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Energy performance of glazings in European climates

Heinrich Manz*, Urs-Peter MentiLucerne University of Applied Sciences and Arts, Technology & Architecture, Technikumstrasse 21, CH-6048 Horw, Switzerland

a r t i c l e i n f o

Article history:Received 5 April 2011Accepted 11 June 2011Available online 2 July 2011

Keywords:Thermal transmittanceTotal solar energy transmittanceClimateFaçade orientationSolar gainThermal loss

a b s t r a c t

Windows can cause significant thermal energy gains or losses in buildings. Focusing on wintertime,a simple method for analyzing and discussing energy flows through glazings is presented. The impact ofthe glazing quality, the façade orientation, and the severity of the climate on the ratio of solar gains tothermal losses through glazings are shown. As regards the passive solar heating of buildings, the glazingquality is represented by the ratio of the total solar energy transmittance to the thermal transmittance(g/U). The severity of the climate is determined by the ratio of the interior-exterior temperaturedifference to the solar irradiance (Dq/I). In this study, the method is based on monthly mean values ofinterior-exterior temperature difference and solar irradiance. Not at least because the approach isstraightforward, it might also be valuable for educational purposes.

For eight case study locations in Europe namely Bucharest, London, Madrid, Moscow, Rome, Stock-holm, Warsaw and Zurich, charts are presented, which display condensed information on the energyperformance of glazings. At a given location, the gain-to-loss ratio varies by a factor of roughly 30depending on the glazing quality and the façade orientation. At all locations and façade orientations,modern triple glazings perform best and enable net gains at south façades in December even in Moscowand Stockholm.

� 2011 Elsevier Ltd. All rights reserved.

1. Introduction

The history of manufacturing glass can be traced backmore thanfive thousand years. For centuries, glass has been used to fabricatetransparent elements for building façades, which allow daylight toenter interior spaces while at the same time providing weatherprotection.

Because of the request of modern architecture for transparencyand the advancement of glazing technologies in the past couple ofdecades, glass has been increasingly used and is today a keymaterialin façade technology. When employing glazings in modern build-ings, numerous physical and physiological aspects need to beconsidered such as visual contact between interior and exterior, useof daylight, optimizing and controlling solar energy gains, mini-mizing thermal losses, optimizing thermal comfort, minimizingglare, noise protection, air and driving rain tightness, and fire safety.

Research and development in the field of glazing technology hasso far e to name just a few major trends e focussed on

- minimizing the heat transfer through glazings aiming at thereduction of the heating demand and optimizing the thermal

comfort in buildings in cold climates: Low emissivity coatings,gas fillings, spacers, evacuated cavities and aerogel glazings areimportant topics here [1e8].

- characterization of glazings and shading devices by means ofadvanced measurement techniques such as hot boxes, solarcalorimeters, spectrophotometers, goniometers etc [9e11].aiming ultimately at reliable data for planning purposes.

- modeling and simulation of glazings and shading devices andrespective validations aiming at reliable planning software [12].

- optimizing solar protection while still providing gooddaylighting and, therefore, selective transmission of solarradiation: High transmittance in the visible and low trans-mittance in the infrared wavelength range [13].

- optimizing the switching properties for solar energy trans-mission with non-mechanical devices [14,15].

- specialized façade constructions such as double-skin façades[16e19].

As a consequence, a large number of research papers, professionalarticles andbooksonarchitectural glazings is available todayewhichcannot, of course, be cited adequately here e and it seems that theinterest in this fascinating building material is even on the increase.

However, the extensive use of glass in building façades is alsoassociated with problems such as overheating of the building in

* Corresponding author. Tel.: þ41 41 349 3915.E-mail address: [email protected] (H. Manz).

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0960-1481/$ e see front matter � 2011 Elsevier Ltd. All rights reserved.doi:10.1016/j.renene.2011.06.016

Renewable Energy 37 (2012) 226e232


summertime, lack of visual and thermal comfort, and increasedthermal losses in wintertime. Despite major technologicaladvancements in insulating glazing technology in recent years,windows still constitute weak spots in the building envelope interms of energy loss in cold climates. The thermal transmittance ofwindows tends to be much higher than that of the neighboringopaque elements. For instance, the thermal transmittance targetsspecified by the Swiss energy standard for buildings [20] for wallsand roofs incorporating state-of-the-art technologies are six timeslower than that for windows (0.15 Wm�2 K�1 and 0.9 Wm�2 K�1

respectively). In addition to being an important parameter withrespect to thermal losses, glazing can also be significant in terms ofsolar energy gains.

