 2017, http://www. pilkington.com. (Last access is on 30 May 2016). [12] E. Cuce, P.M. Cuce, Vacuum glazing for highly insulating windows: recent developments and future prospects, Renew. Sustain. Energy Rev. 54 (2016) 1345–1357. [13] N.V.D. Bossche, W. Huyghe, J. Moens, A. Janssens, M. Depaepe, Airtightness of the window?wall interface in cavity brick walls, Energy Build. 45 (2012) 32–42. [14] M.H. Sherman, R. Chan, Building airtightness: research and practice, in: Lawrence Berkeley National Laboratory Report No. LBNL-53356, Lawrence Berkeley National Laboratory, Berkeley, U.S, 2004. [15] Foundation, A Practical Guide to Building Airtight Dwellings, 2009, Published by IHS BRE Press on behalf of the NHBC Foundation, June 2009, ISBN 978-1-84806-095-1. [16] B.C. Webb, R. Barton, BRE Report BR448 Airtightness in Commercial and Public Buildings, 2002. [17] Energy Saving Trust, CE248 Achieving Airtightness in New Dwellings: Case Studies, 2007, Edition. [18] K. Hilliaho, E. Makitalo, J. Lahdensivu, Energy saving potential of glazed space: sensitivity analysis, Energy Build. 99 (2015) 87–97. [19] J. Sadauskiene, L. Seduikyte, V. Paukstys, K. Banionis, A. Gailius, The role of air tightness in assessment of building energy performance: case study of Lithuania, Energy Sustain. Dev. 32 (2016) 31–39. [20] F.R.D. Alfano, M. Dell’Isola, G. Ficco, F. Tassini, Experimental analysis of air tightness in Mediterranean buildings using the fan pressurization method, Build. Environ. 53 (2012) 16–25. [21] T. Kalamees, Air tightness and air leakages of new lightweight single-family detached houses in Estonia, Build. Environ. 42 (2007) 2369–2377. [22] A. Sfakianaki, K. Pavlou, M. Santamouris, I. Livada, M.N. Assimakopoulos, P. Mantas, A. Christakopoulos, Air tightness measurements of residential houses in Athens, Greece, Build. Environ. 43 (2008) 398–405. [23] D. Sinnott, M. Dyer, Air-tightness field data for dwellings in Ireland, Build. Environ. 51 (2012) 269–275. [24] E. Cuce, Toward multi-functional PV glazing technologies in low/zero carbon buildings: heat insulation solar glass – latest developments and future prospects, Renew. Sustain. Energy Rev. 60 (2016) 1286–1301. [25] Nottingham Climate, 2017, http://www.metoffice.gov.uk/public/weather/ climate/gcrjm8jf7/. (Last access is on 13 July 2016). [26] M. Dastbaz, C. Gorse, Sustainable Ecological Engineering Design. Selected Proceedings from the International Conference of Sustainable Ecological Engineering Design for Society (SEEDS), Springer International Publishing, Switzerland, 2016. [27] A.L. Pisello, F. Cotana, A. Nicolini, C. Buratti, Effect of dynamic characteristics of building envelope on thermal-energy performance in winter conditions: in field experiment, Energy Build. 80 (2014) 218–230. [28] C. Buratti, E. Moretti, E. Belloni, F. Agosti, Thermal and acoustic performance evaluation of new basalt fiber insulation panels for buildings, Energy Procedia 78 (2015) 303–308. [29] C. Buratti, E. Moretti, E. Belloni, F. Agosti, Development of innovative aerogel based plasters: preliminary thermal and acoustic performance evaluation, Sustainability 6 (9) (2014) 5839–5852. [30] C. Buratti, E. Belloni, E. Moretti, D. Palladino, M. Vergoni, Thermal behaviour and energy saving evaluation of innovative reinforced coatings, Energy Procedia 82 (2015) 480–485. [31] S.B. Riffat, E. Cuce, Aerogel with its outstanding features and building applications, in: Eleventh International Conference on Sustainable Energy Technologies, 2–5 September 2012, Vancouver, Canada, 2012.
