Energy and comfort analysis of glazed double skin facades for refurbishments in temperate climates

engineered transparency. International Conference at glasstec, Düsseldorf, Germany

21 and 22 October 2014

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Energy and comfort analysis of glazed double skin

facades for refurbishments in temperate climates

Francesco POMPONI*, Kenneth IPa

, Simeon OXIZIDISa

and Poorang A.E. PIROOZFARa

* Faculty of Science and Engineering, University of Brighton

Division of Built Environment & Civil Engineering

Lewes Road – BN2 4GJ – Brighton, United Kingdom

[email protected]

a

Faculty of Science and Engineering, University of Brighton

Abstract

Double skin facades (DSFs) are gaining momentum in Europe as a low-energy design

technique for the refurbishments of office buildings due to their potential to act as a thermal

buffer in winter and maximise the use of natural ventilation in summer. Existing research

suggests that DSFs are capable of significant reductions to the energy consumption of

buildings as well as improvements to the thermal comfort achieved. However, the design of

a DSF is highly complex due to the parameters and variables involved. By means of the

dynamic energy simulation software IES VE 2013, energy analyses have been undertaken

to evaluate the energy consumption under the variation of different aspects (namely,

shading devices, type of glazing of the outer skin, operable inlets and outlets areas, and

orientation). To assess the risk of overheating in DSFs, energy analyses have always been

coupled with comfort assessments based on the adaptive method suited to the free-running

nature of most of existing buildings. Eventually, the contribution of each aspect towards

both energy saving and overheating has been assessed.

Keywords: double skin facade, energy analysis, thermal comfort, natural ventilation

1 Introduction and research background

According to the European Union [1], buildings account for 40 % of the total energy

consumption in Europe. In this respect, all major governments in Europe have set targets to

reduce building related greenhouse gases (GHG) emissions. Ferreira et al. [2] assessed that,

in meeting GHG targets, existing buildings plays a crucial role and their sustainable

refurbishment is one of the major challenges in the future.

mailto:[email protected]



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At European level, Ebbert and Knaack [3] reported that 65 % of the non-residential

buildings in Western Europe have been constructed before 1978 and present an urgent need

for improvement. In refurbishments, Ardente et al. [4] indicated the improvement of the

envelope as the most beneficial action for energy savings and CO2 emissions reduction. To

adequately manage interactions between the inside and outside of a building, Shameri et al.

[5] stated a double-skin facade (DSF) represents one of the best options.

In renovations, a DSF consists of a second glazed skin added in front of the actual building

facade, from which it is separated by an air cavity that acts as a ventilation channel and may

accommodate shading devices. The benefits of DSFs are many such as providing a thermal

buffer zone, allowing for natural ventilation, solar preheating of indoor ventilation air, night

cooling ventilation, sound-, wind- and pollutant protection (even with open windows),

protection for shading devices, a higher degree of daylighting and an improved aesthetic of

the building they are applied to – see, among others, Streicher et al. [6].

In evaluating the suitability of DSFs for refurbishing offices, Pomponi et al. [7] reviewed

published studies of DSFs performances in temperate climates, showing this technology is

capable of significant reductions in operational energy consumption, with values in the

range of 30 %-60 %. Yet, current research seems focusing on detailed analyses of specific

design parameters (such as optimal position and orientation of shading devices) while a

broader approach to assess the influence of different aspects on the DSF performance is still

missing.

In this respect, this paper reports findings from an ongoing research on the performance of

DSFs for office refurbishments. The aim is to assess the influence of different aspects on

the energy savings achievable and the summer thermal comfort of the examined scenarios.

The focus on indoor comfort is due to the risk of summer overheating which can be

significant in buildings with DSFs (Gratia and De Herde [8]).

2 Methodology

To carry out the study, a location and a representative building model had to be chosen.

Additionally, aspects to be assessed and scenario to be simulated had to be established.

These aspects will now be dealt with in turn.

