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

Basement: Whether/how to insulate

Siffe | Posted in Energy Efficiency and Durability on

Hi, I’m a consumer building in Zone 5A (NC mountains, elev. 3700′). I’ve read a lot on here about insulating basements, and the more I read the more confused I get! The questions are should I insulate, and if so, how.

Background: half of the house will be over basement, likely unfinished, probably unconditioned. One wall is fully below grade, two walls are mostly below grade, and the fourth wall is fully above grade. The basement will contain HVAC equipment (HP or mini, ducts), water heater, etc. The other half of the house will be on slab-on-grade.

1. I don’t want any sort of foam in the interior (too many off-gassing horror stories). Are there other options with no framing on the walls? I know about Thermax, which is foil-lined, but it still worries me.

2. If I insulate the exterior, what’s the best way to cover the insulation which is above grade?

3. Should I insulate the basement ceiling? If so, would blown-in fiberglass work there?

4. Should I insulate the basement walls at all? Just the ceiling? From what I understand, it may not be worth the expense in terms of energy savings if the basement is unconditioned.

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

    Q. " I don't want any sort of foam in the interior (too many off-gassing horror stories)."

    A. I have never heard a single "off-gassing horror story" about any type of rigid foam -- only about spray foam. However, if you are determined to keep all foam away from the interior of your house, then you should insulate your basement on the exterior.

    Q. " If I insulate the exterior, what's the best way to cover the insulation which is above grade?"

    A. That question is answered at length in my article, How to Insulate a Basement Wall.

    Q. "Should I insulate the basement ceiling?"

    A. No. Since you have a walkout basement that includes HVAC equipment, it's important for you to insulate your basement walls. Once you've insulated your basement walls, there is no need to insulate your basement ceiling.

    Q. "Should I insulate the basement walls at all?"

    A. Yes, you should. Especially since you have a basement with one wall that is fully exposed. Much of the heat produced by your water heater and HVAC equipment will be conditioning your basement whether you want it to or not. You need to keep all your equipment within your home's conditioned space -- and your basement will be losing a tremendous amount of heat to the outdoors during the winter.

  2. Siffe | | #2

    Thanks, Martin. Your table says R-10 for Zone 5, but should the R value be higher for the above grade walls?

  3. GBA Editor
    Martin Holladay | | #3

    According to the 2009 International Residential Code, the minimum R-value for above-grade walls in your climate zone is R-20 (or R-13 plus R-5 exterior rigid foam).

    Since this is a rather basic question, I'm beginning to wonder whether your project has an experienced professional involved. Most builders are aware of code requirements for insulation; if your builder isn't, that's worrisome.

  4. Expert Member
    Dana Dorsett | | #4

    Most insulated concrete form (ICF) vendors start at ~R20 these days, (some go as low as R16) and often worth the premium paid from a "time is money" point of view on new construction. Most ICFs are made from expanded polystyrene beads (EPS), which is blown with pentane. The pentane encapsulated by the closed cells of the EPS during that process is highly volatile, and off-gasses VERY quickly, and is more than 99.9% displaced by air within the first month of manufacture. By the time it's shipped to your site it's at undetectable levels.

    If your area is in a high termite-risk area you may have to use select compacted sand backfill to keep termites from accessing the foam as a tunneling highway, and use metal flashing between the foundation sill that extends over the foam on both the interior & exterior, but in most places those measures would not be necessary.

    Putting R5-R7.5 (either EPS or extruded expanded poly styrene aka "XPS"- the pink or blue or green or gray stufft that has no obvious bead-structure) under the slab is also advisable, since that keeps the temp of the slab close to the room temperature. If you don't take that measure the slab is likely to remain below the outdoor air dew point in summer, which can create mold conditions at the finish-floor or the bottoms of boxes, etc. stored on the floor.

    Even if it doesn't have short term "paypack" on a net-present-value of heating season savings, the sub-slab R counts for both comfort and lower mold risk, lower risk to stored goods, etc. It also saves on the power required for mechanical dehumidification, since you can keep it at 60% RH without mold risk rather than ~50%, a factor rarely accounted for in the financial analysis. It may only be $15-25/year on dehumidifier- power savings, but $25 pays for about 30-40 square feet of sub-slab insulation. Add that to the also marginal heating savings and it's pretty easy to make a longer term financial argument without factoring in the damage to stored goods or co-pays for the allergy-doc visits, etc.

    Avoiding the "musty basement" aspects of uninsulated basements is argument enough for me, YMMV.

  5. Siffe | | #5

    Dana, thanks for your reply.

    Martin, our builder has been doing Energy Star homes for years. He knows his stuff; I am just trying to get myself more educated before we get into those sorts of details. Thanks!

Log in or create an account to post an answer.


Recent Questions and Replies

  • |
  • |
  • |
  • |