Contents lists available at ScienceDirect
Energy and Buildings journal homepage: www.elsevier.com/locate/enbuild
Role of airtightness in energy loss from windows: Experimental results from in-situ tests Erdem Cuce a,b a b
Department of Mechanical Engineering, Faculty of Engineering, University of Bayburt, 69000 Bayburt, Turkey Central Research Laboratory, University of Bayburt, 69000 Bayburt, Turkey
a r t i c l e
i n f o
Article history: Received 22 September 2016 Received in revised form 16 November 2016 Accepted 7 January 2017 Available online 13 January 2017 Keywords: Double glazed windows Heat loss Air leakage Airtightness U-value Energy saving
a b s t r a c t The rate of air leakage related energy loss from glazed areas is unequivocal especially in older and poorly installed windows. Therefore, in this research, a comprehensive experimental investigation is done to analyse the importance of air leakage on overall heat transfer coefficient (U-value) of conventional air filled double glazed windows. The tests are conducted in a typical UK dwelling of Nottingham fitted with traditional air filled double glazed windows. One sash of the test window is sealed internally with a special transparent cover to provide excellent airtightness whereas the second window sash is left as it is to represent the ordinary case. The experiments are conducted in April 2016, and dynamic co-heating test methodology is applied to evaluate the rate of enhancement in the U-value of airtight window sash. The results indicate that the airtight window sash has a notably lower U-value compared to the ordinary window sash due to the impact of airtightness and reverse heat flux during noon time owing to the greenhouse effect between transparent cover and internal glazing. The overall U-value of ordinary window sash is found to be 2.67 W/m2 K, whereas it is 1.79 W/m2 K for airtight window sash. It is observed that about 33% of reduction in heat losses can be achieved via airtight windows. © 2017 Elsevier B.V. All rights reserved.
1. Introduction Total energy consumption in the world has a steadily increasing trend despite of intensive measures taken worldwide and consensus on exhaustion of energy resources as well as growing importance of environmental issues [1,2]. It is clear from the latest reports based on energy consumption models that buildings are of significant relevance for notably rising global energy use [3]. About 40% of total world energy use is attributed to building sector [4]. This remarkable impact of buildings on energy consumption is a consequence of poor structural features and insufficient thermal insulation characteristics of existing building elements notably windows [5]. Glazed areas are responsible for the greatest energy loss from building envelope [6]. Approximately 47% of total heat loss through a typical residential building can be attributed to the windows due to their considerably higher U-values compared to other building elements [7–9]. Table 1 illustrates the latest performance figures of the three most common fenestration products in market [10,11]. It is clear from the data that even Argon filled double glazed windows equipped with suspended films are responsible for
E-mail address: [email protected] http://dx.doi.org/10.1016/j.enbuild.2017.01.027 0378-7788/© 2017 Elsevier B.V. All rights reserved.