The location selected for the study is London due to its representativeness of the temperate

climate with mild winters and moderately warm summers. Therefore, a building model has

been developed which represents, in terms of facade characteristics, 20 % of large UK

offices (Figure 1). The building is 100% naturally ventilated.



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Figure 1: View of the building model in IES VE

The hypothesis has been of a facade retrofit where all the thermal elements have been

retained and improved except windows and window frames which have been replaced with

new ones. U-values for the simulations (Table 1) have been adopted from the 2010 edition

of the UK Building Regulations.

Thermal element Thermal transmittance (U-value)

Double Glazed Windows + Frames 1.8 W/m² K

Wall (external insulation) 0.30 W/m² K

Flat roof 0.18 W/m² K

Table 1: U-values used in the simulations

The DSF studied is a multi-storey type with a cavity width of 1m. The multi-storey is one

of the commonest types of DSFs, and it is characterised by the cavity that spans on the

entire building without intermediate divisions (Oesterle et al. [9]). Additionally, inlets and

outlets are generally based only at the bottom and the top of the cavity.

The aspects considered and the different scenarios assessed are reported in Table 2.

Aspects Assessed Scenarios

Shading devices in the cavity Presence/absence and type (venetian/roller blinds)

Value of solar irradiance to lower the

shading devices (SD trigger)

100 – 600 [W/m²] (steps of 100)

Type of glazing of the outer skin Five different types as shown in Table 3

Operable areas of inlets and outlets 10 – 90 % (steps of 10 percentage points)

Orientation of the DSF Only N and S are considered in this study



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Summer overheating Thermal discomfort as per CIBSE [10]

Table 2: Parameter assessed in the dynamic simulations

More specifically, in terms of glazing types, the choice tried to encompass a selection as

diverse as possible in terms of both spectrophotometric parameters and energy parameters

(Table 3).

Gla

zin

g

spectrophotometric

parameters

energy parameters

τν ρν τe ρe αe g SC

(light

transmit.)

(light

reflect.)

(solar direct

transmit.)

(solar

direct reflect.)

(solar

direct absorpt.)

(solar

factor)

(shading

coefficient = g/0.87)

Type #1 86% 8% 72% 8% 20% 0.78 0.90

Type #2 32% 13% 30% 11% 59% 0.44 0.51

Type #3 38% 34% 46% 29% 33% 0.55 0.63

Type #4 80% 11% 65% 9% 26% 0.69 0.79

Type #5 45% 32% 50% 25% 25% 0.56 0.64

Table 3 - Glass types analysed (Terminology as per BS EN 410)

It is worth specifying the operational mode of the bottom opening. In fact, Gratia and De

Herde [8] assessed the best conditions to open the inlets of the cavity in temperate climates

and maximise natural ventilation, which happen when the wind is blowing from the side

opposite to the one with the DSF (Figure 2).

Figure 2: View of the model from the top



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Therefore, the simulations have been set up with a mathematical formula which opens

automatically the inlets of the DSF when the wind is blowing from any direction within the

range indicated in Figure 2. For what concerns the outlets, the formula opens them when

either the outside temperature is higher than 10 °C or the one in the cavity is higher than

20 °C. When both inlets and outlets are closed the buffer behaviour is realised which

corresponds to 0 % of operable areas. Finally, 100 % of operable area is never achieved due

to the presence of frames.

The simulated DSF has an aluminium supporting structure with walkway grills which

obstruct the flow in the cavity. Besides, the roughness of the inner walls introduces an

additional flow resistance. Both these elements have been taken into account by reviewing

the flow coefficient and correcting the equivalent areas for the air to flow (IES VE [11]).

Finally, to assess the overheating the adaptive method has been used which suits the free

running nature of the buildings. The assessment method is given in CIBSE [10] and it is

built on the European Standard EN 15251:2007 but includes more detailed criteria aimed at

assessing severity, frequency, and intensity of overheating. In this paper, internal rooms

which fail the TM52 assessment are classified as uncomfortable and are represented as a

percentage of the total occupied rooms.

