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A critical review of the energy savings and

cost payback issues of double facades

David StriblingBuro Happold Glasgow

Byron StiggeBuro Happold New York

Summary

This paper presents a dynamic thermal modelling study to show estimated energy savings and

projected payback periods for various double faade construction systems in different climates and

orientations. It concludes that although the economic case for a double faade based purely on energy

cost savings may be marginal, many other factors must be considered. Climate, construction type,

construction cost, and energy cost significantly contribute to the feasibility of each unique case and

must be assessed for each building.

Introduction

Double facades are an effective means of buffering and controlling heat, light, air and noise through a

building envelope. They do, however, have a premium cost associated with them compared to

conventional facade systems. Justification of their inclusion in a building design, therefore is typically

on the basis of energy efficiency and associated cost savings. Qualitative benefits of solar control,

moderated surface temperatures, noise reduction, reduced glare, reduced heating/cooling demand,

moderated access to fresh air, aesthetic purity and increased daylighting are generally seen only as

intangible bonus benefits.

The principle of a double skin faade is an additional layer of glass offset from the conventional

curtain wall forming an interstitial space that acts as a thermal buffer. Blinds are typically incorporated

into the void space to prevent solar heat gains from entering the occupied space. Blinds may be

automatically or manually operated. On the outer surface of glazing, operable vents are located top and

bottom to prevent the void from overheating in the summer. In the winter the vents are generally

closed to trap heat in the void and reduce heat loss through the interior windows. In the mid-season


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condition, the inner curtain wall normally has

operable windows for natural ventilation. An

additional benefit is that modulating outer vents

can be used to control the void temperature and

thus extend the period suitable for natural

ventilation.

Despite this sophistication, detailed energy

savings analyses rarely conclude double skin

facades have affordable paybacks at todays

relatively low energy prices although there is no

doubt that they do provide better overall energy

efficiency. The aim of this paper is to

investigate which scenarios may approach

affordable payback periods and to what extent

these are influenced by different factorsFigure 1 Typical double faade components

To this end, dynamic thermal modelling (DTM) has been applied to the problem using the TAS

software from EDSL (1). DTM techniques employ a three dimensional virtual model of the building

and calculate the time based thermal loads on a building based on its fabric, solar shading and

historical weather data. It has been used previously in the application of double faade analysis and

validated against detailed CFD analysis in this respect (2).

Faade Construction Costs

A double skinned faade is a significant capital investment and an estimate of construction cost is

necessary in order to make a calculation as to the likely payback period. There are wide variations

depending on the level of sophistication of the faade, location, construction method and contractors

experience. Many suppliers give ranges because of these factors but the figures assumed for the Las

Vegas, USA analysis have been 300/m2($50/ft

2) for a conventional curtain wall faade with low-e

glass and 800/m2($130/ft2) for a simple, flat, double faade with operable vents, blinds and windows

for a large building and manufactured in a factory. These figures are estimates rather than quotes and

are based on experience and conversations with suppliers for the mid west US. As an example of


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regional variations, the same faade installed in New York might cost 50% more, in the UK 40% more

and in Germany 20% more.

Orientation

A basic office building model was

simulated with the intent of isolating the

annual HVAC energy savings of a double

faade over a conventional faade for

various orientations. In order to do this a

model of a four-storey office block was

created with solid walls on three sides. The

fourth side was a fully glazed faade which

was rotated through 8 different orientations

with annual energy consumption calculated

for each and for three different climates. Figures 2, 3 and 4 summarise the model geometry whilst the

other modelling assumptions are detailed in Appendix A. London was selected as a mild, cloudy

climate (Figures 5 and 6). Las Vegas was selected as a hot, dry sunny climate (Figures 7 and 8). And

Winnipeg, Canada was selected as a cold climate (Figures 9 and 10).

Figure 2 - Plan diagram of simulation model

investigating effect of orientation on

energy savings of double facade

It should be noted that these results are not representative of actual office energy consumption figures

due to the simplistic nature of the model. The results are intended purely as a comparison between the

energy savings associated with each orientation for one design of double facade.

