April 1, 2012
By Geoff McDonell, P.Eng., LEED AP
A curtain wall is a cost-effective building exterior enclosure system, and can be an esthetically pleasing envelope for many building types. From high-rise residential to office buildings, glass curtain wall cladding can provide architecturally eye-catching and highly marketable views for occupants. However, it is not an exterior cladding normally associated with high thermal and solar gain performance for reduced building energy use.
The last two decades of architectural design culture in many areas of Canada has led to an increasing trend for the maximized use of vision glass, with some use of insulated spandrel panels and, in some cases, ceramic fritting for solar gain control. The most common commercial building curtain wall systems consist of sealed double-glazed windows in an aluminum frame that may or may not incorporate any kind of thermal break. Insulated spandrel panels generally consist of an insulated metal pan installed in the same framing system as the windows.
Even in some of the country’s most extreme climates, it has been acceptable practice to continue to use double-glazed window panels, albeit with improved thermal and solar performance with low-emissivity (low-e) coatings and argon fills. The local designers usually pay attention to specifying a better degree of window thermal performance in more extreme winter climate zones, mainly to prevent indoor surface condensation and frost.
Nationally, Canada has had an energy code (i.e. Model National Energy Code for Buildings [MNECB]) since the early 1990s, but this has only been applied and enforced for federally owned buildings on land under federal jurisdiction. Ontario is the sole province to have adopted it as an enforceable section of its provincial code. The City of Vancouver, unique in having its own municipal charter, has incorporated American Society of Heating, Refrigerating, and Air-conditioning Engineers (ASHRAE) 90.1, Energy Standard for Buildings Except Low-rise Residential Buildings, as part of the Vancouver Building Bylaw for the last 20 years. It will be enforcing ASHRAE 90.1-2010 sometime in 2012. On the other hand, British Columbia has only recently incorporated ASHRAE 90.1-2004 with an amendment to its provincial code in 2008. As incredible as it might seem, no other provinces have an enforceable building energy performance standard referenced in their building codes at this time.
The construction community must wrestle with the differing building design standards in each province, as well as face the fact that, sooner than later, all provinces will be including and enforcing some kind of energy performance requirement. The question is, how will the drive for improved energy performance affect curtain wall systems and designs?
Changing requirements at home and abroad
One of the key issues included in ASHRAE 90.1 (as well as the newly released 2011 National Energy Code for Buildings [NECB]) is a prescriptive maximum ratio of 40 per cent vision glass to opaque wall—the window-to-wall ratio (WWR). When the design employs a higher ratio, then a performance compliance path must be followed, requiring a detailed energy model to show the combination of other building systems (including mechanical and electrical) perform in such a manner to achieve equal or better energy efficiency compared to the reference building model designed with the prescriptive requirements. In fact, the trend going forward is to further reduce the WWR to 30 per cent in some climate zones.
ASHRAE 90.1-2013 will be released by 2014 and is anticipated to be 20 per cent more stringent than the 2010 standard. The U.S. Department of Energy (DOE) has required all states adopt ASHRAE 90.1-2010 (or an equivalent performance) by 2013. It will be interesting to see how this move affects Canadian energy codes, as it appears we tend to follow the Americans, considering a great deal of the glazing products are based on U.S. company supply chains. This definitely sets a bottom level for Canadian building energy standards as a potential ‘business-as-usual’ baseline for any building project starting design this year.
Many of the current European energy codes—such as the German Energy-saving Ordinance (ENEV 2009/German standard DIN V 18599), along with those for Sweden, Norway, and Finland—are about 40 to 50 per cent better than ASHRAE 90.1-2010, and an average of 80 to 90 per cent more stringent than the 1997 MNECB. Additionally, there are other prescriptive requirements for building envelope detailing and construction to minimize thermal bridging, as well as to prescribe minimum insulation values, window performance, and airtightness.
The European Union building energy use codes are being amended to reach a goal of “nearly zero energy” use off the grid by December 31, 2020, with the building energy primarily generated onsite with renewables. Stricter enforcement and oversight will be part of the European energy code developments toward 2020. (Visit www.europeanclimate.org/documents/LR_%20CbC_study.pdf for more information. The tables on pages 80–87 are helpful).