3 Results

Preliminary assessment of the shading devices showed they are necessary if overheating is

to be avoided. In this respect, light coloured venetian blinds perform far better than roller

blinds. This depends on the nature of the device which is composed of swivelling slats able

to avoid glare while still allowing daylight which reduces the lighting energy consumption.

The most interesting results from the set of simulations run are presented below. Datasets

have been divided for the North and South cases. In terms of energy consumption the best

and worst cases for each orientation have been identified (Table 4). The gap between the

highest and lowest energy consumption achievable by the manipulation of the parameters

considered has been tagged as 100 %. Therefore, the contribution of each of the aspects

assessed can be presented in terms of percentage (Figure 3). For what concerns comfort, the

average influence of each parameter on overheated rooms along with its maximum and

minimum values have been assessed (Figure 4).



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Orientation

Worst config.

assessed [kWh/m²]

Details

Best config.

assessed [kWh/m²]

Details

Net

difference [kWh/m²]

NORTH 191.2

Glass Type #4

143.1

Glass Type #5

48.1 (=

25.15%)

Inlet: 10% Inlet: 70%

Outlet: 90% Outlet: 10%

SD Trigger: 600

[W/m²]

SD Trigger: 100

[W/m²]

SOUTH 169.7

Glass Type #5

130

Glass Type #1

39.7 (=

23.39%)

Inlet: 10% Inlet: 90%

Outlet: 90% Outlet: 10%

SD Trigger: 100 [W/m²]

SD Trigger: 500 [W/m²]

Table 4: Overview of worst and best configurations assessed

Figure 3: Influence of each parameter on the energy reduction



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Figure 4: Influence of each parameter towards creating or preventing overheating (dots = N, squares = S)

4 Discussion

Figure 3 and 4 show the significant differences existing between a North and South DSF.

The solar irradiance value that lowers the shading devices barely influences the South DSF

both in terms of comfort and energy. In fact, regardless of different settings, a 10 % of

uncomfortable rooms in summer cannot be avoided. In the North case, instead, it influences

both comfort and energy due to the absence of significant solar irradiance. Differences in

glass types are instead influential on the South DSF as the solar and thermal properties of

different glazing are able to reduce both energy consumption and overheating. This results

in a 7 % contribution towards energy reduction, and the possibility to reduce uncomfortable

rooms down to 7 %, which is the lowest discomfort threshold for the South case under

which none of the simulated scenarios ever performed.

However, bottom and top openings are the most interesting and influential parameters. In

fact, they regulate the airflow within the cavity by affecting air temperature, air velocity,

and the possibility to preheat indoor air in winter and naturally ventilate the rooms in

summer. Such a great influence has been investigated in more detail for a specific South

case by analysing and plotting the outside air temperature against average air temperature in

the cavity (Figure 5) and average air velocity in the cavity (Figure 6) for a full year in two

scenarios: bottom fully open (in both cases) and top open 10 % and 90 % respectively.

Figure 5 shows the differences in the outlets do not influence the correlation between cavity

and outside temperatures which is strong in both cases. It additionally clearly shows the

buffer behaviour of the DSF in the cold season. In fact, in both cases, two ‘clouds’ can be

identified out of the regression line. If one looks at 10 °C as outside temperature, it can be

observed the temperature in the cavity is often around 20 °C, with benefits on the energy

consumption due to the reductions of heat losses through the inner skin of the building.



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Figure 5: Two scenarios with different outlets areas in the South DSF

Figure 6: Influence of outlet opening areas on air velocity in the cavity

Figure 6 highlights how a smaller available area for the air to flow significantly increases

the average air velocity throughout the cavity in warm/hot days. This augmented chimney

effect allows to drag more heat out of the cavity and the rooms in summer, thus improving

the indoor comfort. In winter, if the cavity is open due to milder outside temperatures, a

smaller area implies less cold air coming in the cavity. The higher air temperature of the

cavity can be used to heat the rooms or to reduce the heat loss through the inner skin if the

internal windows are closed. Either way, this in turn reduces the energy consumption.