a significant amount of energy loss from building envelope [12]. In this respect, cost-effective and eco-friendly solutions are required to mitigate energy losses in buildings due to windows. Heat loss through windows occurs in a number of ways as shown in Fig. 1. Air leakage can be considered one of the greatest contributors to energy loss from conventional windows, especially in older and badly installed fenestration products. Even in temperate climates, air leakage related heat losses account for about 20% of total energy loss from windows [13]. In colder climates, the significance of airtightness in overall thermal insulation performance of windows becomes much more noticeable [14]. From this point of view, achieving a reasonable level of airtightness is of vital importance not only for the overall energy efficiency of dwellings but also for the thermal comfort of occupants [15]. Poor airtightness might be responsible for about 40% of heat loss from building fabric depending on the temperature difference between indoor and outdoor [16]. There are different building standards in developed countries regarding minimum airtightness levels which need to be achieved. The level of airtightness is given as the quantity of air (in m3 ) that leaks into or out of the building per hour, divided by the unit surface area (in m2 ) of the building fabric at a differential pressure of 50 Pa. In the UK, maximum air permeability is determined to be 3 m3 /h m2 through building standards, whereas it is 6 m3 /h m2 for Netherlands, 1.5 m3 /h m2 for Canada and 1.8–3.8 m3 /h m2 for
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Table 1 Overall heat transfer coefficients (U-values) of commercial glazing products. U-value (W/m2 K)
Pilkington [11]
Ref. [9]
Air filled double glazed window Air filled double glazed window with low-e Argon filled double glazed window with low-e
2.70 2.00 1.80
2.53 2.10 1.90
Germany [17]. The amount of air leakage in a typical window is affected by design and quality of construction, wind speed and direction. Micro gaps around window sashes play an important role in air infiltration. In addition, air can easily leak directly to the outside or into the cavity through micro gaps between the wall reveals and window frame. Despite the pronounced impacts of airtightness on overall thermal insulation performance of conventional windows, there are very few quantitative works conducted in literature aiming to determine the rate of energy loss due to air infiltration trough glazed areas. Hilliaho et al. [18] carry out a sensitivity analysis and evaluate the energy saving potential of glazed balconies in northern climatic conditions through five key performance factors covering airtightness. Their results reveal that airtight windows can provide about 15% energy saving owing to the remarkable enhancement in overall U-value of the windows. Sadauskiene et al. [19] investigate the role of airtightness in overall building energy performance through a case study conducted in Lithuania. The impact of airtightness is evaluated for 27 single-family detached houses built during 2007–2011. The building samples are split into three categories (A–C class) according to the building energy performance rates. The results reveal that A class buildings, which have an air change rate below 0.6, require notably lower heating demand compared to B and C class buildings. The heating demand of an A class building with an air change rate of 0.55 is found to be 130.50 kWh/m2 · year, whereas it is 255.41 kWh/m2 · year for a C class building with an air change rate of 2.99. Alfano et al. [20] evaluate the airtightness in Mediterranean buildings by using fan pressurization method. The impact of air leakage on energy use and thermal comfort of occupants is analysed for different architectural typologies. The tests conducted on wooden frame windows indicate that the retrofitting of conventional windows with appropriate sealing rubbers reduce the air infiltration by 25% resulting in about 50% lower heating demand on dwellings located in Southern Italy. Kalamees [21] performs a field work on 32 lightweight single-family detached houses in Estonia and evaluate them in terms of airtightness performance. The air leakage from each house is determined via a standardized blower door method. It is reported in the research that the overall U-value of windows in Estonian buildings cannot exceed 2.1 W/m2 K according to the Estonian building standard. However,
Fig. 1. The ways of heat loss through conventional windows.
it is found that only 41% of the test houses can fulfil this requirement because of insufficient airtightness, which results in notable rises in heating demand due to the extreme weather conditions in winter. Sfakianaki et al. [22] carry out airtightness and infiltration measurements in 20 residential buildings of Athens. Two different airtightness measurement methods (the tracer gas decay method and the blower door method) are utilised in the tests. It is observed from the measurements that airtightness level is highly affected by the total frame length. In this respect, design, material and construction features of window frames are of significant relevance to total energy loss from building envelope. Sinnott and Dyer [23] conduct some field measurements on old and new buildings in Ireland to assess the airtightness performance. Air leakage tests indicate that the air permeability of pre-1975, 1980’s and 2008 dwellings is found to be 7.5, 9.4 and 10.4 m3 /h m2 , respectively. The most airtight dwelling is observed to be from 1961s with 5.1 m3 /h m2 , which means new dwellings cannot automatically be presumed more airtight than older buildings. It is reported in the research that a proper retrofitting can have a positive impact on airtightness in residential buildings, and airtightness can be enhanced by 35%. It is clear in previous literature that airtightness is a significant performance parameter of windows, and overall energy efficiency and indoor comfort of any dwelling is greatly affected by the level of airtightness achieved in glazing products. There are several ways of enhancing the level of airtightness in windows, and one of them is to retrofit the existing windows internally with special transparent covers. Comprehensive literature survey indicates that there is no attempt done so far to analyse the impact of such transparent covers on airtightness of existing conventional windows through in-situ tests. Therefore in this research, a comprehensive experimental analysis is done to demonstrate the significance of air leakage on the U-value of conventional air filled double glazed windows. Insitu tests are conducted in a traditional UK dwelling, and ordinary and airtight window sashes are simultaneously tested for a reliable and realistic approach.