Figure 3 - Section diagram of simulation

model for baseline office

building without a double

facade

Figure 4 - Section diagram of simulation

model for office building with a

double facade


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0

30

60

90

120

DF

Base

DF

Base

DF

Base

DF

Base

DF

Base

DF

Base

DF

Base

DF

Base

N NE E SE S SW W NW

EnergyConsumption

(kWh/m2)

Heating Fans

Cooling Pumps

Figure 5 LONDON: Energy consumption intensity results for 8 orientations for conventional

faade (Base) and double faade (DF)

0%

10%

20%

30%

N NE E SE S SW W NW N

Orientation

HVACEnergySavin

gs

Percentage cost saving

Percentage energy saving

Figure 6 LONDON: Percentage energy savings through faade for different orientations

For London, the most significant savings in HVAC energy consumption are achieved with SW and S

facades and are in the order of 23%. This is because the void captures solar gain in the winter

providing an insulating layer and rejects solar gain in the summer. Other orientations have less solar

gain and thus heat up less in the winter and block less solar gain in the summer. Nevertheless, even

the N faade shows 14% savings in energy consumption due to a greatly increased overall U-value.


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0

50

100

150

200

DF

Base

DF

Base

DF

Base

DF

Base

DF

Base

DF

Base

DF

Base

DF

Base

N NE E SE S SW W NW

EnergyConsumption

(kWh/m2)

Heating FansCooling Pumps

Figure 7 LAS VEGAS: Energy consumption intensity results for 8 orientations for

conventional faade (Base) and double faade (DF)

0%

10%

20%

30%


Orientation

HVACEner

gySavings



Figure 8 LAS VEGAS: Percentage energy savings through faade for different orientations

Las Vegas sees more energy savings from a double faade (in the region of 27%) because it is a sunny,

hot climate and double facades do well to reduce cooling loads from solar gain.


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Figure 9 WINNIPEG: Energy consumption intensity results for 8 orientations for conventional

faade (Base) and double faade (DF)

Cooling Pumps

0

100

200

300

DF

Base

DF

Base

DF

Base

DF

Base

DF

Base

DF

Base

DF

Base

DF

Base

N NE E SE S SW W NW

EnergyConsumption

(kWh/m2)

Heating Fans

0%

10%

20%

30%


Orientation

HVACEnergySavings



Figure 10 WINNIPEG: Percentage energy savings through faade for different orientations

Winnipeg shows higher annual energy consumption than both London and Las Vegas with

predominantly a heating load due to the cold climate. Energy savings shown are best on the South

Eastern side and are quite modest at 12%. This suggests one or a combination of two things. That the

design of faade selected in this analysis works more efficiently in a warm climate with high cooling

load than it does in a cold climate and/or the fact that because solar heat gains are low in this climate,

that the low-e faade is already more efficient than it is in Las Vegas or London.


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Optimal Climate

In order to determine what types of climate achieve the

best energy savings through the use of double facades

another model was simulated which was more

representative of a typical office building. Using a

South-West orientation, which was generally accepted as

offering the most benefit from a double faade in the

previous analysis, the building was simulated through 7

cities in different climate zones throughout the world.

The model geometry is summarised in figures 11, 12 &

13.

Figure 11:Plan diagram of simulation

model investigating energysavings of double faade in

various climates

Figure 13:Section diagram ofsimulation model for

office building with a

Figure 12:Section diagram of simulationmodel for baseline office building

without a double facade


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0

50

100

150

200

250

300

LE DF LE DF LE DF LE DF LE DF LE DF LE DF

En

ergyConsumption(kWh/m2)

Fans

Heating Cooling Pumps

Winnipeg

Munich

New York MiamiLas Vegas

Rome

London

Figure 14 Energy consumption comparison for a conventional faade (LE) and a double faade

(DF) for different locations and climates

The results of the analysis are shown in Figure 14 with an indication of the predicted energy

consumption for the double faade versus a conventional double glazed faade for different global

locations. Those generally with the greater energy consumption show the most potential for savings. It

should also be noted that the same faade design has been used in all locations when in fact there may

well be a different design for a warm climate as opposed to a cooler climate.

Payback Calculation

Although double facades are meant to save energy and hence be more environmentally friendly, it is

energy cost savings and payback period that building owners are typically most interested to know.

For this calculation it was necessary to make assumptions on the prices of gas and electricity for each

of the locations considered. The figures used in the analysis are given in table 1.

The simple payback calculations in this paper are based on HVAC energy saving cost savings

assuming the same type of air conditioning system for the typical and double faade.


