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Images Courtesy of Skidmore Owings and Merrill LLP, Chicago

The following article was published in ASHRAE Journal, April 2009. ©Copyright 2009 American Society of Heating, Refrigerating and AirConditioning Engineers, Inc. It is presented for educational purposes only. This article may not be copied and/or distributed electronically or in paper form without permission of ASHRAE.

Figure 1: Pearl River Tower, Guangzhou, China, is a 984 ft (300 m) commercial tower with a net-zero energy design goal. It has mechanically ventilated double skin façades; underfloor air distribution; overhead radiant cooling; wind and solar renewable energy; and more.

Ventilating Façades 16

ASHRAE Journal

a s h r a e . o r g

April 2009

By Ray Sinclair, Ph.D., Member ASHRAE; Duncan Phillips, Ph.D.,P.E., Associate Member ASHRAE; and Vadim Mezhibovski, Ph.D.

F

or tall buildings, architects often prefer the envelope to be deliberations must be informed by energy and environmental covered with a high percentage of relatively clear glass goals for the project, heating, overall building cooling and (Figure 1). This choice can challenge goals of achieving ventilation configurations, daylighting, aesthetics, code requireenergy efficiency, thermal comfort and visual comfort within ments, and wind loads along with capital, maintenance and the building around the perimeter. cleaning cost expectations. This section discusses some of the Optimal energy performance requires an appropriate balance configuration issues in designing a DSF. of opaque walls and glazing. High performing glazing choices include low-e, argon-filled double-glazed units; triple-glazed Cavity Height units; glazing with a spectrally selective coating; glazing conThe DSF cavity height may be a single story, several stories, figurations with external or internal blinds; and the double skin or the height of the whole building. Although a multistory DSF façade (DSF), which combines a number of these features. has an advantage in a naturally ventilated cavity due to increased The DSF, also known as a ventilated façade or active curtain- stack-effect induced airflow rates, a single story laterally subwall, is an alternative when design constraints exist such as a divided cavity (horizontal and vertical compartmentalization) high percentage of glazing area; no external shades; transpar- may be a preferred option taking into account fire, smoke, odors, ent glass choices; or a façade material that requires protection and noise separation considerations. The risk of a tall multistory from the elements. single-cavity DSF is that unacceptably high air temperatures can DSFs are frequently associated with sustainable building. occur at the top of the cavity (assuming upward flow) and this The premise is that they may permit greater daylighting without can cause relatively high gains to the occupied space. compromising thermal performance of a building’s envelope. These façades have increased design, construction, and maintenance costs, The double skin façade, also known as a ventilated façade so their use is more popular in places where more stringent energy codes exist (e.g., Europe). However, probor active curtainwall, is an alternative when design conlems can occur in some climates if the façade is not designed well. A design straints exist such as a high percentage of glazing area; strategy that works in one climate (e.g., temperate and overcast regions of Europe) may not be directly applicable no external shades; transparent glass choices; or a façade to others. The DSF most often has two glazmaterial that requires protection from the elements. ing units with solar control devices between (Figure 2). This cavity is ventilated to extract the heat gain. Four ventilation strategies are illustrated in Figure 3. An engineer- Cavity Depth The depth of the cavity is determined by a number of paraming analysis must predict thermal performance under a full range of climatic conditions and include coupling to energy- eters including the aesthetics, types of shading devices/blinds, efficient ventilation and cooling strategies at the perimeter of access to the cavity for cleaning, and the ventilation strategy including arrangement and flow rates. the building. This article identifies some important aspects of how DSFs are implemented, potential benefits and risks, and what param- Cavity Width Lateral separation of the cavity between occupied spaces eters can be used to optimize their performance. Results from detailed engineering analyses are presented to demonstrate the is beneficial for cavity ventilation. It is also helpful for noise benefits of accurate determination of gains that affect thermal control if windows from the occupied spaces open into the comfort and loads that affect equipment sizing. It is vital that cavity. In some cases, lateral separation can lead to more this improved envelope modeling feeds into whole-building- stable airflow within the cavity, leading to reduced risk of local hotspots. simulation models. Configurations

