Vapour Barrier in residential details is missing? Also: Spray Foam
unhappy_meal
| Posted in Energy Efficiency and Durability on
Hi. Wondering why Vapour Barrier is often omitted in your detail library for cold climates, with batt/blown insulation usually drawn? See the attached example detail.
Also: what are your feelings for rigid insulation location (in the attached detail) if some roof rafters are replaced by hollow steel frame? Is “under” still better?
Thank you!
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Replies
I can't speak for GBA, but in most of the US interior side vapor barriers cause more problems than they solve, and it is not a code requirement. The NBC definition of "vapour barrier" is in fact a Class-II vapor retarder, not a true vapor barrier.
Having a sufficient ratio of exterior R/total-R is a more resilient method of preventing rot levels of moisture build up in the structural sheathing.
Steel framing is extremely thermally conductive relative to wood framing, severely robbing the cavity fill of it's potential performance. Continuous insulation layers (whether exterior or exterior) are thus higher performance than layers thermally bridged by the framing. If combined with fiber insulation located on the interior side of the assembly, any exterior rigid insulation needs enough R-value to provide sufficient dew point control at the foam/fiber boundary.