GBA Logo horizontal Facebook LinkedIn Email Pinterest Twitter X 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 Crawlspace Floor?

tdbaugha | Posted in Energy Efficiency and Durability on

Planning a pretty good house build. R30 walls, R60 lid, European windows and doors, etc. Seems pretty common to insulate your slab in homes with basements, but not common whatsoever to insulate the floor of a conditioned crawl space. Climate zone 6, crawl will be ~48” deep. My current crawl that I built 2 years ago has a 12mil reinforced poly on the ground and then 3” CCSF on the walls and rim joist. The space is not cold at all and I only supply a trickle of conditioned air to the space. How much heat am I losing to the ground with zero insulation? Theoretically a lot, delta T of approximately 20 degrees F and essentially zero R value of sand at .1/inch. Take a U value of .5 and you’re losing 20,000 BTU/hr in a 2000 sqft crawl space (.5*20*2000). That doesn’t seem to match reality so what am I missing?

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
    Michael Maines | | #1

    Sand actually has some insulating ability, roughly R-0.5/in, and it will absorb heat so your delta T is probably less than 20°F. Beyond that, there are a lot of variables that make it hard to accurately calculate heat loss to the soil. The downside to insulating the first floor is if that leaves your crawlspace cooler, which could increase the risk of moisture accumulation. Why not just put some mineral wool on the crawlspace floor and leave the space fully coupled with the main level?

  2. brendanalbano | | #2

    When I encapsulated my crawlspace, the company put down a thin layer of foam below the vapor barrier. It was 3/4" thick and branded "TerraBlock". Probably R-3 or so. It's really squishy and flexible, so they could roll a big sheet of it up and fit it through the tiny crawlspace access hatch. I'm not sure how much it's really doing for energy efficiency, but it certainly makes crawling around down there much more comfortable because the earth is padded!

  3. tdbaugha | | #3

    Not sure why the title was renamed “slab”? There is no slab poured in crawl spaces in my market, nor do I see the reason for one. Typical detail is to simply lay down 6mil poly and call it a day.

    The delta T is definitely 20ish degrees, just depends where that temperature transition is. If it’s 45 degrees 20” below the surface, then you could say I have 20” or R2 of insulation. If it’s 45 degrees only 2” below the surface, then there is essentially zero insulation. My google searching showed sand at R0.1. I supposed if you had a layer of pea stone it would be better since it has more voids? Maybe there is a “heat loss to the earth” formula somewhere?

    I’ll have to check out the TerraBlock, that sounds cool!

Log in or create an account to post an answer.


Recent Questions and Replies

  • |
  • |
  • |
  • |