Ensuring Energy Efficiency: Specifying windows and curtain walls for cold climates


CC_Oct13_HR-30The codes
It is outside this article’s scope to give an exhaustive overview of energy code updates in cold climates. However, it will put performance requirements in context.

The 2011 National Energy Code of Canada for Buildings (NECB) provides minimum requirements for the design and construction of energy-efficient buildings with respect to the building envelope, HVAC, service water heating, lighting, and the provision of electrical power systems and motors. It applies to new buildings and additions.

This code’s prescriptive requirements for fenestration U-factor are set for six zones, ranging in Celsius Heating Degree Days (HDDs) from less than 3000 HDDs in Zone 4 to greater than 7000 HDDs in Zone 8, as found in Appendix C of the National Building Code of Canada (NBC). The zone names in NECB generally are consistent with those used in ASHRAE 90.1, Energy-efficient Design of New Buildings Except Low-rise Residential Buildings. However, Canadian Zone 7 is sub-divided into 7A and 7B to provide a finer 1000-HDD distinction. Prescriptive vertical fenestration maximum U-Factor requirements range from 2.2 W/m2·K (0.4 Btu/hr·sf·F) to 1.6 W/ m2·K (0.3 Btu/hr·sf·F), from southernmost to northernmost zones.

Prescriptive maximum fenestration-and-door-to-wall ratios range from 20 to 40 per cent, depending on the zone. Continuous air barriers are required, and limits set on fenestration air leakage.

In the northern tier of the United States, the International Energy Conservation Code (IECC) sets a 40 per cent limit on glazed window-to-wall ratio (WWR) if opting for the prescriptive compliance path. Maximum SHGCs, U-factors, and air leakage rates are set for each product type (e.g. curtain wall, storefront, entrance doors, etc.) in each climate zone. (Zones 7 and 8 are for Canada.) The other possibility, ASHRAE 90.1, allows up to 50 per cent WWR if opting for the prescriptive compliance path. Maximum U-factors and SHGCs are required to be more stringent as percentage of glazed wall area increases.

Non-prescriptive compliance options also are available, limiting glazed area through the application of default values (in the United States), using whole-building energy modelling, or applying trade-off provisions with other building components.

Too often, the Division 08 specifications do not reflect the applicable energy code requirements or the performance basis of the building permit. This can lead to costly revisions and change orders. Co-ordination between architectural, engineering, and construction disciplines is critical.

Interior accessory windows
When existing windows are weathertight, and operation for ventilation is not a requirement, the addition of high-quality interior accessory windows can be a viable option to window replacement. With appropriate caution to avoid between-glass condensation, these economical add-on units improve control of sound, energy, air, and light.

As they are often fitted with between-glass blinds, interior units are side-hinged or removable for cleaning/servicing access. Integral between-glass blinds minimize maintenance, while managing light and glare. Combined with the advantages of natural light, outside views, and thermal performance, these qualities may aid buildings seeking green certification, under programs such as the Canada Green Building Council’s (CaGBC) Leadership in Energy and Environmental Design (LEED) rating system.

Further, when possible, leaving existing windows in place means the exterior appearance of historical buildings can be virtually unaffected.

Conclusion
Reliance on fossil fuels, as well as an increasing recognition of the societal costs of greenhouse gas (GHG) emission and acid rain, makes rising energy costs a trend unlikely to abate in our lifetimes. Further, windows in buildings are an important contributor to North America’s gross energy consumption. However, selection of high-thermal-performance glass and framing, coupled with close attention to integrated design and the use of natural daylight, can make windows and curtain walls positive contributors to building energy performance on the way to zero-energy buildings.

Notes
1 The performance-based, material-neutral North American Fenestration Standard (NAFS)—AAMA/WDMA/CSA 101/I.S.2/A440—defines four performance classifications: R, LC, CW, and AW. AW-rated fenestration products commonly are used in high-rise and mid-rise buildings. Additional details can be found at www.aamanet.org/general/2/407/performance-class-overview. (back to top)
2 Visit www.windows.lbl.gov. THERM’s precursor, FRAME was developed in Canada by Enermodal Engineeering, which still offers a similar modelling programe, FRAMEPlus. Visit www.enermodal.com/window-rating.html offers an in-depth explanation of applicability. (back to top)

AuthorPicSteve Fronek, PE, leads Wausau Window and Wall Systems’ new product development, marketing, field service, technical support, and general research. He is the immediate past-president the American Architectural Manufacturers Association (AAMA), and has served on 16 of AAMA’s committees and task groups, including guiding the evolution of thermal performance standards as they exist today. Fronek is a member of the Lawrence Berkeley National Laboratory’s (LBNL’s) High-performance Building Façade Solutions Public Advisory Committee and a LEED Green Associate. He can be contacted via e-mail at sfronek@wasauwindow.com.

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