GBA Logo horizontal Facebook LinkedIn Email Pinterest Twitter Instagram YouTube Icon Navigation Search Icon Main Search Icon Video Play Icon Plus Icon Minus Icon Picture icon Hamburger Icon Close Icon Sorted

Community and Q&A

Vapor Retarder for Exterior Renovation

Chad Thiel | Posted in General Questions on

Hello GBA,

I live in Michigan (Zone 5 but near Zone 6) and have started an exterior renovation to my home. I plan to replace siding, exterior sheathing, and the cavity insulation as part of the renovation. The interior drywall will not be removed. I will be using R15 Rockwool Mineral Wool in the stud bays, Zip sheathing, 2″ polyiso rigid foam exterior insulation, 3/4″ rainscreen, and fiber cement siding. My question pertains to a vapor retarder, is the interior wall paint sufficient for a vapor retarder? My home was built in 1965 so there are many layers of interior paint. Would a better approach be to use a product like CertainTeed Membrain or would that add any risk? If this type of product is recommended, should I cut strips and stable between stud bays on interior side of wall? I’m assuming installing it over the studs from the exterior is not an option. Thanks

GBA Prime

Join the leading community of building science experts

Become a GBA Prime member and get instant access to the latest developments in green building, research, and reports from the field.

Replies

  1. Bryan Coplin | | #1

    According to the below, you have sufficient continuous insulation for zone 6, even derating polyiso for temperature:
    https://www.greenbuildingadvisor.com/article/calculating-the-minimum-thickness-of-rigid-foam-sheathing

    Ordinary latex paint should provide enough vapor control. If you are still uneasy, vapor retardant paint is available, and would be much less costly and labor intensive to retrofit.

    The use of exterior insulation can complicate flashing details, so pay close attention during installation. There is a far greater moisture risk from flashing defects than from inward vapor diffusion.

    https://www.greenbuildingadvisor.com/article/how-to-install-rigid-foam-sheathing

    https://www.greenbuildingadvisor.com/article/installing-windows-in-a-foam-sheathed-wall

    1. Chad Thiel | | #3

      I opted for the belt and suspenders approach and applied the retardant paint. Thanks so much. with respect to flashing, I plan to flash everything back to the ZIP sheathing.

  2. Jon R | | #2

    Don't neglect the testing that will verify your air sealing.

Log in or create an account to post an answer.

Community

Recent Questions and Replies

  • |
  • |
  • |
  • |