In practice, architects and engineers need simple and clearguidance in building design. The starting point for this studywas, therefore, to develop simple design charts, which show theimpact of the local climate, the glazing quality and the façadeorientation on the ratio of solar gains to thermal losses inwintertime.

2. Methodology

In terms of energy flows, glazings can be characterized by twoparameters: Firstly, the total solar energy transmittance g, whichdenotes the share of the incoming solar energy, which is convertedinto heat inside the indoor space. Secondly, the thermal trans-mittance U that describes how much heat is transferred throughthe glazing per square meter and Kelvin temperature differencebetween inside and outside. Therefore, the parameter g charac-terizes the solar gain of a glazing, whereas U stands for the thermalloss of a glazing.

As regards the energy flows through a glazing in a buildingfaçade, three cases can be distinguished:

g$I>U$Dq net energy gains (1)

g$I ¼ U$Dq energy gain ¼ energy loss (2)

g$I < U$Dq net energy loss (3)

where I denotes the solar irradiance and Dq stands for thetemperature difference between interior and exterior. The ratio ofsolar gains to thermal losses, a, is given by:

a ¼ g$IU$Dq

(4)

If glazing properties and climatic boundary conditions are puteach to different sides of the equation, one obtains:

gU

¼ a$DqI

(5)

As regards the passive solar heating of the building, g/U standsfor the glazing quality: A high value of g results in high solar gains,a low value of U in low thermal losses. Dq/I characterizes theseverity of a climate: Large temperature differences between

interior and exterior lead to high thermal losses, high values ofsolar irradiance result in high solar gains.

Discussing Eq. (4), again, three cases can be distinguished:a > 1 net energy gains.a ¼ 1 energy gain ¼ energy loss.a < 1 net energy loss.Eq. (5) is a simple linear equation and can be used to discuss the

impact of the glazing quality and the severity of the climate on thegain-to-loss ratio of a glazing. At a given location, the climaticparameter Dq/I is different for different façade orientations.

The net gain through a glazing is given by

_qnet gain ¼ g$I � U$Dq (6)

Eq. (4) can also be written as

g$I ¼ ða� 1Þ$U$Dqþ U$Dq (7)

and, therefore, the net gain is also given by

_qnet gain ¼ ða� 1Þ$U$Dq (8)

3. Case study locations

Eight case study locations were selected, which are e in a veryrough approach e evenly distributed over Europe (Fig. 1) andrepresent to some extent the diversity of European climates. Theselocations comprise of Bucharest, London, Madrid, Moscow, Rome,Stockholm, Warsaw and Zurich. In Table 1, longitude, latitude andaltitude above sea level of case study locations in Europe can beseen.Where some locations are very close to the sea (London, Romeand Stockholm) particularly Moscow is far away from large bodiesof water and experiences, therefore, a more continental climate.Geographical and meteorological data for all case study locationswere taken from the meteorological database Meteonorm [21].

Fig. 1. Case study locations in Europe.

Table 1Longitude, latitude and altitude above sea level of case study locations in Europe (Data from [21]).

Bucharest London Weather C. Madrid Barajas Moscow Rome Ciampino Stockholm Bromma Warsaw Zurich SMA

Altitude above sea level (m) 91 77 582 156 131 11 130 556Longitude (�) 26.22 �0.12 �3.55 37.62 12.58 17.95 20.98 8.57Latitude (�) 44.50 51.52 40.45 55.83 41.80 59.35 52.27 47.38

H. Manz, U.-P. Menti / Renewable Energy 37 (2012) 226e232 227


4. Climate and meteorological data at case study locations

Tables 2 and 3 provide an overview of monthly mean values ofsolar irradiation and temperature at case study locations in Europe.

Whereas Bucharest, Madrid and Rome have horizontal solarirradiations of more than 5000MJ/m2 per year, at all other locationsirradiation is less than 4000 MJ/m2. London experiences the lowestirradiation of 3352 MJ/m2 per year. At all locations, irradiation islowest in December. The lowest horizontal solar irradiation inDecember has Stockholm with only 22 MJ/m2, Rome experiencesthe highest value with 180 MJ/m2.