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Table 1 Energy prices assumed in payback analysis

Electricity Gas

(/kWh) ($/kWh) (/kWh) ($/kWh) Off Peak Rate

United Kingdom 0.07 $0.105 0.025 $0.037Electricity0.04/kWh

12am and 7am

USA (Nevada) 0.04 $0.06 0.013 $0.02

USA (New York) 0.133 $0.20 0.047 $0.07

USA (Florida) 0.053 $0.08 0.017 $0.028

Canada 0.038 $0.055 0.012 $0.018

Italy 0.085 $0.13 0.028 $0.042

Germany 0.09 $0.14 0.03 $0.045

Table 2 summarises the energy cost payback for the 7 cities modeled and shows that the ultimate

feasibility is largely dependent on energy price. The more expensive energy locations such as New

York, Rome and Munich show the best paybacks with the lower energy prices of Canada extending

the payback period significantly. London shows a longer payback period partly due to a lower energy

price, partly due to a less extreme climate and partly due to more expensive construction costs

Table 2 Comparison of payback periods for double facades in different locations

London Las Vegas Winnipeg New York Miami Rome Munich

Faade area 1,040m2 1,040m2 1,040m2 1,040m2 1,040m2 1,040m2 1,040m2

Add. faadecost

700/m2 500/m2 500/m2 800/m2 600/m2 650/m2 600/m2

Add. capitalinvestment

728k 520k 520k 832k 624k 676k 624k

Energy costsaving

1.01/m2 1.31/m2 0.83/m2 2.57/m2 1.17/m2 2.18/m2 1.88/m2

Floor area 3,000m2 3,000m2 3,000m2 3,000m2 3,000m2 3,000m2 3,000m2

Annual costsaving

3,030 3,930 2,490 7,710 3,510 6,540 5,640

Payback

period

240 Yrs 132 Yrs 208 Yrs 108 Yrs 177 Yrs 103 Yrs 111 Yrs


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From the results in table 2 one conclusion that applies to all locations is HVAC running costs are very

low compared to the capital cost of the double facade. With an expected building life of 50 to 70 years

these projected paybacks would make double facades seem to be a financially poor decision. There are

three things that control this, the cost of construction, the energy savings, and the energy price. The

first two of these can be considered in the design and although the third is largely a factor of market

forces. In the following section an analysis is carried out for double faades located in Las Vegas and

London taking these factors into consideration.

Optimal Faade Design & Cost

Previous analysis has shown that the projected payback period for the considered design in Las Vegas

is of the order of 132 years. Through simulation however, the baseline double faade design has been

optimised to achieve 95 years. This has been achieved through optimizing the solar shading properties

and control of the blinds and the temperature setpoints of the automatically opening vents and

windows. The next challenge, therefore, is to reduce the construction cost of the faade and

functionality to optimise the trade-off between energy savings and construction cost.

Additional features such as BMS controlled vents, BMS controlled blinds, daylight dimming, BMS

controlled operable windows, shut-off dampers to air supply when windows are open, low-iron glass

and an occupiable (wider) void all contribute to energy savings. But all of these features contribute to

additional construction cost, as demonstrated in figures 15 & 16. For a given climate and orientation a

different set of these functionalities will produce an optimal payback period. This principle is

demonstrated in theory by figure 17.

Figure 15: Capital investment for a double

facade per faade area vs.

functionality features of faade

Figure 16: % HVAC Energy Savings

for a double facade vs.

functionality features of

faade


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Figure 17: Payback for a double facades vs.

functionality features of faade

Five models with various double faade systems were run for Las Vegas to demonstrate this theory.

Model description and capital cost assumptions are summarised in table 3.

Table 3 Summary of simulations to investigate effect of faade design

Model Description Additional capital costover typical faade

1 Fully controllable faade optimised to give 95 year payback 600/m2 $90/ft2

2 Model 1, but changing motorized vents to permanently open vents 500/m2 $75/ft2

3Model 2, but changing blinds to static tilted blinds with solartransmittance = 0.6

400/m2 $60/ft

2

4Model 3, but changing operable windows on internal faade tofixed, closed windows

350/m2 $53/ft2

5

Model 4, but a full height, four storey double faade with inlet onlyat the bottom of the first floor and outlet only at the top of thefourth floor

300/m2 $45/ft2


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0.00

1.00

2.00

3.00

300 400 500 600

Additional Investment (/m2 - facade)

Energy

Cos

tS

av

ing

(/m2-

floor)

0

50

100

150

200

Simp

lePay

bac

kPerio

d

(Years)

Energy cost saving

Payback

Figure 18: HVAC Energy Cost Savings and payback

period for a double facade with different

features

The results of the analysis are shown in figure 18. Here, the characteristic U shape described in

figure 17 is apparent. At the lower point of the U shape, the payback period does, in fact, reach an

optimum at an additional investment of 500/m2($75/ft2) over a conventional double glazed low-e

curtain wall system, achieving an 86 year payback. Note this is purely an analytical analysis for the

given assumptions for Las Vegas. The optimal model had permanently open vents, which is

reasonable for such a hot climate, but would not be reasonable for a cold climate where closing thevents is very important.

