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Double-skin facades.
Subject:
Buildings (Heating, cooling and ventilation)
Buildings (Energy use)
Facades (Design and construction)
Facades (Usage)
Authors:
Roth, Kurt
Lawrence, Tyson
Brodrick, James
Pub Date:
10/01/2007
Publication:
Name: ASHRAE Journal Publisher: American Society of Heating, Refrigerating, and Air-Conditioning Engineers, Inc. Audience: Academic Format: Magazine/Journal Subject: Construction and materials industries Copyright: COPYRIGHT 2007 American Society of Heating, Refrigerating, and Air-Conditioning Engineers, Inc. ISSN: 0001-2491
Issue:
Date: Oct, 2007 Source Volume: 49 Source Issue: 10
Topic:
Event Code: 470 Use of energy
Geographic:
Geographic Scope: United States Geographic Code: 1USA United States

Accession Number:
171213315
Full Text:
Many architects--and their clients--prefer buildings with all-glass facades. In most cases, such buildings use a single-skin facade consisting of fixed glazing (windows) that forms the outer surface of the building. Relative to buildings with largely opaque facades, they tend to have higher space conditioning loads from heat transfer through the building envelope because windows pose less resistance to heat transfer than insulated walls.

Recently, building designers in the U.S. have begun to use double-skin facades (DSF) to attempt to improve the thermal energy performance of facades of buildings with high glazing fractions. DSF first came into use in the 1970s and have primarily been used in European buildings since. (1,2) Compared to a single-skin facade, a DSF consists of an external glazing offset from an internal glazing integrated into a curtain wall, often with a controllable shading system located in the cavity between the two glazing systems. Typically, the external glazing is a single layer of heat-strengthened safety or laminated safety glass, while the interior layer consists of single- or double-pane glass with or without operable windows. (2,3)

Air can flow through the cavity via natural or mechanical ventilation and is used to help moderate building thermal loads. Most commonly, outdoor air flows into the bottom of the cavity and exhausts from the top of the cavity outdoors. During the cooling season, throttling flaps at the inlet and outlet of the cavity remain open to allow airflow through the cavity to exhaust heat that builds up in the cavity. If the DSF includes dynamic sun shades in the cavity, they are usually deployed during the cooling season. Consequently, instead of heating the building interior, the solar radiation heats the air in the cavity, causing the air to buoyantly rise out of the cavity. (2,3) Thus, the DSF reduces the heat gain of the building. Because the buoyancy of the heated air increases with cavity height, DSFs are typically used in multistory buildings on one or more sides of the building that receive appreciable sun. In addition, when combined with operable windows that open into the air cavity, a DSF can provide natural ventilation to cool spaces adjacent to the DSF without (or with much lower levels of) mechanical cooling when the outdoor air enthalpy is less than the indoor air enthalpy.

During heating season, the DSF cavity inlet and outlet vents close to prevent airflow, trapping solar gains in the cavity and reducing daytime heat losses through the facade. (1,2) In addition, the sunshades remain open during the day to allow more solar radiation to enter the building and are closed at night to retain heat (i.e., the reverse of the operational scheme during cooling season).

Energy Savings Potential

As described earlier, DSFs have the potential to reduce building heating and cooling energy consumption in several ways. However, effectively adding another glazing to the window decreases its visible transmittance and solar heat gain coefficient, decreasing the potential energy savings from daylighting (using sunligth to light indoor spaces instead of electric lighting) and solar "free" heating during the heating season, respectively.

In practice, it is not clear that DSFs realize appreciable energy savings. This is true on two levels. First, there is a dearth of real-world performance data and rigorous whole building energy simulations for DSF performance, preventing a clear assessment of the whole building energy impact of DSFs; this mirrors earlier conclusions. (2,4) Second, past applications of DSF may well have been suboptimal in their design and climatic use (e.g., many uses in heating-dominated and less sunny Germany).1,4 One simulation of a museum in Denver with a DSF found that it reduced cooling and heating energy consumption by a few percent relative to a DSF without ventilation (similar to triple-pane glazing). Relative to double-pane insulated glazing, the DSF reduced heating and cooling loads by approximately half. (5)

Due to the number of variables affecting DSF performance, effective control is crucial to realizing the full energy-saving potential of DSFs. That is, effective control requires sophisticated control algorithms based on current building and weather conditions, otherwise, the DSF may increase energy consumption. (6)

Market Factors

DSF face several major barriers to greater use, notably high first cost, major implementation challenges, and significant uncertainty about their value. To a substantial extent, DSFs owe much of their deployment to date to clients' desires for landmark buildings that visibly project a technologically forward-looking and sustainable image. As such, many projects using DSFs have paid limited or negligible attention to cost. (2) One source indicates that both designing and constructing a DSF is expensive, noting a range of cost estimates from various sources of $135 to $360/[m.sup.2] ($13 to 33/[ft.sup.2]) and $680 or more per [m.sup.2] ($63 or more per [ft.sup.2]). Other sources estimate even higher incremental costs, $900 to $1,800/[m.sup.2] ($84 to $167/[ft.sup.2]) and $500 to $1,000/[m.sup.2] ($46 to 93/[ft.sup.2]). (2,3) The high cost premiums of DSF result in long payback periods and reinforce claims that buildings often incorporate a DSF to enhance an organization's image rather than to save energy. (2)