At all locations, January is the coldest month. The coldest loca-tion in wintertime is Moscow with a mean temperature in Januaryof �5.9 �C. But also Bucharest, Stockholm, and Warsaw have meantemperatures in January below zero. In Madrid, London, and Rome,the mean temperatures in January are between 5.7 �C and 7.2 �C.Hence, at all locations considered in this study, using solar gains forpassive heating of buildings in wintertime is an issue of concern.

The climate at the different case study locations can be brieflysummarized as follows. The southern European locations of Madridand Rome experience mild winters, warm summers, and quite highsolar irradiation over the whole year. The northern and easternEuropean locations of Moscow, Stockholm, Warsaw have cold

winters with little irradiation and mild summers. In Londonwinters and summers are mild and there is relatively little irradi-ation throughout the year. Winters are relatively cold in Zurich,summers are mild and the weather is slightly sunnier than inLondon. Bucharest experiences cold, sunny winters and warmsummers.

Tables 4e6 show the monthly mean vertical solar irradiance atdifferent façade orientations and the climatic parameterDq/Ii, wherei˛fSouth; West; North; Eastg. A constant interior temperature of20 �C was assumed for this study. The ratio Dq/Ii is highest forvirtually all locations and façade orientation in December. In otherwords, climatic boundary conditions aremost severe in December interms of solar gains and thermal losses through glazings. Therefore,the meteorological data for December were used for all figuresshown in this article.

5. Glazing data

As described earlier, glazings can be characterized in terms ofenergy flows by two parameters: Firstly, the total solar energytransmittance g and, secondly, the thermal transmittance U. Bothparameters depend on the glazing type, i.e. the number of glasspanes, the distances between the glass panes, the filling gas and the

Table 2Horizontal solar irradiation at case study locations in Europe (Data from [21]).

Horizontal Irradiation (MJ/m2) Bucharest London Weather C. Madrid Barajas Moscow Rome Ciampino Stockholm Bromma Warsaw Zurich SMA

January 158 68 238 58 209 36 65 101February 238 115 277 126 263 94 115 166March 385 241 508 266 443 245 256 306April 504 360 551 382 554 396 374 414May 662 486 734 569 691 590 551 529June 709 515 803 605 727 626 569 558July 727 508 828 580 778 594 569 590August 641 439 724 479 684 468 493 518September 461 306 540 281 515 281 306 353October 320 184 378 148 364 130 180 212November 162 86 230 58 227 43 72 112December 122 54 176 36 180 22 47 79Year 5080 3352 5976 3578 5620 3517 3586 3928

Table 3Temperature at case study locations in Europe (Data from [21]).

Temperature (�C) Bucharest London Weather C. Madrid Barajas Moscow Rome Ciampino Stockholm Bromma Warsaw Zurich SMA

January �1.5 6.5 5.7 �5.9 7.2 �1.6 �2.0 0.6February 0.9 6.9 7.0 �6.3 7.6 �1.7 �0.5 1.9March 5.4 8.6 10.6 �1.4 10.4 0.9 2.3 6.0April 11.3 10.5 12.7 7.1 13.2 6.1 8.7 9.0May 17.7 13.8 16.4 12.6 18.3 11.1 14.5 13.9June 21.2 17.0 23.1 17 22.9 15.7 16.9 17.4July 23.2 18.5 25.3 20.2 24.6 18.7 18.9 18.0August 22.5 19.2 25.0 17.0 24.9 18.1 18.6 18.5September 16.6 16.3 20.4 11.1 20.4 13.1 13.4 14.0October 11.2 12.8 14.8 5.5 16.9 7.3 8.6 10.0November 5.8 9.1 8.6 �1.1 12.6 3.0 3.3 4.3December �0.4 6.8 5.7 �5.7 8.4 �0.6 �1.4 1.4Year 11.2 12.2 14.6 5.8 15.6 7.5 8.4 9.6

Table 4Monthly mean irradiance at different façade orientations and climate parameter Dq/Ii in Zurich.

