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

Insulating ground floor of closed crawl space?

Debra_Ann | Posted in Energy Efficiency and Durability on

My contractor wants to insulate the floor (ground) of our closed/conditioned crawl space, in addition to the foundation walls. I have never seen that recommended anywhere, and doubt the cost/benefit ratio would be worth it. Anybody know of any situations that would warrant this?

This would be in climate zone 4a, in a mixed humid zone. As we don’t plan to use a lot of AC, I would like to seal the crawl space from the house air and use a dehumidifier down there instead. Otherwise, we would be introducing unconditioned air into the crawl space. But I’ll need to discuss this with the building inspector. Thanks for any thoughts/suggestions.

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. user-2310254 | | #1

    So the crawlspace does not have a concrete "rat" slab. Is the soil covered with heavy-duty liner (sealed to the stem walls)? What type of insulation are you using on the foundation?

    You are typically required to install a register or forced air vent or exhaust fan to condition the space.

  2. Debra_Ann | | #2

    No rat slab. Properly sealed heavy duty liner, and R10 foil covered foam boards on foundation. I'm in communication with the building inspector about options for conditioning the space.

    My main question is regarding insulating the plastic covered floor of the crawl space. I've never heard of that before.

  3. 382 | | #3

    For the same reason you have insulated the stem walls, specifically that portion below grade, you need to insulate the ground . A sealed crawlspace may eliminate the moisture problems associated with vented crawlspaces but it is still unconditioned space. Likewise, you could place insulation between the floor joists.

  4. user-2310254 | | #4

    I just reread Martin's article on unvented crawl spaces (, and he notes that ceiling insulation is not necessary. I did not see anything about insulation on top of the ground/liner but suspect it will not provide much value. You might want to check out the article. The sidebar also includes links to other crawl space related information.

  5. GBA Editor
    Martin Holladay | | #5

    I disagree with Jason. In your climate zone, there is no way that energy savings resulting from the installation of rigid foam insulation on a crawl space floor will ever be high enough to justify the cost of the insulation -- not in 200 years. All you need is the vapor barrier (polyethylene) over the dirt.

    For more information on this topic, see Building an Unvented Crawl Space.

  6. 382 | | #6

    Martin, what are your thoughts on insulation between the floor joists?

Log in or create an account to post an answer.


Recent Questions and Replies

  • |
  • |
  • |
  • |