About the Authors

Evaluate the goals, benefits, and risks of incorporating a DSF with the full design team. The design of the DSF involves decisions on geometric parameters, glass selection, ventilation strategy, shading, and passive or active control strategies. These

Ray Sinclair, Ph.D., is a vice president and consultant in building science. Duncan Phillips, Ph.D., P.E., is a senior consultant in integrated building design. Vadim Mezhibovski, Ph.D., is an energy/HVAC specialist. The authors work for Rowan Williams Davies & Irwin Inc. (RWDI), Guelph, ON, Canada.

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Mechanically Ventilated Airflow

Outer Glazing

Naturally Ventilated

Inner Glazing

Troom = T7

Room

T6

Ambient

T4

T5 Room

Tambient = T1

T3

Ambient

T2

Shading Device

Mechanically Ventilated Supply

Ventilated Exhaust

Room

Ambient

Room

Hotspots or hot regions within the cavity can occur in wide cavities in which the vertical airflow does not remain uniform across the cavity width. Minor asymmetries in geometry, solar gains, and minor pressure differences (perhaps owing to wind, for example) can cause the low-speed airflow to “slosh” to one side of the cavity, allowing one or more zones to be poorly ventilated. This results in higher surface temperatures. These flow features can be unstable, making them difficult to accurately predict.

Ambient

Figure 2: Example of a double skin façade configuration.

Figure 3: Internal and external ventilation configurations of DSFs. Cavity Ventilation

The cavity can be ventilated either naturally or mechanically. Ventilation rates must be sufficient to limit high temperatures associated with solar gains but not so high that the fan energy costs impact total building energy use. The optimal ventilation rate is the lowest rate where building energy consumption is minimized, and inside glass temperatures are as close to the inside dry bulb as possible. The latter will provide comfort conditions for occupants sitting adjacent to the glass. If the cavity is naturally ventilated, the local dust, humidity, and pollution conditions must be understood. Although the fan energy costs associated with naturally ventilating the cavity will be low, the increased maintenance may eliminate any potential benefit associated with this. In many cases, the relatively warm cavity air is discharged to the atmosphere. In some cases with a mechanically ventilated cavity, the warm cavity air is used for heat recovery or potentially drying a desiccant wheel. For optimal energy performance, the cavity airflow rates should vary with time of day, as well as external and internal conditions. For example, in hot climates, when air is drawn mechanically from the interior building space it may be advantageous to draw proportionately larger flow from the side of the building with the greatest solar exposure. The air drawn into the cavity should be of return-air quality either warmed by interior loads or well mixed. Care should be taken that supply air does not short circuit to the cavity intakes. 18

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Glazing Selection

The glazing system design for a DSF depends on the climatic conditions of the project site, preferred ventilation and blind operating modes, and internal space requirements. A façade ventilated with outdoor air usually has the insulating glazing unit (IGU) at the interior side, as a thermal break, and a noninsulating (single pane) at the exterior side. A façade ventilated with indoor air usually has the IGU (double) at the exterior side and a single pane at the interior side. Environmental Factors Affecting Design

The local climate of the building site and the chosen orientation of the DSF are important. The frequency and duration of occurrence of combined conditions of particular wind speeds, wind directions, temperature, humidity, cloud cover, and levels of direct and diffuse solar radiation must be factored into design decisions. There is a need to agree on the extremity of design day conditions and acceptance of less optimal performance for limited periods to optimize capital costs. Condensation risks, dust ingress, exterior noise, and other local influences often result in an incompatibility of certain design options. Wind pressures on the building due to site location, building shape and surroundings, and thermal stack effect of the building and the cavity itself, can affect cavity ventilation and building infiltration/exfiltration. In hot, humid climates, for example, these factors can affect the DSF design and the cooling strategy a s h r a e . o r g

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Risks of a Double Skin Façade