Zurich SMA Irradiance South(W/m2)

Dq/IS(m2K/W)

Irradiance West(W/m2)

Dq/IW(m2K/W)

Irradiance North(W/m2)

Dq/IN(m2K/W)

Irradiance East(W/m2)

Dq/IE(m2K/W)

November 65.3 0.240 30.6 0.513 15.3 1.026 30.6 0.513December 53.8 0.346 25.5 0.728 14.8 1.258 20.2 0.923January 65.9 0.295 32.3 0.601 18.8 1.031 26.9 0.722February 98.2 0.184 52.1 0.348 29.8 0.608 50.6 0.358

H. Manz, U.-P. Menti / Renewable Energy 37 (2012) 226e232228


number and properties of the coatings used. As a rule of thumb, anincreasing number of panes leads to decreasing values of U and g.The physical reason for that is that more cavities provide betterthermal insulation whereas more light is reflected due to moreglassegas interfaces. In Fig. 2, each dot represents a glazing type. Alldata for U and gwere taken from references [22e24]. Glazings AeDwere chosen for this study. Glazing A is a modern insulating glazingunit with three panes (3-IGU), both cavities filled with krypton andtwo low emissivity coatings. Glazing B is also a modern insulatingglazing unit but with two panes (2-IGU), the cavity is filled withargon and one low emissivity coating. Glazings C is an “old” doubleglazing without any coatings and with an air filled cavity. Glazing Dis an “old” single glazing without any coatings. Table 7 show thenumerical values of U and g for the four different glazing types used.

6. Results

Fig. 3 shows an array of lines according to Eq. (5) with the gain-to-loss ratio as a parameter in the range of a¼ 1/16 to a¼ 8. For themonth of December, the ratios of temperature difference to solarirradiance, Dq/Ii, at the façade orientations south, west, north andeast in Zurich, as well as the ratio g/U of glazings AeD wereemployed. Each dot determines the gain-to-loss ratio of a specificglazing type at a façade orientation in Zurich. It can be seen in Fig. 3that even in December glazing A features a net energy gain at allfaçade orientations except for north. Glazing B provides a net gainonly at the south façade whereas for all other orientation a net lossresults. Glazing C and D show net losses at all façade orientations.

Figs. 4e10 display in the sameway the gain-to-loss ratios for theglazing types AeD at different façade orientations and at other casestudy locations.

Because of the wide range of the climate parameter Dq/Ii at thedifferent locations, two different scales were used. Firstly, a range ofDq/Ii from 0 to 5.5 m2 KW�1 for Stockholm, Moscow and Warsaw,and secondly, a range from 0 to 2 m2 KW�1 for Bucharest, London,Madrid, Rome and Zurich.

Fig. 4 shows that in Bucharest glazings of type A feature netgains at all façade orientations whereas glazings of type B displaynet gains only at south, west and east façades.

Table 5Monthly mean irradiance at different façade orientations and climate parameter Dq/Ii in Moscow.

Moscow Irradiance South(W/m2)

Dq/IS(m2K/W)


Dq/IW(m2K/W)


Dq/IN(m2K/W)


Dq/IE(m2K/W)


Table 6Monthly mean irradiance at different façade orientations and climate parameter Dq/Ii in Rome.

Rome Ciampino Irradiance South(W/m2)

Dq/IS(m2K/W)


Dq/IW(m2K/W)


Dq/IN(m2K/W)


Dq/IE(m2K/W)


0

0.2

0.4

0.6

0.8

1

0 1 2 3 4 5 6

Tota

l sol

ar e

nerg

y tra

nsm

ittan

ce g

(-)

Thermal transmittance U (W/m2K)

Double glazing

A

B

CD

Single glazing

Triple glazing

Fig. 2. Thermal transmittance U and total solar energy transmittance g of differentglazing types (Data from [22e24]). Glazings A, B, C and D were used in this study.

Table 7Thermal transmittance U and total solar energy transmittance g of the four differentglazing types used in the Figs. 2e9 (Data from [22e24]).

Nr. Glazing type U (W/m2K) g (�) g/U (m2K/W) Data source

A 3-IGU 0.4 0.47 1.175 [24]B 2-IGU 1.0 0.60 0.600 [24]C Double 2.9 0.77 0.266 [23]D Single 5.9 0.87 0.147 [22]

Fig. 3. Gain-to-loss ratios at different façade orientations and for different glazingtypes in Zurich.



