Joe Lstiburek: air seal both sides of the insulation

While we didn't attend this past summer's Building Science Camp we appreciated hearing about it from folks like Daniel Morrison and Allison Bailes.   Of particular interest was this tweet from Allison Bailes/Energy Vanguard quoting Joe Lstiburek of Building Science Corporation, and event impresario.

Why is this quote so important? Simply because it clearly asserts that airsealing just one side of the enclosure -  a common practice to meet air tightness requirements - does not provide an optimized condition. Far from it. Why? Thermal bypass.

Thermal bypass describes heat loss that gets around intended thermal insulation, including by windwashing, air infiltration, and convective loops.  Studies have shown that such conditions can dramatically reduce the effectiveness of  installed thermal insulation.   Thermal bypass can encourage condensation, wetting and damage. This is true not only for fiberous insulation but foam insulation too. The one sure approach to avoid the problems associated with thermal bypass is to install a continuous and robust air tight layer both inboard and outboard of the thermal insulation.

Thermal  bypass: the impact of natural and forced convection upon building performance.

Diagrams by Mark Siddall

We are happy to adopt Joe's admonition but we are compelled from experience to then add:   don't rely on foam to form your air control layers.   Foam installations are too often unpredictable and unreliable.  Be careful to choose materials that consistently and predictably achieve air tightness and maintain it for years to come.  Of course, we are happy to help.  Just drop us an email at

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