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Community and Q&A

Insulating Crawl Space with Existing Vapor Barrier/Radon Mitigation

alex39 | Posted in GBA Pro Help on

Should the existing vapor/radon barrier be removed, rigid insulation added to the walls and earth floor, and then reinstalled (on the ‘warm’ side)? Or would rigid insulation with taped seams be adequate from a radon sealing perspective? Would it need to be foil faced insulation from a fire rating perspective since it will be left exposed (as opposed to covering with drywall)? The rim joist will be closed cell spray foam (existing batt insulation to be removed). 

The goal is to insulate the floor and perimeter walls of the crawl space to improve the climate for the finished space above. All of this while keeping radon in mind and avoiding a ‘double vapor barrier’ that could potentially cause moisture issues. The other goal is to then add a floating osb subfloor to make sure space more conducive for storage. 

Details:

This is a crawl space below 4 bedrooms and 2 bathrooms in a 1960’s single story ranch home. Southeast Wisconsin, located in climate zone 6A (south end of it). The crawl space is approximately 5 ft in height and the perimeter walls are 12″ CMU without insulation on the interior or exterior. Of the 5 ft in height, approximately 4 ft below grade with the rim joist at/above grade. One side of the crawl space is directly adjacent to a full height basement and is accessible through two, 4’x5′ openings. HVAC duct work uninsulated. Being essentially open to the adjacent full height basement that is partially finished, I would consider the crawl space a ‘conditioned’ space. It is my understanding the existing plastic was installed to serve both as a vapor barrier and radon mitigation (there is interior perimeter drain tile and a sump).

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Replies

  1. FrankD | | #1

    A regular foil facing doesn't provide fire protection. There are one or two brands of polyiso (ex. Thermax) that have a built-in ignition barrier, so they can be left uncovered in a crawlspace that is accessed only for maintenance. However, since you want to use the crawlspace for storage, you will need what the building code calls a thermal barrier, such as 1/2" drywall or 3/4" plywood or OSB.

    I think you will get more bang for the buck by putting all the foam on the walls rather than splitting it between walls and floor. You would install the foam board over the vapor barrier. It would probably be best to use unfaced EPS or fiber-faced polyiso, and then a paperless drywall for mold resistance.

    If you decide not to use the crawlspace for storage, then you could use Thermax and tape the seams. You would probably have to close off the openings to the basement though. If you have two or more layers of foam, you only need to use Thermax for the last inner layer.

  2. alex39 | | #2

    Thanks FrankD for the input, much appreciated. I read a lot of great articles/links from the GBA website regarding closed crawl spaces, but couldn't find specific guidance regarding adding insulation to an already sealed crawl space.

    In short.......basically leave the existing vapor barrier, apply rigid insulation over on the walls, and cover the insulation with a thermal barrier since I want to use the space for storage.

    Any reason not to use XPS vs EPS or polyiso?

    1. FrankD | | #3

      You could use XPS too. But for the same R-value, you'll likely find that EPS is less expensive, and it also has less environmental impact.

      1. alex39 | | #4

        Got it. Thanks!

    2. Expert Member
      BILL WICHERS | | #5

      Reclaimed polyiso is the cheapest/best way to go here if you have that option available. I used what is basically "slightly irregular" fiber faced polyiso (which I got cheap from a place that sold manufacturer seconds polyiso), then painted the interior with fire retardant paint which was sufficient for my purposes here.

      Attach the insulation using Plasti-Grip insulation anchors which are easy to install here: just place the polyiso against the wall, drill through the polyiso into the wall, then tap in an achor through the hole and into the wall. Works great, and it's relatively fast and easy to do.

      Bill

      1. alex39 | | #6

        Good ideas. Thanks for the input Bill!

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