It can be seen in Fig. 5 that in London glazings of type A have netgains at south, west and east façades whereas glazings of type Bdisplay net gains only at south façades.

Fig. 6 shows that in Madrid modern glazings of types A providenet gains at all façade orientations. Glazings of type B display netgains on south, west and east façades.

According to Fig. 7, modern glazings of types A and B provide netgains at all façade orientation in Rome.

The Figs. 8e10 for Moscow, Stockholm and Warsaw display thatin cold European climates only south oriented modern tripleglazing of type A feature positive net gains. For all other façadeorientations and glazing types net losses are obtained.

Old double glazings of types C lead to net losses in all climatesand at all façades e except for south oriented façades in Bucharest,Madrid and Rome.

Old single glazings of types D lead to net losses in all climatesand at all façadese except for south oriented façades inMadrid andRome where minor net gains result.

In a given climate, the mean net energy flux through a specificglazing into the interior space can be computed according to Eq. (8).

In Zurich, the mean temperature difference between interiorand exterior is Dq¼ 18.6 K in December. Assuming glazing of type Aand a¼ 3 (south), an average net gain of 15W/m2 is obtained. If theglazed area is large enough, a quite significant heat gain results forthe interior space (Note: Taking the room geometry into account,this value can e.g. be compared with the maximum heating powerper floor area of 10 W/m2 as defined for passive houses).

For Bucharest, assuming a ¼ 5 (south) and glazing A, a net gainof 33 W/m2 results.

For Moscow, assuming a ¼ 2 (south) and glazing A, the net gainis 10 W/m2.

For Madrid, assuming a¼ 8 (south) and glazing A, the net gain is40 W/m2.

Figs. 3e10 show that at a given location, the gain-to-loss ratiovaries by a factor of roughly 30 depending on the glazing qualityand the façade orientation. In Stockholm (Fig. 9), the gain-to-lossratio varies by a factor of around 60.

0

0.2

0.4

0.6

0.8

1

1.2

1.4

0 0.5 1 1.5 2

g/U

(m2 K/

W)

Δθ/I (m2K/W)

Bucharest, December

Gain Lossα = 1/2

α = 1/4

α = 4α = 2

α = 1/8α = 1/16

α = 8 α = 1Glazing A (3-IGU)

Glazing B (2-IGU)

Glazing C (Double)Glazing D (Single)

Sout

h

Wes

tEa

st

Nor

th

Fig. 4. Gain-to-loss ratios in Bucharest.

0

0.2

0.4

0.6

0.8

1

1.2

1.4

0 0.5 1 1.5 2

g/U

(m2 K/

W)

Δθ/I (m2K/W)

London Weather C., December

Gain Loss

α = 1/2

α = 1/4

α = 4α = 2

α = 1/8α = 1/16


Glazing B (2-IGU)


Sout

h

Wes

t

East

Nor

th

Fig. 5. Gain-to-loss ratios in London Weather C.

0

0.2

0.4

0.6

0.8

1

1.2

1.4

0 0.5 1 1.5 2

g/U

(m2 K/

W)

Δθ/I (m2K/W)

Madrid Barajas, December

Gain Lossα = 1/2

α = 1/4

α = 4α = 2

α = 1/8α = 1/16


Glazing B (2-IGU)


Sou

th

Wes

tE

ast

Nor

th

Fig. 6. Gain-to-loss ratios in Madrid Barajas.

0

0.2

0.4

0.6

0.8

1

1.2

1.4

0 0.5 1 1.5 2

g/U

(m2 K/

W)

Δθ/I (m2K/W)

Rome Ciampino, December

Gain Lossα = 1/2

α = 1/4

α = 4α = 2

α = 1/8α = 1/16


Glazing B (2-IGU)


Sout

hW

est

Eas

tN

orth

Fig. 7. Gain-to-loss ratios in Rome Ciampino.



7. Discussion

The energy performance of glazings in European climates wasanalyzed. A simple method for quantifying energy flows throughglazings was presented, which is based on steady-state modeling ofsolar gains and thermal losses. Focusing on wintertime, the impactof the glazing quality, the façade orientation and the local climateon the gain-to-loss ratio was shown. As the climate is most severeat all European locations in December, meteorological data for thismonth were used.