2. Methodology Air and Argon filled double glazed windows are the most common fenestration products in market as a consequence of their well-documented manufacturing process and considerably better thermal insulation feature compared to conventional single glazing [24]. However, their current U-value range is not promising due to some design, material and construction related performance parameters, and the airtightness is of vital relevance. Therefore in this experimental research, air filled double glazed windows are taken into consideration to investigate the impact of airtightness on the overall thermal insulation performance of conventional windows. A typical two-storey dwelling located in Nottingham, UK is selected for the in-situ tests as shown in Fig. 2. The test window of the dwelling consists of two sashes in which one sash is internally retrofitted with a 1 mm thick special transparent cover for airtightness and the neighbour sash is left as it is to represent the pre-retrofit case. The aforesaid special transparent cover is a polyethylene based sheet specifically developed for the internal sealing of fenestration products. This polyethylene cover has 72% visible light transmittance, which is very promising for efficient daylighting. Thermal conductivity of the material is given to be 0.33 W/mK, whereas the specific heat capacity is 1.9 kJ/kgK. The special transparent cover also provides a flexible operating temperature range from −60 to 90 ◦ C. A dynamic co-heating test methodology is performed to the test house in April, and the thermal insulation characteristics of conventional air filled double glazed windows are determined at pre and post retrofit case. The
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Fig. 2. Traditional two-storey dwelling in the UK for the in-situ tests.
role of airtightness in the overall U-value of the test window is evaluated through a systematic experimental approach. Schematic of the airtight and ordinary window sashes with measurement details is given in Fig. 3. 3. Experimental work The tests are conducted in April over three complete days under the climatic conditions of Nottingham, UK. A typical two-storey UK dwelling of Nottingham having conventional air filled double glazed windows is utilised for the airtightness tests. Nottingham has temperate and erratic weather conditions representing the characteristic climatic features of continental regions. The heating season is still active in April in which the co-heating tests are conducted. According to the Nottingham Weather Centre at Watnall [25], the average high and low temperature in April over the last three decades is reported to be 12.5 and 4.3 ◦ C, respectively. The measurements presented in this research also accord with the average climatic data of Nottingham. The test window has an entire glazing thickness of 24 mm (6 mm pane + 12 mm air gap + 6 mm pane) and its frame is made of polyvinyl chloride (PVC). One sash of the test window is sealed internally with a 1 mm thick special transparent cover to enable required airtightness while the second window sash is left as it is to characterise the ordinary case. The experiments are conducted in April 2016, and dynamic co-heating test approach is performed to evaluate the rate of enhancement in overall thermal insulation performance of airtight window sash. Dynamic co-heating test methodology is based on providing sensible temperature difference between indoor and outdoor air and calculating the overall heat transfer coefficient of the window holistically by considering day and night time data rather than doing analysis with the data from conditioned environments [4,26]. In this respect, sensible temperature difference between indoor and outdoor air is provided by occupants when the heating need arises. The occupants utilise thermostatic wall mounted panel heaters for heating purposes, and they continue to live in the test house during the experiments. The measurement sensors utilised in the experiments are shown in Fig. 4. HFS-4 thin film heat flux sensors, which are specially fabricated for glazing tests and standard K type thermocouples are implemented to the window sashes centrally. Thermocouples are protected from direct sunlight by covering them with reflective and thermal insulation tapes. HFS-4 thin film heat flux sensors already have a K type thermocouple to be able to measure the internal glazing temperature as well as heat flux
Fig. 3. Schematic of the airtight and ordinary window sashes with measurement details.
