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

Air-Sealing and Insulating Floor Over Vented Crawlspace

JohnS415 | Posted in Energy Efficiency and Durability on

I have a 1959 building, with vented crawlspace. Zone 3M. Northern California.
Subfloor is T&G.
Looking to air seal and insulate the floor joist cavities.
Looks like that might be impossible because of all the gaps between each 2×8 T&G board.

I have a 10 MIL moisture barrier on the dirt crawlspace floor.

I was hoping to caulk the inside of the rim joist, and batt insulate the joist cavity. Is this worth it? Looking for something I can do myself, but also will actually be financially wise.

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. brian_wiley | | #1

    Have you considered sealing and conditioning the crawlspace at the walls instead of at the underside of the floor?

    It seems like it’d be less work as you already have the 10-mil vapor barrier on the floor. Assuming radon isn’t a concern, taping those seams and then adding rigid insulation on the walls would be a pretty decent way to go about things.

  2. Expert Member
    NICK KEENAN | | #2

    Joe Lstiburek's advice is here:

    For a vented crawlspace, he recommends polyiso foam attached to the bottom of the joists, with seams taped, as an air and moisture barrier.

  3. Expert Member
    BILL WICHERS | | #3

    You're much better off airsealing and insulating the WALLS of the crawlspace and not the floor of the rooms above. If you already have a 10 mil vapor barrier on the crawlspace floor, you're halfway to an encapsulated crawl space. Airseal and insulate your rim joist, then insulate the walls with rigid foam and you're good to go.

    Is there any particular reason why you'd want to insulate the underside of the floor instead of the walls of the crawlspace?


  4. JohnS415 | | #4

    The only reason I was going to insulate the underside of the floor, was because this is how I have done it in previous homes over 12 years ago. It also seems to be one of the recommended solutions by some of the fiberglass manufacturers. I suppose that is why I am asking the question here. I will take a look at the info here. Thanks.

    1. Expert Member
      BILL WICHERS | | #5

      It's usually easier to seal the perimeter walls instead of the floor, and you'll typically be able to do a better job that way too. The other issue with floor is that the batts inevitably fall out from between the joists over time and then you start losing insulating value.

      If you're able to do a crawlspace encapsulation style project and insulate the walls you'll end up with a better overall result, probably with less effort too, and it will hold up better over time.


  5. brian_wiley | | #6

    One thing to add to is that insulating at the walls will likely result in much less square feet of insulation being used.

    I recently conditioned my crawlspace at the walls. The house is 1285 sq feet, or roughly 32'x40'. The crawlspace walls are 2 feet high, so I ended up with 288 sq feet to insulate versus the roughly 1200 sq feet of the floor.

Log in or create an account to post an answer.


Recent Questions and Replies

  • |
  • |
  • |
  • |