Install vapor barrier or not?
I am in the process of restoring a 100+ year old farmhouse, which mostly entails finishing the second story which was not winterized when we bought the house (we live in the upper midwest, zone 6/7). The floors of this second story were insulated in the 1950’s with vermiculite and fiberglass batts; I have since removed that insulation and am now focusing on insulating the knee-walls and ceiling of the second story.
The original construction is 2X4 with shiplap exterior siding and then bevel pine siding. no insulation or interior siding currently.
So far on the knee-walls I’ve added 2×2 stock to the 2×4’s to make a deeper cavity for insulation; I’ve added R13 fiberglass within the cavity and have installed 2″ rigid foam between the studs.
For the ceiling I’ve added the 2″ foamboard between the rafters and overlayed it with two layers of R19 fiberglass in a crisscross fashion in the attic space above.
The roof is vented with soffit vents and a ridge vent.
My question is whether to apply the 6 mil poly across the face of the ceiling/walls now before I install interior siding, which will be wood siding (cedar, pine, mixed hardwoods)
I don’t believe I have a completely air-tight construction with what i’ve done so far, so adding the poly would help maintain air tightness. But I also don’t want to hinder drying to the interior if I do get moisture in the wall cavities and attic space.
Your feedback is much appreciated
thanks
Levi
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Replies
Thots.
Roofs leak.
An axiom of roof ownership.
H2O will find its many ways.
Your best allies are drying and knowing.
The xps is functioning as exterior insulation, Class 2 vapor retarder (2” produces 0.75 perm.) and weather resistive barrier, though it be in 14” swaths.
Optimize the leak resistance, and encourage any fugitive water to drain out so it doesn’t backwater in the soffit, but spills out your soffit vents.
ABTG’s “Wood Wall Calculator” can help you identify and optimize viable strategies for the space available.
Adding any membrane for added water resistivity doesn’t sit well.
To control indoor moisture you can use vapor retarder paints to get a semi permeable to middling semi impermeable Vapor retarder on your walls. I recollect both SW’s and BM’s are self-priming.
Good luck
Levi,
An air-barrier behind wood covered walls is a must. The ideal solution would be a variable-perm membrane, but I'd have no qualms about using poly either. In a heat dominated climate like 6 or 7 the vapour drive is to the outside, and with a vented roof that's the direction drying will occur.