5 Conclusions

Existing buildings account for a significant amount of energy consumption and also often

have poor indoor thermal comfort. For these reasons, many of them will be refurbished in

the coming years. Interventions on the building facade are often the most effective way to

Inlet 90% - Outlet 10% Inlet 90% - Outlet 90%

Inlet 90% - Outlet 10% Inlet 90% - Outlet 90%



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reduce energy consumption and enhance indoor comfort. In this respect, DSFs are

increasingly gaining momentum in Europe as a low-energy design technique. Knowledge

on the influence of different DSF design parameters on energy consumption and

overheating phenomena is still in its infancy. This paper represents a first attempt to fill this

gap. Energy analyses have been coupled with comfort assessment to address the

contribution that each of the aspects considered has towards energy savings and

overheating. Results revealed that inlets and outlets of the cavity appear to be the most

influential parameters among those assessed for what concerns both energy consumption

and indoor comfort. Not all the possible combinations of the aspects considered have been

analysed and this represents a limitation of the present study. Further research should

complete such analysis and address a broader number of parameters, like - for instance - the

cavity width or the type of supporting structure of the DSF, which may affect the airflow.

6 References

[1] EU, Directive 2010/31/EU of the European Parliament and of the Council of 19 May

2010 on the energy performance of buildings (recast), in, Official Journal of the

European Union, 2010, pp. L153/113-L153/135.

[2] J. Ferreira, M.D. Pinheiro, J.d. Brito, Refurbishment decision support tools review—

Energy and life cycle as key aspects to sustainable refurbishment projects, Energy

Policy, 62 (0) (2013) 1453-1460.

[3] T. Ebbert, U. Knaack, Office refusbishments by means of facades with integrated

building services, in: Proceedings of Building Stock Activation, Tokyo, Japan, 5-7

November 2007, 2007.

[4] F. Ardente, M. Beccali, M. Cellura, M. Mistretta, Energy and environmental benefits

in public buildings as a result of retrofit actions, Renewable and Sustainable Energy

Reviews, 15 (1) (2011) 460-470.

[5] M.A. Shameri, M.A. Alghoul, K. Sopian, M.F.M. Zain, O. Elayeb, Perspectives of

double skin facade systems in buildings and energy saving, Renewable & Sustainable

Energy Reviews, 15 (3) (2011) 1468-1475.

[6] W. Streicher, R. Heimrath, H. Hengsberger, T. Mach, R. Waldner, G. Flamant, X.

Loncour, G. Guarracino, H. Erhorn, H. Erhorn-Kluttig, M. Santamouris, I. Farou, S.

Zerefos, M. Assimakopoulos, R. Duarte, Å. Blomsterberg, L. Sjöberg, C. Blomquist,

On the Typology, Costs, Energy Performance, Environmental Quality and

Operational Characteristics of Double Skin Facades in European Buildings, Advances

in Building Energy Research, 1 (1) (2007) 1-28.



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[7] F. Pomponi, K. Ip, P. Piroozfar, Assessment of Double Skin Facade Technologies for

Office Refurbishments in the United Kingdom, in: A. Passer, K. Höfler, P. Maydl

(Eds.) Sustainable Buildings – Construction Products & Technologies, Graz, Austria,

2013, pp. 1098-1109.

[8] E. Gratia, A. De Herde, Natural ventilation in a double-skin facade, Energy and

Buildings, 36 (2) (2004).

[9] E. Oesterle, R. Lieb, M. Lutz, W. Heusler, Double-Skin Facades. Integrated

Planning, Prestel, Munich - London - New York, 2001.

[10] CIBSE, TM 52: 2013 - The limits of thermal comfort: avoiding overheating in

European buildings, Great Britain, 2013.

[11] IES, MacroFlo Calculation Methods. Technical Manual, 2012.

Energy and comfort analysis of glazed double skin facades for refurbishments in temperate climates

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