While a DSF can provide several benefits, it is possible that a DSF that is not designed and operated correctly may cause problems in the operation of the building, or provide no benefit at all despite the additional cost: •• Poor design can lead to excess thermal gains to the occupied spaces and building overall. This can result because the choice of glazing, shading device(s), and control strategy is incorrect or the ventilation rate is too low. Another result can be poor visual comfort resulting in blinds being lowered and artificial lights turned on. The combination of these factors can also conspire against the building’s performance. •• Noise, odors, or smoke from potential fire can be transported through the cavity system if there are lateral or vertical connections between façade segments. These contaminants also can be transported through the DSF system due to poor construction practices. •• Depending on the climate, it is possible for condensation to form on any or all surfaces in the DSF during particular meteorological conditions and time of day. 20

ASHRAE Journal

Glazing Blinds

Blinds LW

Diffuse Solar

LW

Convection

Convection

Outdoor

Convection

LW

Convection

A high performance façade, as can be achieved with a properly designed DSF, can realize the following additional benefits: •• Reduced mechanical plant capital costs. If a sufficiently accurate engineering analysis is performed, predicted peak loads may be lower than those for a traditional façade given the architecture (i.e., same glass area and aesthetic requirements). This allows selection of smaller HVAC equipment. •• If a reduced load is achieved and incorporated with radiant cooling and efficient supply air delivery, such as underfloor or displacement ventilation, then floor-tofloor heights may be reduced. This can lower the building capital cost or provide additional floors of leasable space in high rise towers, for example. The design of the Pearl River Tower in Guangzhou, China (Figure 1), reduced the floor-to-floor height by approximately 1 ft (0.3 m). With more than 70 floors, five additional floors of leasable space were added. This meant that the payback calculation favored the integrated design that incorporated the DSF. •• Energy savings resulted from reduced cooling demand from perimeter sensible gains and effective daylighting leading to reduced lighting requirements. •• Increased occupant thermal comfort adjacent to the façade compared to that of a traditional façade of equivalent glass area and transparency. •• Improved aesthetic of a more visually transparent façade. •• Improved sound insulation from exterior sources as a result of the third pane of glass.

Glazing

Convection

Potential Benefits of a High Performance Façade

Incident Direct Solar

Convection

that may be used in the perimeter occupied spaces (i.e., via supply air or radiant cooling).

Indoor

LW

Conduction

Conduction Airflow (Natural or Mechanical)

Figure 4: Paths of heat transfer in a double skin façade illustrating direct and diffuse solar radiation, conduction, convection, and long-wave radiation.

•• If the façade is open to outdoors, the surfaces within the cavity will gather dust and require cleaning. Coupled with condensation risks, the cavity could become soiled. This adds to the ongoing maintenance costs and aesthetic concerns. As well, access to the cavity for cleaning must be included in the design. Appropriate seals must be in place around the access panels to minimize exfiltration and infiltration issues. Requirements of Performance Analysis

The thermal performance of a DSF depends on many design variables. This section discusses the physics involved. A schematic of the heat and air transport mechanisms is presented in Figure 4. The complexity of the physics, coupled with the uncertainties in many parameters (e.g., heat transfer coefficients), highlights why modeling DSFs is difficult and why some approaches show mixed results. Conduction must be taken into consideration in solids, describing heat transfer through glass, concrete, wood, or other materials forming the inner or outer DSF construction. Care must be taken not to merely use the manufacturers’ stated conductance “U-values” directly, as is often done in simpler modeling approaches. Conductivity and material thicknesses must be applied directly in conduction calculations. The heat transfer between a solid surface and air is a function of both forced and natural convection processes. This convective a s h r a e . o r g

