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 a Slab Foundation in Mild Climate

benneaf3 | Posted in General Questions on

I’m building a little rental house and I’ll probably do a slab foundation as opposed to the crawlspace foundations that are mostly used in the area.  I’ve read all about insulating slab foundations in cold weather areas, but I’m in East TN, just below Knoxville.  We tend to have mild winters and probably have as many cooling days (or more) than heating days…although this time of year a given day can be both a heating and cooling day…or one type of day following another…I ramble and digress.

I know I need a good layer or two of taped 6 mil plastic to keep moisture out of the slab, but in my area do I need any insulation at all? 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. Expert Member
    ARMANDO COBO | | #1

    I suppose it depends on how EE you wan to be, and if you live in CZ3 or CZ4. On CZ3 the 2015-2018 IRC requires no slab insulation, but R10 for 2 ft in CZ4. On the 2021 IRC, CZ3 requires R10 for 2 ft and R10 for 4 ft. Read up on 2015-2018 IRC Table N1102.1.2 (R402.1.2) or 2021 IRC Table N1102.1.3(R402.1.3).

    1. benneaf3 | | #2

      I looked at the table and honestly am not sure I am deciphering it correctly. For C4 (my climate zone) it shows 10ci, 4ft. From the notes I interpret the 10ci as R10 and 4' deep. That just seems nuts. I built a SIPS house back in 2009 and had the footings engineered and they said they really over did it and those footings where no where near 4' deep. That house was on a crawl space, not a slab though. However, I would think a slab wouldn't have to go as deep for the footer as a crawl...

  2. Expert Member
    ARMANDO COBO | | #3

    You can take it up with ICC. There's been 4 code changes since 2009, all increasing EE requirements, so I'm sure if you haven't kept up with code, I can see that it could be a shocker to you.
    I've never done a frost-protected shallow foundation, but you could check into that. The code shows a prescriptive way to do it. Maybe someone else here at the GBA with FPSF experience can give you a hand. Good luck.

    1. benneaf3 | | #4

      Thank you

Log in or create an account to post an answer.


Recent Questions and Replies

  • |
  • |
  • |
  • |