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

Can closed cell spray foam be applied over dirt in a crawl space.

GBA Editor | Posted in Energy Efficiency and Durability on

I have done in energy audit on the home yesterday, that is 30 x 30 square. Half of it is a full basement with a concrete floor the other half is a crawlspace with dirt. The foundation walls are stone and cement, the customers complain of moldy smell mainly in the summer. There is no insulation in the floor joists, the house is very cold. Would you recommend spraying closed cell foam over the dirt in the crawl space to seal the moisture, also coming up the walls to the sill plate just in the crawlspace area? Then in the unfinished basement area do the ceiling? I appreciate your opinion.
Thanks Pete,

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.


  1. GBA Editor
    Martin Holladay | | #1

    Save the closed-cell spray foam insulation for the basement walls and the crawl space walls. The dirt floor of the crawl space should be covered with 6 mil poly or Tu-Tuf.

  2. Doug McEvers | | #2

    Go with Martin's suggestion even though the phony foamers may tell you otherwise.

  3. Expert Member
    Michael Maines | | #3

    Pete, to answer your question, there is nothing wrong with spray foaming the dirt floor. Here's an article explaining the process. But you'll get a lot more bang for the buck focusing on insulating and air sealing the walls and rim joists, and keeping groundwater away from the foundation.

  4. Riversong | | #4

    While you're at it, you may want to nail up an inch or two of XPS foam board under the floor joists, both to warm up the floor above and to decouple the framing from both the basement moisture and the radiant temperature of the floor. That should warm the framing and subfloor enough to keep the wood dry and mold-free.

    And don't forget to spray foam on the band joists to both air seal and insulate that all-important perimeter.

Log in or create an account to post an answer.


Recent Questions and Replies

  • |
  • |
  • |
  • |