The Envelope Corner: The Corners of the Envelope

By Jim D’Aloisio | Friday, April 30, 2021

If your building has walls and a roof, then it has wall-to-roof intersections. The details at these intersections, or what could be called the “corners of the envelope,” vary from project to project. They need to be well-designed for acceptable structural performance but can sometimes be challenging to control both convective and conductive heat flow. Let’s take a look.

Continuous convective resistance — the air barrier system — can be confounded by penetrations for roof edge blocking support, parapets, corbels, and perimeter projections such as sunshades. How to affect a continuous airseal varies with the details, but usually transition tape, sealant, or mastic is needed.

Conductive resistance can be compromised by thermal bridging of elements that pass through the insulation. One challenge is that such heat loss is difficult to quantify without using a program such as THERM that calculates the overall U-factor for the modeled section.

We have found that many designers do not realize that the Energy Code applies to the entire envelope, including the roof-to-wall intersection. This is a particularly challenging condition to check against Code since it involves the combination of wall and roof thermal requirements. We recommend analyzing a section that has equal lengths of wall and roof, so the prescriptive-required U-factor is the average of the wall and roof values. For a commercial building in Climate Zone 5, this is (0.064 + 0.032) / 2 = 0.048.

In the past year, we have investigated four conditions in various locations around the country, where severe interior moisture was caused by thermal bridging: either interior condensation or water intrusion after thermal bridging caused snowmelt and ice dams, or both. In each case, the remedial work to address the problem conditions was much more costly, and less effective, than if the thermal bridging detail had been modified prior to construction, during the design phase.

Even if no interior moisture manifests itself, thermal bridging at the wall-to-roof intersections can cause significant thermal losses, resulting in greater utility bills and carbon emissions, and reduced thermal comfort.

Klepper, Hahn & Hyatt can assess these conditions and, if problematic, can propose alternatives to minimizing thermal bridging problems.

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