GBA Logo horizontal Facebook LinkedIn Email Pinterest Twitter Instagram YouTube Icon Navigation Search Icon Main Search Icon Video Play Icon Audio Play Icon Headphones Icon Plus Icon Minus Icon Check Icon Print Icon Picture icon Single Arrow Icon Double Arrow Icon Hamburger Icon TV Icon Close Icon Sorted Hamburger/Search Icon

Community and Q&A

Minimum foam thickness for basement floor

Blutowski | Posted in General Questions on

I live in zone 5, Western PA and will be finishing part of my basement. I have very little wiggle room to maintain code height for the finished assembly. My plan is to use glued and taped olyiso with two floating layers of 7/16 OSB staggered with a floating floor on top. Can I get away with 1/2″ foam and likely avoid summer condensation issues?

And though I don’t want to use cork for my basement floor, would this help enough to make up for the cost differences of cork flooring? I could increase the foam thickness to 3/4″, but no more.

Any advice 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. User avater
    Dana Dorsett | | #1

    Don't use polyiso on the slab under a subfloor. It's slightly hygroscopic, with the potential to take on water semi-permantently in that stackup. Use EPS (or XPS, if you must) instead.

    I'm not clear on what "...would this help enough to make up for the cost differences of cork flooring? " means.

    The cheap napkin math analysis: A half inch of EPS or XPS is about R2, which is still enough to make a difference in how much humidity the subfloor takes on. Your deep subsoil temps are about 50F, the summertime room air temp is likely to be 70F or warmer 3' from the floor, maybe as low as 65F at the floor. The dew point of 70F, 60% RH air is about 55F, the dew point of 70F 50% RH air is about 50F. As long as the dew point of the basement air stays below 60F the wood won't get saturated-wet. With ~R1 of subfloor and ~R2 between the subfloor & slab the bottom of the subfloor would run no cooler than ~60F.

    So as long as you dehumidify the basement to something less than 60% RH the risk to the subfloor from summertime humidity is low.

    With 3/4" of EPS you'd be at about R3, which would keep the bottom of the subfloor a bit warmer, and give you a bit more margin.

  2. Steve Knapp CZ 3A Georgia | | #2

    You have to moisture issues in your basement, correct? You are considering carpet or cork as flooring options, correct?

  3. Blutowski | | #3

    Thank you Dana. This is reassuring; I could go 3/4" and feel pretty ok about it. Regarding the cork, I was thinking that the cork may add insulating capacity, though perhaps not by enough to make it worth the higher cost of the cork. Or maybe it being vapor permeable means it's not really helpful?

    The floor is dry, and the only carpeting I would consider is an area rug.

  4. Nick Welch | | #4

    Very timely, as I am currently agonizing over whether I should use 1/2" or 3/4" or 1" EPS in my basement floor. The stackup will be:

    * Delta FL (let's say it's R-0.5 as a reasonable guess)
    * EPS (1/2", 3/4", or 1", i.e. R-2, R-3, or R-4)
    * 3/4" OSB (R-0.9)
    * Various/undetermined finished flooring materials - let's say R-0.5 as a baseline

    So that's a total of R-3.9, R-4.9, or R-5.9.

    I'm in zone 4C, Portland OR, and I'm guessing my sub-soil temperatures (the slab is at about 6-7ft below grade) are 50F or maybe 55F if I'm lucky.

    What's the difference in underfoot temperature given my different potential foam thicknesses? Or how do I go about calculating it myself? We generally wear socks around the house and the current slab temperature is definitely uncomfortable in winter. I'd like to make it comfortable but I'm also fighting against ceiling height (7' from slab to joists) so I feel a lot of pressure to minimize the floor height build up. I may even have to trim doors/jambs/casing with the thicker foam.

  5. User avater
    Dana Dorsett | | #5

    Blutowski; Putting insulating flooring materials such as cork or carpet ABOVE the subfloor is the opposite of what you want to do, since that lowers the temperature of the subfloor, raising it's average moisture content.

    Nick Welch: In Portland OR outdoor dew points in summer average well below 55F even during the most-humid weeks of summer:

    So in your case it doesn't take a whole lot of R under the subfloor to keep all the wood above 55F, mitigating the mold risk. Even the Delta-FL(or some other vapor barrier) would probably do it (and no foam) from a moisture control perspective.

    But even R2 under a subfloor + 1/2" of wood laminate flooring is a huge improvement in barefoot comfort in a 65-68F room compared to 52-55F concrete. Still, in zone 4C installing R6-R8 is still financially rational on energy use savings over the full lifecycle of a house. It's not a huge uptick in barefoot comfort from R2, but some. If you have enough headroom for an inch, use it.

    Is your basement wet enough to really NEED the Delta-FL (instead of a polyethylene or EPDM vapor barrier)? Unless your slab looks visibly wet or you have a history of seepage that would require a drain path over the top of the slab you can probably skip it, and use that space for another R1 of foam or another 1/4" of headroom.

  6. Nick Welch | | #6

    Yeah, unfortunately, there are still two occasional small dribbles when it rains heavily. One is from the chimney which I'm actively working to diagnose and fix. The other only happens when a downspout extension occasionally falls apart -- I need to get around to securing it better. So while I should be able to resolve these last remaining sources of dribbles, I wouldn't feel confident enough to rely on them never happening again. I will sleep much better eating that 1/4" for resiliency's sake.

  7. User avater
    Dana Dorsett | | #7

    Keep the Delta-FL!

    Don't sweat the summertime moisture accumulation issue- that's pretty much a non-issue in the PNW, even though it's very real issue on the right-half of the US map. (I had no clue what summertime humidity was about when I first left the Puget Sound region for the midwest & further east!)

    Cold feet & energy use issues are the same everywhere the ground temps are below 65F.

  8. Nick Welch | | #8

    Is there a generic way to calculate the steady-state temperature of an indoor surface, given I know the indoor/outdoor temps and R-value of the assembly?

Log in or create an account to post an answer.


Recent Questions and Replies

  • |
  • |
  • |
  • |