As regards energy performance of glazings in European climatesthe following conclusions can be drawn based on this study:

- At all case study locations, modern triple insulating glazingunits (glazing A) exhibit the highest gain-to-loss ratios.

- Only modern triple insulating glazing units (glazing A) guar-antee net energy gains at all locations at south façades.

- Modern double insulating glazing units (glazing B) display onlyat certain locations and mainly at south façades net gains.

- “Old” double and single glazings (glazings C and D) lead to netlosses at all façades (except for most southern locations).

- At all locations, gain-to-loss ratios are best at south facades andworst at north facades. West and east façades perform simi-larly, with only minor differences between locations. (Note: Asall locations considered are on the northern hemisphere, solarirradiance is highest on the south and lowest on the northfaçade. Of course, on the southern hemisphere, the oppositewould be true.)

- Depending on the location, absolute net gains of modern tripleinsulating glazing units are on average roughly in the range of10 W/m2e40W/m2 at south façades in December. These fluxesbecome significant in low energy buildings if glazing areas aresufficiently large.

At a given location, solar irradiance and temperature vary asa function of time. Hence, also net fluxes vary and change the sign,even during the course of a single day. The utilization of the solarenergy gains requires sufficient activatable thermal mass in theinterior space such as concrete ceilings, brick walls etc. Buildingswith high time constants e well insulated, air tight buildings witha large amount of thermal mass e are well-suited to dampentemperature fluctuations and to maintain the room temperaturewithin the comfort limits.

Limitations to passive solar energy use in buildings, of course, dooccur particularly in urban areas due to shadowing by neighboringbuildings. Nevertheless, this study shows the importance of solarenergy gains for saving energy for heating in buildings in temperateand cold climates. For the sake of completeness it has to be addedthat an extensive use of glass in the building envelope can also leadto an overheating of the building in summertime and result in poorthermal comfort and/or cooling loads, which has to be avoided bymeasures such as an appropriate glazed area and shading devices.

The presented findings are not surprising and in accordancewith results of previous investigations. Therefore, the major pointof this study is that employing the ratio between total solar energytransmittance and thermal transmittance (g/U) for characterizingthe quality of a glazing and the ratio between interior-exteriortemperature difference and solar irradiance (Dq/I) for character-izing the severity of the climate, the energy performance of glaz-ings at different façade orientations can be discussed in a graphicway by means of the presented figures and the ratio between solar

0

0.2

0.4

0.6

0.8

1

1.2

1.4

0 1 2 3 4 5

g/U

(m2 K/

W)

Δθ/I (m2K/W)

Moscow, December

α = 1/2

α = 1/4

α = 2

Gain

α = 1/8

α = 1/16

α = 1/32

Loss

Glazing A (3-IGU)

Glazing B (2-IGU)


Sout

h

Wes

t

East

Nor

th

Fig. 8. Gain-to-loss ratios in Moscow.

0

0.2

0.4

0.6

0.8

1

1.2

1.4

0 1 2 3 4 5

g/U

(m2 K/

W)

Δθ/I (m2K/W)

Stockholm Bromma, December

α = 1/2

α = 1/4

α = 2

Gain

α = 1/8

α = 1/16α = 1/32

Loss

Sout

h

Wes

tEa

st

Nor

th

Glazing A (3-IGU)

Glazing B (2-IGU)


Fig. 9. Gain-to-loss ratios in Stockholm Bromma.

0

0.2

0.4

0.6

0.8

1

1.2

1.4

0 1 2 3 4 5

g/U

(m2 K/

W)

Δθ/I (m2K/W)

Warsaw, December

α = 1/2

α = 1/4

α = 2

Gain

α = 1/8

α = 1/16

α = 1/32

Loss

Sout

h

Wes

tEa

stN

orth

Glazing A (3-IGU)

Glazing B (2-IGU)


Fig. 10. Gain-to-loss ratios in Warsaw.



gains to thermal losses (a). To the authors knowledge, thisstraightforward approach of displaying the energy performance ofglazings is novel. Not at least because of the simplicity of themethod, it might also be valuable for educational purposes.

Acknowledgements

The authors gratefully acknowledge the financial support of thisproject by the Lucerne University of Applied Sciences and Arts,Technology & Architecture. In particular, we would like to thankProf. Dr. René Hüsler.

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author's personal copy energy performance of glazings in european climates

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