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Existing poly vapor barrier in retrofitted wall assembly: What to do?

Ryan Breisach | Posted in General Questions on

We are the new owners of a 1975 home in a Twin Cities suburb (climate zone 6A). It is a two story home with a contemporary take on salt box style. It has T-111 siding over fiberboard (sometimes called Buffalo board up here) sheathing, fiberglass insulation, 2×4 walls, and poly as the vapor barrier (between the drywall and studs). There is no WRB.

We have numerous areas where water has gotten behind the siding and into the sheathing and plan to strip the house from the outside to the studs this spring, including new windows. Our plan is to use rock wool cavity insulation, plywood sheathing, a self-adhesive WRB as the air barrier, 3-4 inches of ComfortBoard exterior  insulation, a 3/4” rain screen, and likely Boral siding.

Is the original poly a concern in this assembly, given that the new assembly will be vapor permeable? If it is a concern, what would you recommend doing to the poly and how would you install a vapor barrier from  the outside while keeping it on the warm side of the house? We are trying to avoid spray foam if possible.

Ryan, Minnesota

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  1. GBA Editor
    Martin Holladay | | #1

    Q. "Is the original poly a concern in this assembly, given that the new assembly will be vapor- permeable?"

    A. No. Proceed with your plan without further worries.

  2. GBA Editor
    Peter Yost | | #2

    Hi Ryan -

    Note that the new assembly you have specced in your question has excellent drying potential to the exterior and hence no concern with the Class I vapor retarder, the 6-mil poly.


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