data. However, an additional K type thermocouple is fixed next to the each heat flux sensor as illustrated in Fig. 4 for accuracy verification of the measurements. External glazing temperatures are also measured via K type thermocouples, and glazing temperature difference provided by airtight and ordinary window sash is measured time-dependently. Indoor and outdoor air temperatures are measured simultaneously for an easier analysis and interpretation of the results. The special transparent cover used in experiments not only provides airtightness but also enables a greenhouse effect between the internal glazing and indoor environment. In the sunny days of winter season, this greenhouse effect provides a reverse heat flux especially during the noon time, and thus the passive heat gain contributes to enhancing the overall thermal insulation performance of glazing products. The heat flux data is divided by the glazing temperature difference, and the dynamic U-value of airtight and ordinary window sash is determined [4,26]. Hence, the impact of airtightness on overall energy performance of conventional air filled windows is evaluated through a dynamic co-heating test methodology. DT85 Datataker data-logger is utilised for timedependent data triggering. The time step for data acquisition is selected to be 10 s. The accuracy of the measurements in the experimental work is checked through the uncertainty analysis. Through the sensitivity values of the K type thermocouples and the HFS-4 thin film heat flux sensors, total uncertainty for the U-value measurement is found to be less than 1%, which is totally acceptable for such a research. Moreover, the experimental U-value of air filled double glazed
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Fig. 4. Airtight and ordinary window sashes with measurement sensors.
window sash is verified through the previous works in literature and recent performance reports of commercial companies [11]. 4. Results and discussions The demonstration of the results starts with the variation of internal glazing temperature of each window sash with time as shown in Fig. 5. The data achieved from the T type thermocouples of thin film heat flux sensors indicate that special transparent cover is capable of providing a noticeable temperature drop in internal glazing temperature. The reduction in internal glazing temperature up to 4 ◦ C is achieved in the night time as expected and desired. On the other hand, a reverse influence occurs during the day time as a consequence of greenhouse effect, and internal glazing temperature of airtight window sash becomes considerably higher than that of ordinary window sash especially in the noon time. As previously notified, the accuracy of temperature measurements from heat flux sensors is checked through additional thermocouples, and there is an excellent agreement between the results of primary and secondary measurements as illustrated in Fig. 6. On the contrary to the internal glazing temperatures, there is an insignificant difference between the external glazing temperatures
Fig. 5. Internal glazing temperatures of ordinary and airtight window sashes.
of airtight and ordinary window sash. The greatest temperature difference is observed to be about 1 ◦ C as shown in Fig. 7. Similar tendency of external glazing temperatures can be attributed to the much more remarkable convective effects of outdoor air in comparison with that of indoor air. Negligible deviations are noticed in the daytime measurements of external glazing temperature. Following the determination of internal and external glazing temperatures, glazing temperature difference of airtight and ordinary window sash is calculated as illustrated in Fig. 8. As it is well-documented in literature that the rate of heat loss across any building element is driven by the temperature difference [27–31], and it is unequivocal from the night time results that ordinary window sash has notably higher glazing temperature difference than airtight window sash. The greatest night time glazing temperature difference is observed on 22nd April 2016 with 2.6 ◦ C, which is remarkable. During the day time tests, it is promisingly found that the transparent cover provides sensible rise in internal glazing temperature of airtight window sash owing to the greenhouse effect enabled between the internal glazing and the indoor environment. This greenhouse influence results in reverse heat flux inside the airtight window sash, and the thermal energy content of the warm air in
Fig. 6. Accuracy confirmation of internal glazing temperatures of ordinary and airtight window sashes through secondary measurements.
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Fig. 7. External glazing temperatures of ordinary and airtight window sashes.
