Is that 3/8″ strip of insulation at the window sill really preventing condensation — inside the wall or on the interior surface — or, as some argue, is it blocking drainage and better left out altogether? What does the physics actually tell us? Studies conducted by NRC beginning around 2010 shed light on many of these questions. Under controlled winter conditions, researchers examined how rough opening insulation, air leakage, pressure differences, and drainage detailing affect interior surface temperatures and condensation potential. The findings may not align with long-held assumptions.

 

This session steps past opinion and looks at how heat, air, and moisture actually behave at the rough opening. We’ll examine condensation risk, the role of air sealing versus thermal resistance, and why drainage at the sill may be just as critical as insulation. If we have to prioritize between marginal thermal gains and reliable water management, where should we stand? The 3/8″ question may be small — but the implications are not.

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Presented by Darrell Paul, BTRboard Inc.