Energy Cost Influences Payback

Energy costs influence payback as seen clearly back in table 2. But energy costs vary greatly across

the world and with time making it a moving target, so it is important to look at historic price trends

and future price predictions for perspective on payback calculations. Simple payback calculations

done with todays energy price will not be accurate for very long.


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$0.00

$0.05

$0.10

$0.15

$0.20

$0.25

$0.30

1967 1977 1987 1997 2007 2017 2027 2037

EnergyPrice($/kW

h) Ave. Electric ity Price (in 1996$)

Ave. Gas Price (in 1996$)

Weighted Energy Price (82% elec,18% gas)

20 yr payback

40 yr payback

60 yr payback

Figure 19: Energy cost for Nevada 1960-2001 (in constant

1996 dollars) compared to threshold values for

target payback periods.

For the optimum Las Vegas design (calculated above to have an 86 year payback at todays energy

prices) the energy split is 12% gas, 88% electricity due to the predominance of cooling in this climate.

A weighted average energy price for the model building has therefore been calculated and plotted on

the graph in figure 19 together with threshold prices that would achieve 20, 40 and 60 year simple

paybacks. This analysis shows that the historic gas and electricity prices for Las Vegas, Nevada (4) in

constant 1996 dollars would provide between 50 and 90 year simple paybacks.

If energy prices reach $0.27/kWh (0.40/kWh) for electricity and $0.09/kWh (0.135/kWh,

$2.60/therm) for gas the payback would be 20 years. These energy prices are close to that of New

York City today.


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In a similar exercise, the faade design for London was optimised through simulation to give an 90

year payback at todays prices of 0.07/kWh ($0.105/kWh) for electricity and 0.025/kWh

($0.04/kWh) for gas. The energy split for the modelled building in London is 59% gas, 41% electricity

resulting in a weighted, current energy price of 0.044/kWh ($0.066/kWh). Figure 20 shows the trend

of the energy price for the UK normalised to real 1995 prices (5) together with the thresholds for 20,40

& 60 year paybacks.

0.00

0.05

0.10

0.15

0.20

0.25

1970 1980 1990 2000 2010 2020 2030

EnergyPrice(/kWh) Ave. Electricity Price (in 1995)

Ave. Gas Price (in 1995)

Weighted Energy Price (41% elec, 59% gas)

40 yr payback

20 yr payback

60 yr payback

Figure 20: Energy cost for UK 1970-2001 (in constant

1995 sterling) compared to threshold values

for target payback periods.

The graph shows a similar trend to the US with higher energy prices in the early eighties making the

case for double facades more feasible. During this period, the projected payback approached 40 years.


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Other Important Influences on Cost Payback

Due to the space restrictions of this paper, other very important influences on total life-cycle payback

of double facades cannot be fully elaborated on, but are listed below and expanded on in many

references on double facades (3)(6). Many are soft issues which improve worker productivity, by far

the largest portion of an office buildings life-cycle cost. Little quantifiable research has been done

on these topics and this is an area in great need for future research.

Additional Maintenance Costs 4 window surfaces to clean instead of 2, motor replacement, louvre

maintenance, etc.

Reduced Mechanical Plant Capital Costs peak load reductions allow smaller chillers, boilers, air

handlers, ducts, or even different HVAC systems altogether.

Glare Control operable blinds block direct solar glare and accept diffuse light

Moderated Glass Surface Temperatures blinds block direct solar rays from striking the inner glass

preventing it from heating to upwards of 60C (140F). In the winter, the warm void heats the

inner glass reducing drafts and cold radiant exchange.

Operable Windows in High-Rise Buildings the void buffers wind pressures which otherwise make

operable windows very gusty and disruptive in tall buildings.

Acoustical Buffering the vents and void dampen noise improving acoustics near noisy roads,

airports, factories or rail lines.