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Fig. 10. Heat flux from airtight window sash.
the enclosure transmits into the indoor environment yielding to notable enhancements in overall thermal insulation performance of the airtight window sash. The results of heat flux measurements are shown in Figs. 9 and 10 for ordinary and airtight window
sash, respectively. The impacts of transparent cover on reverse heat flux are unequivocal from the time-dependent data. Following the determination of heat flux values, dynamic U-value of each window sash is calculated. Fig. 11 indicates the overall dynamic U-value of ordinary window sash. A regression analysis is done to determine the average U-value for an easier understanding and interpretation of the results. It is observed from the calculations that the average Uvalue of the ordinary window sash is 2.67 W/m2 K, which is in good accordance with the previous literature. Pilkington [11] presents a theoretical U-value of 2.80 W/m2 K for the same frame and glazing features. Cuce [1] also finds an experimental U-value of 2.53 W/m2 K for a similar window sample. At the post retrofit case, the U-value is considerably enhanced as a consequence of airtightness. The special transparent cover provides excellent airtightness as well as beneficial greenhouse influence resulting to an average U-value of 1.79 W/m2 K as illustrated in Fig. 12. In other words, it is observed that about 33% of reduction in heat losses can be achieved via airtight windows under reasonable differential temperatures given in Fig. 13. It can be understood from the characteristic results of the insitu tests that airtightness is of vital importance to mitigate energy losses from building envelope. Remarkable enhancements in the average U-value of conventional air filled double glazed windows can be achieved through transparent covers, which are ideal for
Fig. 9. Heat flux from ordinary window sash.
Fig. 11. Dynamic U-value of ordinary window sash.
Fig. 8. Glazing temperature difference of ordinary and airtight window sashes.
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Fig. 12. Dynamic U-value of airtight window sash.
Fig. 14. Visual quality of airtight and ordinary window sash.
Fig. 13. Dynamic indoor and outdoor air temperatures during testing.
internal retrofitting of glazed areas for winter season. The cost of such transparent covers is quite low, and the visual quality after retrofit is satisfactory even in case of condensation as shown in Fig. 14. Although it is not in the scope of this research, that would be useful to give additional information to the readers about the separate airtightness tests conducted on window sashes investigated in this paper. It is well-documented in literature that air permeability of a window refers to the amount of air that travels through the window system in its closed position. Air permeability testing relies on the quality of the systems sealing, engineering and manufacturing to ensure that all opening segments seal together properly in order stop as much air travel through the window as possible. There are several ways of measuring airtightness such as tracer gas decay method and Blower Door method. In this research, Blower Door method is utilised to check the airtightness rates of window sashes with and without special transparent cover. The name of the method comes from the fact that in the common utilization of the technology there is a fan fixed in a door [30]. Blower Door data predicts airflows at a variety of pressures and notably at a 50 Pa pressure difference. The advantage of this method is that the results are less affected by environmental conditions [31]. As the research presented in this paper is a part of a large-scale research project, several comprehensive airtightness analyses are done for different transparent covers and window products. The results from
the comprehensive airtightness tests are illustrated in Table 2. The measurements are done in accordance with EN ISO 13829 standard. Air leakage rates from separate tests for window sashes with and without special transparent cover clearly indicate that air leakage and thus air leakage related energy losses from glazed areas can be completely prevented in buildings by utilising appropriate transparent covers. The effectiveness of sealing is also verified via thermal anemometers like illustrated in Fig. 15. It is observed that there is no air penetration through the frame or glazing after retrofit as the special transparent cover entirely surrounds the window. It needs to be noted that the special transparent cover utilised within the scope of this research for airtightness purpose does not only prevent air leakage from the entire window but also provides greenhouse effect inside the glazing. The additional air gap between the indoor air and the internal glazing enables greenhouse effect in the daytime of sunny days resulting in sensible temperature rise in enclosed air. This temperature rise enables passive heat gain into the indoor air through the glazing, and this impact contributes to the overall heat transfer coefficient of the windows. The average U-value of air filled double glazed window sash is found to be 2.67 W/m2 K, while it is 1.79 W/m2 K for the airtight window sash. Table 2 Air leakage rates at different air pressure differences. Before retrofit Air leakage rate (Lt/s/m2 )
50 Pa
75 Pa
100 Pa
Test 1 Test 2 Test 3
1.288 1.285 1.290
1.521 1.518 1.524
1.775 1.772 1.778
After retrofit Air leakage rate (Lt/s/m2 )
50 Pa
75 Pa
100 Pa
Test 1 Test 2 Test 3
0.014 0.011 0.012
0.032 0.027 0.030
0.055 0.047 0.051
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Fig. 15. Standard thermal anemometers to check the air leakage rates.
