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

Walkout Basement Vapor Barrier

Martin_S | Posted in General Questions on
We are looking to finish our unfinished new construction house’s (in the past year) walk-out basement. We’re planning to put a bathroom, bedroom, kitchenette down there (think mother-in-law unit). The sidewalls are half poured concrete and stick built above. The walk-out wall is all stick built. The home is in Zone 6.
I’m thinking of cladding the walls in shiplap panels instead of drywall for easier DIY. Now, I know vapor barriers are not right for below-grade concrete walls as the water vapor can come through the concrete (I’ve been looking at the great resources here). There is a vapor barrier on the above-grade stick-built sections. Is the painted shiplap going to prevent the vapor from dissipating more than painted drywall would (thus preventing the basement from drying inward)? My concern is mold which my wife is very allergic to.

A related question: We are seeing some mention in building journals that its no longer recommended to have a plastic vapor barrier (except in Alaska/Canada) as during humid summer days when running the AC the cold air hitting the warm wall assembly can create moisture which is then trapped between the vapor barrier and the wall covering.  So should I remove the vapor barrier that is on the upper stick-built sections and the walk-out wall that is all above grade?

thank you,
Martin

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. Expert Member
    Michael Maines | | #1

    Martin, it sounds like you're planning to put wood directly over the concrete walls without insulation? That would be a big mistake. You should insulate the interior of the concrete walls with foam. I generally avoid foam when possible for health and environmental reasons but I always use it in finished basements. In climate zone 6, the 2021 IRC model code requires R-15 continuous insulation (or R-5 continuous insulation with R-13 "cavity" insulation--installed between studs.) I usually use rigid foam in the form of 3" foil-faced polyisocyanurate, or sometimes closed-cell spray foam. As a renovation, you probably won't be required to do that, and you may not even be planning on getting inspections. But it's still important to insulate the concrete for building science reasons.

    The first is heat loss, which is an economic and environmental issue, but also a comfort and health issue. If you don't insulate, moisture will come through the concrete AND the walls will be cool, and you will very likely end up with condensation and mold. If you do insulate with foil-faced polyiso, moisture won't come in from the exterior and the interior surfaces will be warm; you are unlikely to have mold and the space will be more comfortable because you won't be losing body heat through the air to cold walls.

  2. Martin_S | | #2

    Hi Michael,

    Thanks for replying. I'm definitely not going to put wood over the concrete. The posts and articles here have been a great help in making a plan. I'm planning rigid foam over the concrete and then adding a stud wall over that, insulated with something like rock wool or denim insulation.

    What do you think about shiplap vs drywall for the cladding over that stud wall?

    thanks,
    Martin

    1. Expert Member
      Michael Maines | | #3

      Sounds like a good plan. If your interior relative humidity is controlled, either drywall or wood boards will be fine.

Log in or create an account to post an answer.

Community

Recent Questions and Replies

  • |
  • |
  • |
  • |