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Community and Q&A

Sub-slab vapour barrier over or under sill plate vapour barrier?

MichaelAnthony | Posted in General Questions on

Getting ready to install our sub-slab poly and realizing I have a few options for the intersection of all the vapour barriers at the wall sill plate. The slab is our finished floor over Comfortboard and we did ICF foundation. We ran a strip of 10mil poly under the sill plate (as pictured). We also require vapour barrier on the interior walls in my area.

Is there an advantage to positively overlapping the sill poly over the slab poly, or vise versa? A few options are:
1) sill poly positively overlaps the slab poly. I can get a really good bead of sealant and tape the joint below grade.
2) slab poly overlaps sill poly with tape joint near plate, still below grade. Similar to option 1
3) slab poly overlaps the sill poly and continues up wall for 3″. I can’t tape it to sill poly, but I can positively overlap the wall poly over the slab poly and seal and tape that at the sill plate, above grade.

Option 3 sounds like a tighter air seal at the plate. But we wouldn’t be able to use that same technique for the load bearing interior walls.
Any thoughts or unforeseen risks?

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