33% of reduction in heat losses is obtained via airtight windows. It is of course a combined consequence of airtightness and greenhouse effect. However, the dominant enhancement is achieved through the improved airtightness rate of window. This output can be easily verified through previous literature. Alfano et al. [20] clearly notify in their research to investigate the airtightness levels of Mediterranean buildings that air leakage in windows can be reduced by about 37% through proper sealing materials. In this respect, the results presented in this study are in good accordance with the previous literature. 5. Conclusions In this experimental research, the role of airtightness in overall thermal insulation performance of conventional air filled double glazed windows is analysed through comprehensive in-situ tests conducted in a typical dwelling of Nottingham, UK. One sash of the test window is fitted with a special transparent cover for airtightness, whereas the neighbour sash is left as it is to characterise the ordinary case. Dynamic U-values of airtight and ordinary window sashes are determined over three complete days, and the potential enhancements in the overall thermal insulation performance are evaluated. It is concluded from the results that airtightness is a significant performance parameter for fenestration products. The average U-value of air filled double glazed window sash is determined to be 2.67 W/m2 K, whereas it is 1.79 W/m2 K for the airtight window sash. In other words, 33% of reduction in heat losses is achieved via airtight windows. The results reveal that conventional windows still play an important role in energy demand of buildings, and cost-effective solutions such as internal retrofitting of windows in winter season with transparent covers can greatly contribute in reducing the windows related energy losses in dwellings. In further works, the role of airtightness in overall thermal insulation performance of glazed areas will be extended to overall building energy performance through comprehensive experimental and simulation works. Acknowledgements The author gratefully acknowledges the financial support of TÜBITAK (Scientific and Technological Research Council of Turkey) through Grant BIDEB2219 2015/1. References [1] E. Cuce, Development of Innovative Window and Fabric Technologies for Low-carbon Buildings, The University of Nottingham, 2014, Ph.D. Thesis.
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[2] L. Perez-Lombard, J. Ortiz, C. Pout, A review on buildings energy consumption information, Energy Build. 40 (3) (2008) 394–398. [3] V. Bianco, F. Scarpa, L.A. Tagliafico, Analysis and future outlook of natural gas consumption in the Italian residential sector, Energy Convers. Manag. 87 (2014) 754–764. [4] E. Cuce, Toward Thermal Superinsulation Technologies in Buildings: Latest Developments in Glazing and Building Fabric, LAP Lambert Academic Publishing, Saarbrücken, Germany, 2015. [5] E. Cuce, S.B. Riffat, A state-of-the-art review on innovative glazing technologies, Renew. Sustain. Energy Rev. 41 (2015) 695–714. [6] E. Cuce, S.B. Riffat, Vacuum tube window technology for highly insulating building fabric: an experimental and numerical investigation, Vacuum 111 (2015) 83–91. [7] E. Cuce, C.H. Young, S.B. Riffat, Thermal insulation, power generation, lighting and energy saving performance of heat insulation solar glass as a curtain wall application in Taiwan: a comparative experimental study, Energy Convers. Manag. 96 (2015) 31–38. [8] E. Cuce, C.H. Young, S.B. Riffat, Thermal performance investigation of heat insulation solar glass: a comparative experimental study, Energy Build. 86 (2015) 595–600. [9] E. Cuce, C.H. Young, S.B. Riffat, Performance investigation of heat insulation solar glass for low-carbon buildings, Energy Convers. Manag. 