In Canada and the United States, the impact will likely mean realistic design approaches to more energy-efficient curtain walls will come into effect. Examples include:
Over the life of a building, the basic net result of improved window and curtain wall products will be:
Properly tuned WWRs with glazing performance also eliminates commonly used (and often energy-intensive and high-capital-cost) perimeter compensation systems from the HVAC designs—no more perimeter baseboards, fan-coils, or induction units.
Extreme thermal loads inside the building imposed by the envelope (glazing systems, mainly) are commonly handled by supplying higher volumes of warm and cold air into the perimeter zone, where the heating and cooling loads are occurring. The fast-acting thermal transients, caused by poor thermal and solar performance envelopes, require a great deal of energy and responsive controls systems to maintain some level of human comfort. Such systems result in increased maintenance as well, incurring a high lifecycle cost and long-term high-energy expenses to operate.
The other intangible results of using better (i.e. higher thermal and solar performance) window systems will be significant improvements to occupant comfort around the perimeter zones through lowered mean radiant temperature (MRT) due to reduced internal surface temperature differences of the glass, as well as elimination of convective air currents from cold inner surfaces of windows. A design goal for climate-adapted design should be to select the glazing thermal performance based on maintaining an interior glass surface temperature above 17 C (63 F) at peak winter design conditions, and below 27 C (81 F) during peak summer conditions.
In a heating-dominated climate like Canada, this has great energy-reduction benefits—internal heat gains no longer leaking out of the bad envelope can be recovered via air-to-air heat exchanger systems to preheat the required ventilation air supplied to the occupied spaces. Further, there is plenty of colder air available for free cooling most of the year.
Thermal performance of curtain walls must also include improved thermal break designs of the framing and support systems. Additionally, the thermal break design of the framing systems must improve along with the number of panes and/or suspended films being used in the sealed glass windows. Using a poor thermal break framing system can compromise even the best-performing window.
Even in Canada’s climate, solar gain through windows incurs the need for a great deal of energy-demanding cooling systems. The current architectural esthetic and building marketing trend is for as much window area as possible—sacrificing energy efficiency in the name of having nice views.
Curtain wall systems with essentially 100 per cent floor-to-ceiling window area are still being proposed, based on the economic first cost aspect of that type of building cladding system, with the availability of low-energy HVAC systems to compensate for the low thermal performance of an all-glass exterior wall. Triple-glazed, low-e-coated, argon-filled sealed glass elements can compensate somewhat for the extensive window area, but as the codes require better envelope performance, how far will all-glass curtain wall systems have to go?
There are some suspended film sealed window units that, with krypton gas fill, can provide:
While this may be considered exotic by North American sensibilities, it is interesting to note Europe has been using triple- and quadruple-glazed (i.e. suspended film) window systems for the last 30 years, as their energy codes have been driving up the window performance requirements. Further, Chinese production of these window systems is being refined to a very high quality level, which should make economic availability widely available.
The key to successful application of extensive curtain wall systems to meet better solar gain performance (and, therefore, lower energy use goals) is to:
There is always a compromise—glazing optimized for best heating performance (i.e. lowest heat loss), is not the best for cooling reduction performance, and vice versa. The envelope and glass design must be climate-adapted for the project’s specific location and orientation.
Careful selection of window tints will have to be made. For example, use of ‘heat-absorbing glass’ can be a boon in a sunny but cold climate zone, but it introduces the risk of creating higher air-conditioning loads in the summer. This is because the inner surface of the glass can reach very high surface temperatures, creating large radiant heating panels.
Ceramic fritting is another alternative for solar gain control, and can be designed into various patterns and densities for controlling glare and providing better quality daylighting. The use of light-coloured fritting on the inner surface of the outer glass panel can be an effective solar heat gain control element that helps reduce the need for tints; it also lowers the glass temperature on sunny days compared to conventional heat-absorbing heavily tinted glass.
Fritting has to be carefully designed. While the ceramic pattern reduces the direct solar heat gains entering the occupied space, the fritting absorbs heat and causes the glazing to warm up. This potentially creates some radiant heating from the warmed inner glass surface.
