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Community and Q&A

Crawlspace Insulation: Floor and/or Walls

jcastilow | Posted in Energy Efficiency and Durability on

Looking to have spray foam applied in crawlspace. 1950s home with dirt floor crawl space.

Main concern with insulating walls is that poly vapor barrier provides little insulating value, and a lot of heat will be lost through the vapor barrier into the ground. How much of an issue is this? Will the floors in the house be noticeably warmer in the winter if we insulate the walls but not the floors?

Other suggestions? Thanks for any help!

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  1. Expert Member
    BILL WICHERS | | #1

    Usually only the walls are insulated. The poly vapor barrier doesn't really provide any insulating value at all, but the floor is typically warmer than the walls, so you loose less energy to the floor due to the lower thermal differential. If your crawlspace walls are particularly short, it's sometimes beneficial to continue the insulation from the footing out along the floor into the room for a few feet. If your crawlspace walls go down to the frostline or deeper, than insulating the walls is going to get you most of the benefit of insulating your crawlspace.

    I've never seen spray foam installed on a floor, BTW. That isn't to say it can't be done, but I think it would be tricky to get a nice layer. You'd probably be better off using panels of rigid foam on the floor instead. Ideally you'd use rigid foam on the walls too, unless you have an irregular surface (like a stone foundation), where spray foam is really the best option.


    1. willymo | | #3

      Bill are you refering to spray below the floor, or above? I have a basement w/ field stone foundation, that will never be used other than storage. The 1st floor flooring is COLD; flooring is single layer of fir. I am considering 1/2LB spray foam as a way to both insulate and air seal, as well as spray foaming the rim joist. Will cover the foam in the joist bays with rockwool.

  2. Jon_R | | #2

    > will the floors in the house be noticeably warmer in the winter

    Independent of insulation, you can heat an encapsulated crawlspace, providing whatever floor temperature you want.

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