88 (2014) 834–841. [10] E. Cuce, P.M. Cuce, S.B. Riffat, Novel glazing technologies to mitigate energy consumption in low-carbon buildings: a comparative experimental investigation, Int. J. Energy Res. 40 (2016) 537–549. [11] Understanding the Government’s Data on U-values, 2017, http://www. pilkington.com. (Last access is on 30 May 2016). [12] E. Cuce, P.M. Cuce, Vacuum glazing for highly insulating windows: recent developments and future prospects, Renew. Sustain. Energy Rev. 54 (2016) 1345–1357. [13] N.V.D. Bossche, W. Huyghe, J. Moens, A. Janssens, M. Depaepe, Airtightness of the window?wall interface in cavity brick walls, Energy Build. 45 (2012) 32–42. [14] M.H. Sherman, R. Chan, Building airtightness: research and practice, in: Lawrence Berkeley National Laboratory Report No. LBNL-53356, Lawrence Berkeley National Laboratory, Berkeley, U.S, 2004. [15] Foundation, A Practical Guide to Building Airtight Dwellings, 2009, Published by IHS BRE Press on behalf of the NHBC Foundation, June 2009, ISBN 978-1-84806-095-1. [16] B.C. Webb, R. Barton, BRE Report BR448 Airtightness in Commercial and Public Buildings, 2002. [17] Energy Saving Trust, CE248 Achieving Airtightness in New Dwellings: Case Studies, 2007, Edition. [18] K. Hilliaho, E. Makitalo, J. Lahdensivu, Energy saving potential of glazed space: sensitivity analysis, Energy Build. 99 (2015) 87–97. [19] J. Sadauskiene, L. Seduikyte, V. Paukstys, K. Banionis, A. Gailius, The role of air tightness in assessment of building energy performance: case study of Lithuania, Energy Sustain. Dev. 32 (2016) 31–39. [20] F.R.D. Alfano, M. Dell’Isola, G. Ficco, F. Tassini, Experimental analysis of air tightness in Mediterranean buildings using the fan pressurization method, Build. Environ. 53 (2012) 16–25. [21] T. Kalamees, Air tightness and air leakages of new lightweight single-family detached houses in Estonia, Build. Environ. 42 (2007) 2369–2377. [22] A. Sfakianaki, K. Pavlou, M. Santamouris, I. Livada, M.N. Assimakopoulos, P. Mantas, A. Christakopoulos, Air tightness measurements of residential houses in Athens, Greece, Build. Environ. 43 (2008) 398–405. [23] D. Sinnott, M. Dyer, Air-tightness field data for dwellings in Ireland, Build. Environ. 51 (2012) 269–275. [24] E. Cuce, Toward multi-functional PV glazing technologies in low/zero carbon buildings: heat insulation solar glass – latest developments and future prospects, Renew. Sustain. Energy Rev. 60 (2016) 1286–1301. [25] Nottingham Climate, 2017, http://www.metoffice.gov.uk/public/weather/ climate/gcrjm8jf7/. (Last access is on 13 July 2016). [26] M. Dastbaz, C. Gorse, Sustainable Ecological Engineering Design. Selected Proceedings from the International Conference of Sustainable Ecological Engineering Design for Society (SEEDS), Springer International Publishing, Switzerland, 2016. [27] A.L. Pisello, F. Cotana, A. Nicolini, C. Buratti, Effect of dynamic characteristics of building envelope on thermal-energy performance in winter conditions: in field experiment, Energy Build. 80 (2014) 218–230. [28] C. Buratti, E. Moretti, E. Belloni, F. Agosti, Thermal and acoustic performance evaluation of new basalt fiber insulation panels for buildings, Energy Procedia 78 (2015) 303–308. [29] C. Buratti, E. Moretti, E. Belloni, F. Agosti, Development of innovative aerogel based plasters: preliminary thermal and acoustic performance evaluation, Sustainability 6 (9) (2014) 5839–5852. [30] C. Buratti, E. Belloni, E. Moretti, D. Palladino, M. Vergoni, Thermal behaviour and energy saving evaluation of innovative reinforced coatings, Energy Procedia 82 (2015) 480–485. [31] S.B. Riffat, E. Cuce, Aerogel with its outstanding features and building applications, in: Eleventh International Conference on Sustainable Energy Technologies, 2–5 September 2012, Vancouver, Canada, 2012.