Hand in hand with improved solar gain reductions required for high-performance curtain walls comes the need to maintain a high degree of daylighting to offset the electrical loads from powered lighting systems inside the building. Most architects believe having more windows is better for ‘natural lighting,’ but this author sees a lot of floor-to-ceiling glazed buildings on sunny days with the blinds closed. In some cases, the occupants have applied their own solar control devices in the form of foil and drapes.
There is a big difference between daylighting with natural light and having direct sunlight streaming in through a window. Good daylighting can result in esthetically pleasing, appropriately lit spaces while saving energy. A successfully daylit building is the result of a combination of art and science, of architecture and engineering. It is the culmination of an integrated design process, rather than simply a technology installed once the building is complete.
The design key is to harvest good quantities of diffuse, non-glaring natural light through well-specified spectrally selective windows, appropriately sized and located (i.e. ‘tuned’ to the building façade). More window area does not result in much more effective daylighting, and the corollary reduction in electric lighting energy.
The example pictured at the top of this page illustrates a common perception. The windows were based on a four-element window system that had a VLT of only nine per cent. There was a decision made to eliminate any interior shading devices (i.e. blinds and drapes), as the design team feared a lack of daylighting through such a low-VLT window.
A couple summers after the building was leased out, the occupants were complaining about too much daylighting, and it was causing a glare on their computer screens. The entire building was retrofitted with interior sheer drapes to reduce the glare and allow more diffuse natural light into the perimeter offices. The moral is the design esthetic of using a lot of glass for transparency and natural light must be understood and properly applied. (References and links to design resources can be found at www.daylighting.org/design.php. The Canadian Commercial Building Daylighting Guide is available at www.enermodal.com/pdf/DaylightingGuideforCanadianBuildingsFinal6.pdf).
While the evolving building energy codes appear to be pushing building designers to use less windows and glass, this does not mean opportunities for effective natural lighting will have to be compromised. If the clean smooth exterior façade design precludes use of exterior shading devices, then tinted and spectrally selective glazing systems will be required, as well as reduced WWRs. Where exterior shading can be used, then less tint and more window area could be specified, albeit with higher thermal performance criteria to meet the energy performance goals.
Numerous technical studies around the world, including extensive in-house modelling at Cobalt LLP, have shown there is a ‘sweet spot’ for optimal window-to-wall ratios of around 30 to 40 per cent for northern, heating-dominated climates, which results in the best daylighting and envelope thermal performance.
Low-e coatings, gas fills, warm-edge spacers, and thermally broken frames are becoming available at reasonable costs. Driven by the increasing demands of stricter energy code compliance requirements, these available technologies will need to be combined with multiple-layer glazings, dynamically controllable shading, highly-insulated frames, and other options.
An astute building design team should be able to combine the savings from mechanical and electrical systems to pay for the apparent ‘premium’ of very high-performance curtain wall systems, satisfying tight budget constraints. For every dollar spent on the building envelope, a dollar can be saved from the HVAC and lighting systems, and result in significantly lower energy and operating costs for the building’s life. An economically designed structure with lower operating and maintenance costs, and increased indoor comfort ought to be an easy sell for building owners and
New building designs using curtain wall cladding must adjust to the future energy code developments by fundamental design shifts:
Given the gestation period of building designs—from initial concept, through re-zonings and development permit processes—it can take anywhere from two to three years before the building permit stage is reached, so project teams must recognize the evolution of code-driven energy performance requirements, and future-proof their designs to ensure project budgets and cladding systems reflect the built environment’s new realities.
Geoff McDonell, P.Eng., LEED AP, is an associate partner at Cobalt Engineering LLP in Vancouver. He has more than 30 years of experience in mechanical engineering, and is a registered mechanical engineer in British Columbia and Alberta. McDonell specializes in low-energy mechanical systems, including radiant cooling applications, Passivhaus style designs, and assisting architects with building envelope performance evaluations. A LEED AP since 2001, he was a speaker at the first Greenbuild conference. He can be contacted via e-mail at email@example.com.
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