DSFs are also challenging to design and implement. Accurately modeling the energy performance of DSF is inherently complex due to the dynamic interaction of several factors, including glazing thermal and optical performance, natural and forced convection cavity flows, adjacent perimeter zone space conditioning loads, operation and characteristics of shades and blinds, daylight transmission to perimeter spaces, dynamic variations in wind speeds that alter cavity inlet and outlet pressures, and framing system thermal performance. Unfortunately, performance data and mature design tools are not yet publicly available, making it challenging to evaluate the potential performance of DSFs and to develop an effective DSF design for a specific building. In addition, standard design performance metrics that accurately take into account the unique features of DSFs (cavity airflow, the range of vertical temperature profiles in the cavity) do not exist. (1,4) Furthermore, because DSFs are not common practice, state energy code officials are often not familiar with DSF. This can make demonstrating code compliance time consuming. (2) All of these factors increase the cost of considering and designing effective DSFs, impeding DSF deployment.

Furthermore, architects and engineers do not clearly see the value of using DSF. (2) This reflects the aforementioned high cost and energy saving ambiguity, as well as other concerns about performance. For example, improved acoustics has motivated several DSF implementations. (2,4) DSFs can reduce the transmission of outdoor noise sources into the building if the cavity entrances and exists are not too large. However, if the DSF provides natural ventilation, the magnitude of the required airflows tends to increase the size of the cavity entrances and exits and windows open into the cavity. As a result, outdoor sounds can more readily enter the cavity and sound transmission between spaces with windows opening into the cavity increases. If this compromises occupant comfort, building occupants may close their windows and negate potential savings from natural ventilation. (4) To cite another example, claims of increased occupant comfort during heating season due to higher glazing temperatures (which decreases radiative cooling from windows) can be offset by excessive heating in spaces located adjacent to the upper portion of the DSF cavity, (2) particularly in spaces with open windows for natural ventilation. If these factor compromise occupant comfort, building occupants may close their windows and negate potential savings from natural ventilation. (4)

Other concerns about DSF performance include fire safety (fires spreading between floors via the cavity) and maintenance (keeping more glazing surfaces clean and dynamic shading operational. (4)

In sum, it appears that a building with an effectively designed DSF may realize moderate energy savings, albeit at high incremental cost and with extensive design effort. It is likely that other technologies and design approaches can achieve the desired positive attributes of DSFs in a more cost-effective manner.

References

(1.) Doebber, I. and M. McClintock. 2006. "Analysis process for designing double skin facades and associated case study," Proc. SimBuild 2006. http://tinyurl.com/2nyx5f (or http://gundog.lbl.gov/dirpubs/SB06/doebber.pdf).

(2.) Lee, E., et al. 2002. "High-Performance Commercial Building Facades." Lawrence Berkeley National Laboratory. LBNL-50502. http://tinyurl.com/2qwdy4 (or http:// repositories.cdlib.org/cgi/viewcontent.cgi?article=2490&context=lbnl).

(3.) Stribling, D. and B. Stigge. 2003. "A Critical Review of the Energy Savings and Cost Payback Issues of Double Facades." CIBSE/ASHRAE Conference. www.cibse.org/pdfs/8cstribling.pdf.

(4.) Gertis, K. 1999. "Sind neuere Fassadenentwicklungen bauphysikalisch sinnvoll? Teil 2: Glas-Doppelfassaden (GDF)," Ernst & Sohn Bauphysik 21.

(5.) Pappas, A. 2006. "Energy Performance of a Double-Skin Facade--Analysis for the Museum of Contemporary Art, Denver." SOLAR 2006. http://tinyurl.com/2olbe6 (or www.solar2006.org/presentations/tech_sessions/t45-p201.pdf).

(6.) Saelens, D., J. Carmeliet and H. Hens. 2003. "Energy Performance Assessment of Multiple-Skin Facades." International Journal of HVAC&R Research 9(2):167-186.

By Kurt Roth, Ph.D., Associate Member ASHRAE; Tyson Lawrence; and James Brodrick, Ph.D., Member ASHRAE

Kurt Roth, Ph.D., is an associate principal, and Tyson Lawrence is a technologist with TIAX LLC, Cambridge, Mass. James Brodrick, Ph.D., is a project manager with the Building Technologies Program, U.S. Department of Energy, Washington, D.C.
Gale Copyright:
Copyright 2007 Gale, Cengage Learning. All rights reserved.