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Solar Transmissivity

Solar Reflectivity

0.73

0.07

0.29

0.38

Gain (W/m2)

heat transfer is predicted by Solar specifying convection coefThickness Heat Gain U-Value ficients that are dependent on Coefficient   air velocities for each solid-air inch (m) Btu/h · ft2 · °F (W/m2 · K) interface. These air speeds are location dependent and Inner fluctuating. 3/ (0.010) 0.79 0.99 (5.6) 8 Skin Radiation heat transfer occurs between two different Outer 1 (0.025) 0.33 0.31 (1.7) components. Solar energy exSkin ists at short wavelengths while heat transfer between room Table 1: Glazing properties for Pearl River Tower. surfaces (i.e., floor, walls, and windows) occurs at long-wavelength bands. Solar radiation 1,000 must be considered in its direct and diffuse components. Direct solar is most affected by cloud cover and also by atmospheric humidity and air pollution. Diffuse solar radiation is affected 800 by atmospheric conditions and site surroundings that influence the atmospheric and ground reflective components, respectively. Both are impacted by building orientation. 600 The design of a DSF involves comparison of model results for several design parameters over a range of atmospheric conditions, typically a design year of hourly meteorological data. 400 Plots of joint probabilities highlight frequency of occurrence of various coincident events. For example, this meteorological data analysis could define the characteristics of a “hot clear 200 day,” the time, and duration of peak conditions. This can inform design decisions of DSF configuration and also operation and 0 control strategies. 0

4

8

Sample Results of Performance Analysis

Figures 5 to 8 present predicted performance data for one of the analyzed configurations of the south-facing DSF of the Pearl River Tower located in Guangzhou, China, at approximately 23° N latitude. These figures show a collection of different types of assessment measures, using mainly custom purpose modeling tools and a commercial CFD package. The analysis presented in the figures corresponds to a DSF using a double-glazed low-e panel as the outer skin and clear monolithic glass as the inner skin. Glass properties are listed in Table 1. The environmental conditions in the analysis presented correspond to a hot December 21 design day. Figure 5 shows the total amount of solar energy (direct + sky-diffuse + ground-reflected components). Figure 6 provides a comparison of the DSF performance with and without blinds. In both cases, the cavity airflow rate is the same. However, the without blinds case results in more than twice the energy to enter the room (gain). These plots also show that the amount of energy that is captured in the DSF cavity (purple line) is reduced without the blinds. It is recognized that the type of blind (roller versus slat) can significantly influence the penetration of solar energy into the room. Figure 7 shows the diurnal variation of average air and surface temperatures in the DSF without shading. The figure shows how the temperature on the various glazing surfaces, as well as the 22

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12 Time (h)

16

20

24

Direct Normal Irradiance

Diffuse Irradiance

Direct Solar Radiation

Ground-Reflected Irradiance

Total Short Wavelength Irradiance

Figure 5: A diurnal cycle of solar radiation components for the Pearl River Tower site in Guangzhou, China. This information fed into thermal analysis performed in the design of the DSF.

temperature of the cavity air, changes during the day. This leads to the observation that the cavity ventilation airflow rate need not be constant during the day. In fact, there is a likelihood of higher energy efficiency if the cavity has varying flow rates throughout the day. The flow rates for a cavity on the north and south of the building will also be different if energy efficiency is to be optimized. Figure 8 highlights how the performance of the DSF can be significantly influenced by the flow rate within the cavity. The figure presents predictions of gain and load to the room, through the DSF, at noon in December for different flow rates (m3/h · m of width of the façade). These results are for the DSF operating without shading devices. The plot shows that gain and load to the room are decreased as the flow rate in the cavity is increased. The cost associated with this is the increased energy picked up in the cavity air. This data can be input into a s h r a e . o r g

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45 Gains Without Blinds

2,200

40

2,000 1,800 1,600 Temperature (°C)

Gain (W)

1,400 1,200 1,000 800 600 400

30

25

200 0

35

0

4

8

12

16

20

24 20

Time (h)

0

Gain from Solar Radiation Gain from Nonsolar Radiation Gain to Cavity Gain from LW Radiation Total Gain to Room

2,000

12

16

20

24

Outside

Outside Glass Surface

Outer Surface of the Inner Glazing Unit

Room

Inner Surface of the Inner Glazing Unit

Cavity Exhaust

Figure 7: Diurnal variation of average air and surface temperatures in Pearl River Tower DSF south-facing façade without shading devices.

