INNOVATION September-October 2014

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The Building Envelope Thermal Bridging Guide A New Tool for Building Design Practitioners

Robin J. Miller

E ver heard some old-timer describe what life was like before AutoCad? That might be exactly what today’s engineers sound like in a year or two, telling the youngsters how difficult it used to be to calculate energy performance in the days before the Building Envelope Thermal Bridging Guide (BETB). In many buildings, heat loss and air leakage across the exterior envelope can account for more than 50% of the total energy load. That’s why, says former energy modeler and current BC Hydro Power Smart specialist engineer Bojan Andjelkovic, P.Eng., who originated the idea for the guide, “we advise building design practitioners that their top priority is to reduce energy loads in their buildings before implementing other measures to reduce energy consumption. We also advise them that constructing buildings with thermally resistant, airtight envelopes should be considered job one.” Up until now, however, it’s been very difficult and time-con- suming for building designers to accurately calculate the thermal performance of building envelopes owing to thermal bridg- ing—pathways of high heat flow through walls, roofs and other insulated building envelope components that allow the heat flow to bypass the insulating layer, effectively defeating the purpose of

the insulation—which is where the BETB Guide comes in. Available online from BC Hydro, the BETB Guide is the result of more than four years of work by engineering firm Morrison Hershfield in collaboration with primary funding partners BC Hydro Power Smart and the Homeowner Protection Office of BC, a division of BC Housing. Other partners include the Canadian Wood Council, Fortis BC and FPInnovations. The guide’s primary goal is to help the BC construction sector design and build more energy efficient buildings by pro- viding easy-to-use methods for understanding and mitigating thermal bridging. “The BETB Guide illustrates that, depending upon the build- ing type and the insulated envelope assembly, actual build- ing heat loss can be up to four times greater than is generally accounted for in conventional design practice,” says BC Hydro Power Smart engineer Gordon Monk, P.Eng., project manager for the guide. “Clearly, future iterations of building energy codes and standards must comprehensively address thermal bridging rather than solely relying on increased insulation levels.” Adds Andjelkovic, “Remember, too, that mitigating ther- mal bridging can do even more than reducing building heating and cooling loads. It can also increase thermal comfort and air

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