Increased Daylighting operable blinds actively bounce light deeper into occupied space. Improved

U-value allows larger windows.

Reduced Emissions greenhouse gases, SOx, NOx and other particulates are reduced as energy

consumption is reduced.

Aesthetic Purity the exterior rain-screen requires no thermal breaks, structural mullions or spandrel

glass providing a visually simple faade. Blocked UV, wind and rain allow a wood framed

interior curtain wall. External blinds provide shading, so clear glass is acceptable.

Conclusions

An extensive analytical investigation has been carried out into the cost versus payback benefits of

double skin facades with respect to their energy saving potential. From this work the following

conclusions can be drawn.

A double faade offers the most energy saving potential on the south and south-west orientations.


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Extreme climates offer more opportunity for energy savings as they require more HVAC energy

and thus have greater potential for savings through improved building envelope.

Energy savings can range from 10% to 50% of HVAC energy, and cost payback can range from

30 to 200 years based on todays local energy prices. Double Facades must be assessed

specifically on their individual merit considering climate, orientation, detailing, construction cost

and energy price.

The economic viability of double faades in a location is not only a result of the climate.

Construction costs and energy price play an even more significant role in the results as there is a

large global variation.

Energy prices will also vary greatly over the life of the building and this should also be

considered. Although todays climate of low energy prices prohibit significant energy cost

savings, one might consider designing todays building for tomorrows energy prices.

The additional benefits of double facades have not been fully explored in this paper an at present

many of these are unquantified. Considering their importance in making a financial case for

double facades, further research is recommended in this area.

References

1 Tas Software Manual, EDSL (Environmental Design Solutions Ltd, UK), 1998

2 F. Wang, M. Davies, B. Cunliffe, P. Heath, The design of double skin faade: modellingstudy on some design parameters affecting indoor thermal conditions, CIBSE NationalConference, 1999

3 Oesterle, Lieb, Lutz, Heusler, Double-Skin Facades: Integrated Planning, Prestel, Munich,2001

4 Energy Information Administration, EIA, Web Page:http://www.eia.doe.gov/neic/historic/historic.htm

5 Department of Trade and Industry "UK Energy Sector Indicators 2001", Web Page:http://www.dti.gov.uk/energy/inform/energy_indicators/2001/

6 D. Arons Properties and Applications of Double-Skin Building Facades, MSc Thesis,Massachusetts Institute of Technology, June 2000
http://www.eia.doe.gov/neic/historic/historic.htmhttp://www.dti.gov.uk/energy/inform/energy_indicators/2001/http://www.dti.gov.uk/energy/inform/energy_indicators/2001/http://www.eia.doe.gov/neic/historic/historic.htm


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Appendix A Modelling Assumptions

Orientation Model

This model was 10m (30ft) deep and 50m (150ft) wide, four stories tall with solid walls and no

glazing on three faces. On the fourth face, the faade was applied as described below.

The baseline building has 70% glazing on facade side (U-value = 1.8 W/m2K, R-3).

The double faade building had a double faade on the glazed side with a 1m (3ft) wide void,

motorized blinds with total solar transmittance of 0.6, operable lower and upper vents 300mm (1ft)

high which are open in the summer months when the void temperature reaches 24C (75F), clear

10mm single glazing on the outer facade, and insulated low-e inner glass (U-value = 1.8 W/m2K, R-3)

on the inner facade. The double faade model had operable windows and are open for natural

ventilation when the void is between 16-22C (61-72F).

Internal Heat Gains

Occupancy - 10W/m2(sensible), 6W/m2(latent) between 8am to 6pm, 7 days per week

Lighting 15W/m2

Equipment 8W/m2

Plant switched on 8am to 6pm heating 20C, cooling to 24C

Night setback heated to 18C in winter

Optimal Climate Model

This model was 15m (45ft) deep and 50m (150ft) wide, four stories tall at the optimal orientation of

SW. The baseline building has 40% glazing on all four sides (U-value = 1.8 W/m2K, R-3) which does

not have operable windows. The double faade building has a double faade on the southwest and

southeast facades. The double faade has a 1m (3ft) wide void, motorized blinds, motorized vents at

500mm width inlet and outlet (0.5ft2/ft), clear single glazed outer glass, and insulated low-e inner glass

(U-value = 1.8 W/m2K, R-3) and operable windows. All internal heat gains and plant times were the

same as described above for the orientation model.

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8 c Stribling