1,800 1,600 1,400 Gain (W)

8

Time (h)

Gains With Blinds

2,200

4

1,800

1,200 1,000

1,600

800

1,400

600 1,200

400 200

1,000 0

4

8

12 Time (h)

16

20

24

Gain from Solar Radiation Gain from Nonsolar Radiation Gain to Cavity Gain from LW Radiation Total Gain to Room

Power (W)

0

800 600 400 200

Figure 6: Pearl River Tower DSF predicted performance with and without blinds for the south-facing façade.

a cost-benefit analysis to determine at which point the reduction in gain entering the room is offset by the additional fan energy driving the air through the cavity and the increased energy within the cavity air. A holistic approach to building design is required. Figure 9 shows a sample prediction from a CFD simulation of short-wave radiation, air speed, and air temperature distributions in a Pearl River Tower DSF cavity. In this simulation, the blinds were explicitly modeled for solar control. The graphics illustrate the complex patterns that simpler analytic models must account for, or risk providing the incorrect feedback. The vertical stratification of hot air in the cavity will increase heat flow into the building at the top of this single story cavity and the surface temperatures that will affect the long-wave radiant field in the occupied space. Multifloor cavities can exhibit 24

ASHRAE Journal

0 0

10

20 30 Flow Rate (m3/h·m)

Gain to Room

Load to Room

40

50

60

Gain to Cavity

Figure 8: Predicted effects of cavity airflow rate on energy flows. This was an early design configuration of the south-facing façade for Pearl River without shading devices.

more pronounced vertical variations in temperature leading to increased gains at the upper levels of the cavities. Comparison of DSF to a High Performance Single Façade

Figure 10 shows predicted maximum monthly gains and loads into a room through both a single and double skin façade configuration (quoted as W/m of width of the façade). These are compared without shading devices for an east-facing façade of a building located at 40º N (Beijing). This analysis was cara s h r a e . o r g

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Maximum Gain and Load to the Room (Single Façade, Triple Pane [W/m]) 1,500

Jan.

Dec.

Feb. 1,200 900

Nov.

600

584 869

438 300 392

600 Oct.

645

883

Mar. 1,245

615

927

1,045

1,074 773

958

1,368

Sept.

ried out early in design as a means to assess the viability of the DSF for this building and climate considering similar design constraints of 100% glazing area and equivalent transparency. The glazing properties are listed in Table 2. The plots present the maximum hourly values for the 21st day of each month for different parameters: the gain that is passed into the room through the façade (blue line) and the load (purple line) that the ventilation system within the room experiences given the thermal lag of the building thermal capacitance. The green line in Figure 10 is the load to the system should the energy captured in the DSF be passed back to the HVAC systems via the cavity ventilation air. In July, for example, the single façade permits a peak gain of 1,349 W to enter the room, and this is converted into a cooling load of 1,072 W. The DSF performance in July, and its impact on the building, depends on how the system is configured. The air ventilating the cavity is assumed to enter the cavity at room temperature. The peak gain that enters the room is 1,050 W, a reduction of approximately 22% over the single façade. The load that the system experiences from the room is 829 W, a reduction of 23%. These are useful performance improvements. However, if the air ventilating the cavity is returned to the HVAC systems and then recirculated again, the total load experienced by the system is approximately 1,242 W that is greater than that for the single façade. The choice of ventilation strategy is important. There are opportunities to use the cavity air in other ways that can provide energy savings. The DSF ventilation air can be mixed with the supply air, instead of using a reheat coil 26

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1,080

1,244 1,031

Figure 9: Example of CFD computer model prediction of airflow and temperature distribution in a DSF. From left to right the images are: (1) predicted solar radiation flux in the cavity; (2) predicted air speed distribution; (3) predicted air temperature distribution; and (4) configuration of mechanically ventilated DSF for Pearl River Tower building with outflow from top of cavity, which draws air from the room into the base of the single-story cavity.

1,353

May

1,071 1,072

1,322

1,346 1,349

Aug.

June

July Gain to Room

Load to Room

Maximum Gain and Load to the Room (DSF), (W/m) 1,500

Jan.

Dec.

