Sustainability Insights

Extreme Measures: Building Resilience, it’s All About the Envelope

By Helen Sanders

With climate change already noticeable through severe weather events and warming summers, architects are challenged with how to design climate adaptability into buildings. Buildings also need to be resilient to extended power outages during extreme weather events, such as 2003’s Superstorm Sandy, which can leave many without power for days and even weeks. During power outages, buildings must provide a barrier to the outside weather without mechanical assistance.

Designing for resilience is as important as designing for energy efficiency.
The design strategies for the two can be complementary and both hinge on
the performance of the building envelope. According to Urban Green¹, thermal-resilient, envelope-centric passive design strategies should include:
• High levels of wall insulation;
• Appropriate window area;
• Highly thermally efficient windows with solar control;
• Air sealing;
• Shading devices; and
• Natural ventilation.

MEASURING RESILIENCE

Two metrics have been developed to measure resilience:
Thermal autonomy is the number of hours a year that a space can provide acceptable thermal comfort through passive means only.
Passive habitability is a measure of how long a building remains habitable during extended power outages coincident with periods of extreme hot and cold weather. The “survivability” temperatures are being debated, but 59° F for cold and 86° F for hot often are used.

CURRENT BUILDING STOCK

A 2014 simulation study of a range of residential building types in New York City, commissioned by Urban Green and conducted by Atelier Ten, showed how the typical and current code-compliant residential building stock cannot maintain a safe temperature during an extended power black-out in seasonal extremes.

In a summer extreme, a typical highly glazed residential tower’s ambient temperature reached 90° F after just one day without power, and over 100° F after five days— higher than the temperature outside. The code-compliant version of this building, and a post-2000 brick highrise with much less glazing, didn’t perform much better either.

Only by using a high-performance envelope (U=0.2 Btu/°f.hr.ft² and SHGC=0.3 fenestration, R20 wall insulation, and 0.08 air changes an hour) could the interior temperature be managed effectively. For the all-glass tower, the temperature reached 82° F after one day, and stayed below 88° F. For the brick tower, the temperature only reached 83° F after five days, stabilizing at 84° F.

Performance of typical buildings during a winter black-out was not much better. The typical all-glass tower (70% window area) and the brick tower (30% window area) fell to 35° F and 37° F in seven days respectively. However, with the high-performance envelope, both were able to achieve a much higher minimum temperature of 56° F, suggesting the resilience solution may not be so much about reducing window area but improving overall envelope performance. The University of Toronto’s Thermal Resilience Design Guide asserts that for typical ranges of window areas (35-65%) “investments in more efficient windows deliver higher performance than investments in more efficient opaque walls.”

The next time a design team considers trading poorer façade performance for higher HVAC system efficiency, mention the concept of building resilience. It’s all about the envelope.

Helen Sanders is in strategic business development for Technoform North America Inc. in Twinsburg, Ohio. Read her blog each month at usglassmag.com/insights.

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