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

Crawlspace insulation question

danielbuntic | Posted in Building Code Questions on

Hi, hopefully somebody can help.

I have 1978 house with a finished basement and i have one small section that’s crawlspace, the floors above the crawlspace in winter are cold, very cold. it had crappy fiber bats under the floor that were just hanging and were a mice superhighway. 

1. i removed all the old insulation.
2. insulated the rim joist with foam board (regular xps pink) will cover with ci max polyiso to meet code. apparently the pink has to be covered up but not the ci max/dow thermax.  
3. planning on using 2″ polyiso boards fire rated. I can only find Johns mansville CI max or Dow Thermax. Both are not easy to find, lucky i live in a metro area so i was able to find small qty x 10 sheats 4×8 with some home building companies, instead of home depot special order of 3 pallets, which was 9k$.
4. All will be covered with 15 mil poly liner and properly sealed/encapsulated

My question is, can the foam board be installed touching the ground or do i need to leave  a gap at the bottom and i understand in some states you need a termite gap 3″ or more on top.  

I checked with the local building  department (not very friendly people) and could not get a straight answer. They said they re going by the ,
2018 International Energy Conservation Code and 2018 International Residential Code 
I tried checking those references but i could not find anything about a termite barrier gap and ground clearance gap. There is a US termite map and it tells you which regions has what termites and ect. 

So pretty much do i need a termite gap and do i need ground clearance for the foam board or can it be installed touching the ground. Also the 15mil  poly liner would you install that over the polysio boards on the wall or underneath ?

My crawlspace walls are roughly 45 in tall, the crawlspace entrance is trough the basement. I live in Libertyville IL, (Chicago Suburb)

I attached 2 photos of a professional company installing these in ground contact and no termite gap. i belive they did this in northern Iowa , same climate as Chicago. 

Any help would be appreciated.

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. Expert Member
    Dana Dorsett | | #1

    >"My question is, can the foam board be installed touching the ground or do i need to leave a gap at the bottom and i understand in some states you need a termite gap 3″ or more on top. "

    The termite question is for your local inspectors to answer.

    It's fine to leave polystyrene in contact with soil, but not polyisocyanarate, which wicks moisture over time. A half-inch gap is plenty of capillary break for the polyiso though.

    If the ground vapor barrier laps extends up the foundation wall and up a few inches (a foot is good) and is sandwiched between your wall-foam and foundation, it's fine to let the polyiso rest on the vapor barrier, just not bare soil.

  2. danielbuntic | | #2

    perfect , thanks for your help, that was quick

Log in or create an account to post an answer.


Recent Questions and Replies

  • |
  • |
  • |
  • |