Feb. 1,200

726

582

600 520 582 696 520 300 703 486 Oct.

1,084

900 803

Nov.

934

1,136

Mar.

787 787

990 749

822 1,069

847 617

981

752

1,217 Apr.

835 1,064 804

Sept.

1,100

1,038

829

826

1,251

1,045

May

1,050 1,205 Aug.

1,235 1,242

June

July Gain to Room

Load to Room Load to System

Figure 10: Comparison of thermal performance of a high-performing single façade and double skin façade. Peak one-hour energy gains and loads are predicted for each month of the year. This work was carried out during the design of a commercial tower in Beijing, east-facing façade, without shading devices. a s h r a e . o r g

April 2009

or it could be used in a heat exchanger. The air can also be used for drying a desiccant wheel if temperatures are high enough for sufficient frequency and duration. In the winter it can serve to prewarm incoming air. Closing Remarks

Thickness

Solar Heat Gain Coefficient

U-Value

Solar Transmissivity

Solar Reflectivity

Btu/h · ft2/°F (W/m2 · K)

inch (m)

  Inner Skin

1/2 (0.013)

0.73

0.98 (5.59)

0.64

0.06

1 (0.025)

0.38

0.28 (1.57)

0.33

0.31

DSF Outer

Skin The selection of the building envelope is one of the most critical aspects of building Single Façade 0.34 0.22 (1.25) 0.28 0.32 1 5/8 (0.042) (Triple Pane) design. This selection will dictate how occupants will behave in the building (e.g., Table 2: Glazing properties for Beijing building. lighting, natural ventilation), Poirazis, H. 2004. “Double Skin Façades for Offices Buildings: the energy demand to manage the external climate, and the appearance of the building. The range of design options of a Literature Review.” Division of Energy and Buildings Design, Lund building envelope can lead to a dramatic difference in the siz- Institute of Technology, Lund University. Report EBD-R-04/3. ing of building ventilation and cooling systems. If the building www.ecbcs.org/docs/Annex_43_Task34-Double_Skin_Facades_A_ Literature_Review.pdf. skin is not done well, then occupant behavior to moderate their Straube, J.F., R.V. Straaten. 2002. “The Technical Merit of Double environment (e.g., drawing blinds and closing windows) can Skin Façades for Office Buildings in Cool Humid Climates.” School dramatically affect the energy use and change the basis of design of Architecture, University of Waterloo, Waterloo, ON, Canada. www. of the building. In this case, at best, this means the building uses civil.uwaterloo.ca/beg/Downloads/DoubleFacadesPaper.pdf. more energy. At worst, the systems are undersized and building occupants will be uncomfortable. Optimal energy savings requires an appropriate balance of opaque walls and glazing. A double skin façade is one means to manage the interaction between the outdoors and the internal spaces. It also provides some architectural flexibility to the design. Energy efficiency and thermal comfort adjacent to the façade requires careful configuration of glass, solar control devices (both for thermal and visual effects), and ventilation of the DSF cavity. A responsive control system for the solar control devices and the ventilation of the DSF is required. The examples presented here show how different operating practices (e.g., flow rates), or design decisions (glazing choices) can affect the ultimate performance of the DSF. Decisions on these elements should be based on analysis that accounts for the detailed physics and inclusion of site climatic data as illustrated in this article.

Bibliography Blomsterberg, Å. et al. 2007. “BESTFAÇADE: Best Practice for Double Skin Façades—Literature.” EIE/04/135/S07.38652. www. bestfacade.com. Lee, E., et al. 2002. “High Performance Commercial Building Façades.” Building Technologies Program, Environmental Energy Technologies Division, Ernest Orlando Lawrence Berkeley National Laboratory, Report No: LBNL-50502. http://gaia.lbl.gov/hpbf/ documents/LBNL50502.pdf. Oesterle, E. et al. 2001. Double-Skin Façades: Integrated Planning: Building Physics, Construction, Aerophysics, Air-Conditioning, Economic Viability. Munich: Prestel Publishing. www.info.hotims.com/23934-30

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