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

Closed-cell spray foam

michemez | Posted in Energy Efficiency and Durability on


We are considering putting closed cell spray foam based in our unvented crawl space based on an energy audit. I got a second quote and the numbers vary wildly and the only difference is the depth of the foam (which makes sense more foam = $). How do I know what is the correct thickness of foam? One quote is for 2 inches on the ceiling and 1.5 inches on the walls and the other is for 3 inches on both. I don’t want to waste money on 3 inch if it isn’t necessary but I don’t want to not see a significant change due to too little spray foam being used. We are in Long Island NY.

Thank you in advance.

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

    You are in Climate Zone 4. The minimum code requirement for insulating basement walls and crawl space walls in your climate zone is R-10.

    I think it's a good idea to assume that closed-cell spray foam has an R-value of R-6 per inch, although many spray foam installers claim higher numbers like R-6.5 per inch. Most code inspectors would probably accept 1.5 inch of closed-cell spray foam as minimally adequate in your climate zone, but the choice is yours.

    Thicker insulation is better than thinner insulation, of course, because it reduces the rate of heat flow through the crawl space walls.

    There is no need to install any insulation on your crawl space ceiling, so you can save some money by omitting the ceiling insulation. For more information, see Building an Unvented Crawl Space.

  2. Expert Member
    Dana Dorsett | | #2

    If the foundation is insulated to IRC code minimums there is no requirement for insulating the crawlspace ceiling (the subfloor of the room above), which may be the lion's share of those quotes!! That part would only need to be insulated if it were a VENTED and UNINSULATED crawlspace. In other words, as long as the foundation walls are insulated from the ground or rat-slab all the way up to the subfloor, the rest of the subfloor doesn't need ANY insulation.

    Long Island is US climate zone 4A. IRC 2015 code minimum for foundation walls calls for R10 minimum in your climate zone, which would be 1.5", for most closed cell foams, but in nearby zone 5A it would be R15 , which would be more than 2".

    NY code may require more, but I didn't think so.

    Some closed cell foams foams are rated ~R6/inch , others ~R7/inch. The "greener" foams are blown with fairly benign HFO1234ze rather than HFC245fa (an extremely powerful greenhouse gas), and run about R7/inch. So 2" of HFO blown foam is pretty close to the amount needed for zone 5, and 1.5" would be good enough for zone 4. Heatlok HFO High Lift, or Lapolla Foam-Lok 2000-4G are 2 such products. There are others, but you have to ask.


    The crawlspace floor should have a heavy vapor barrier (EPDM or heavy polyethylene) that goes all the way to the foundation wall, and up a bit, so that it will be sealed air-tight when the spray foam goes over the portion going up the wall.

  3. michemez | | #3

    Hi Martin-

    Thank you for the information. That definitely cleared it up for me and makes me a little suspicious of the guy that said he was putting 1.5 inch and said it was R13. I'm just curious why I wouldn't want to spray the ceiling. Should I put any insulation in the ceiling? The current problem is that all the rooms above the crawl are very cold.

    Thank you

  4. GBA Editor
    Martin Holladay | | #4

    The reason that you are installing insulation on your crawl space walls is to reduce air infiltration and to bring your crawl space inside your home's thermal envelope. Once the wall insulation is installed, your crawl space will be warmer -- and so will the rooms above the crawl space.

    Since your sealed crawlspace will be inside your home's themal barrier, there is no need for insulation on the crawl space ceiling. The ceiling will separate two areas that are both "indoors." Since both areas are indoors, they don't need to be separated by insulation.

  5. michemez | | #5

    The space is unvented and floor is dirt. I inquire about the vapor barrier. Neither contractor mentioned that.

    Thank you

  6. GBA Editor
    Martin Holladay | | #6

    I urge you to read the article I linked to ("Building an Unvented Crawl Space").

    The article gives lots of details, and should answer your questions.

Log in or create an account to post an answer.


Recent Questions and Replies

  • |
  • |
  • |
  • |