The envelope backstop: preventing poor thermal performance

Twinsburg, Ohio
707 Fifth Tower
Blog by Helen Sanders, Ph.D
707 Fifth Tower
The 707 Fifth office tower in downtown Calgary is an example of an exceptionally high-performance façade utilizing high-performance curtainwall with bent triple pane insulating glass with durable warm-edge spacer. Even with large expanses of glass, the building met both energy targets and thermal comfort targets in proximity to the building envelope. Photo: Steve LeBlanc

Backstops seem to be all the rage these days, what with the “Brexit backstop” in European politics, financial backstops and the many backstops in the sporting world. The term has even crept into building energy codes through the creation of the “envelope backstop” the intent of which is to prevent poor-performing building envelopes. While some oppose this mechanism because of the potential to reduce glazing area, others see significant opportunity for more sustainable and comfortable buildings, and for business growth through accelerating the adoption of high value-added fenestration.

During a recent visit to Washington, D.C., and discussion with my friend Vicente Montes of Curtainwall Design and Consulting, I learned that many buildings are routinely being built across the country with fenestration of lower thermal (U-factor) performance than even the prescriptive code compliance path requires. For example, where the prescriptive code may require fenestration with a U-factor of 0.38 btu/, the actual thermal performance of the fenestration delivered is, say, 0.43 btu/ or higher.

This is because when following the performance code compliance path, the performance of the envelope can be “traded off” with improvements in the performance of internal systems such as HVAC and lighting to deliver the same overall energy performance as the base prescriptive building. Many people argue that it doesn’t matter how building energy target is achieved – that’s what the performance path is all about, and gives the architect design flexibility. If a strategy using poorer performance windows with higher performance HVAC and lighting delivers the required energy performance, this is considered acceptable. 

I would argue that it is does matter how you achieve the target. In the absence of complementary mandatory targets for occupant comfort or other measures of building sustainability, a sole focus on achieving energy performance can lead to unintended consequences of poorer occupant experience and lack of resilience to severe climate events and longer-term climate continue reading click here.

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