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Thermal bridging can impact energy performance

July 02, 2013

In an era of energy efficiency, some new buildings don’t measure up.

“A lot of the older buildings that we (the industry) are trying to restore, maintain and improve are actually already more energy-efficient from a thermal control perspective than a lot of the new buildings we’re constructing,” said Scott Armstrong, manager, building science and sustainability, MMM Group.
Part of the problem with new buildings can be pinpointed to thermal bridging, which significantly impacts the durability, energy performance and occupant comfort of buildings today. 
Underestimating it is tantamount to undermining a building’s performance, Armstrong told a seminar at a building envelope forum in Toronto recently. 
A recent study published by the firm indicated that heating loads can vary by 30 per cent and cooling loads by 200 per cent, based on the thermal and solar control design strategies. What that often means is that a sophisticated and often expensive high-efficiency mechanical system is used to meet Ontario’s new SB-10 code requirements. 
Armstrong said in-situ performance problems are often not easily resolved and can be “extraordinarily expensive to rectify. If we don’t design right and we don’t build right we’re setting ourselves up for this enormous problem.” 
And that problem can develop in only a few years, he said, noting a five-year-old building as an example with serious material degradation issues such as corroding steel beams, crumbling bricks and failing exterior insulation finishing systems. 
“Not understanding membrane continuity, control of moisture at building interface joints and overall thermal performance can lead to a situation no one wants to face.” 
Armstrong told the audience that designers should aim to reduce thermal loads, rather than simply try to compensate for under-designed thermal performance with advanced high-efficiency mechanical equipment.
One challenge with specifying continuous exterior insulation is the task of eliminating thermal bridging, he said, pointing out that the effects of thermal bridging actually increase when adding more insulation. Buildings may target an R-30 wall but end up with an R-12 wall due to thermal bridging. 
“The key is to cut the thermal bridge and let the insulation perform as intended.”
Examples of building details causing thermal bridging are z-girts, steel studs, curtain wall mullions, backpans, insulation pins, and glazing caps. 
Armstrong noted that a relatively new insulation product, vacuum-insulated panels, is being designed by some manufacturers to be installed between two panes of glass as part of an insulating glazing unit (IGU) to improve thermal performance. This foil-wrapped insulation product is sealed in a vacuum blanket and encapsulated within the IGU, protecting it from damage. 
The new technology is promising, he told the crowd, but some field detailing such as slab-edge fire stopping must be addressed. 
Thermal bridging also has a big impact on roofing. “If we only fasten the bottom layer of insulation and we adhere the top layer and finishing membrane and surfacing, we only have about a three per-cent loss in our thermal performance of the insulation.” 
Through fasteners, however, can result in considerably more degradation of the insulation’s performance, Armstrong said. 
“You can’t just necessarily count on the nominal R value of your roof assembly. You have to understand how the roof system is installed, how the various layers of the assembly are secured, and where thermal bridging may be affecting your roof assembly.” 
Overall, Armstrong said it is important to integrate HVAC and building enclosures to produce optimum performance. 
Few new buildings in Ontario can meet the prescriptive requirements of the Ontario Building Code’s SB-10 energy efficiency requirements; designers often opt for the performance route, he said.
“The key is to understand the actual thermal value of your wall assembly and not simply rely on high-performance HVAC equipment to compensate for poor thermal performance,” he said.


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