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Community and Q&A

Do I really have to insulate my basement slab?

alex9999999 | Posted in General Questions on

I am finishing my 1940 basement, with 2 inches of rigid foam on the walls, covered with sheet rock.

My question: should I really insulate the floor? If so, is it avoid condensation on the slab, or because of heat loss?

There’s not a lot of headroom, which is why I’m reluctant to insulate the slab. And if I do, can I get away with 1/2 rigid foam instead of 1″?

Thanks much.

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  1. user-2310254 | | #1

    Where are you located? Are you planning to install flooring on the slab?

  2. ohioandy | | #2

    Do you "really have to" insulate? Well, no, unless some municipal official is looming over your project with a codebook, you don't "have to" do anything. "Should" you insulate? Absolutely, yes, you should. It will make your floor a better floor in all respects, particularly in the two ways you mention, but also it will be more comfortable underfoot, more attractive, and probably reflect very well on your home equity. But it costs money, it uses a whole bunch of materials made from chemicals (OSB, foam, adhesives, etc.) and it pushes your head closer to the ceiling. Only you can weigh out this decision.

    If the present 70-year-old concrete is in good shape, it probably makes the most sense to simply paint it and use area rugs for comfort and warmth. Similarly, if you have even a teeny bit of water entry trouble that might still happen in the future, you don't want to encapsulate bulk water under a new floor.

    If you elect to go ahead and do a new floor, you may as well go with at least an inch of foam, since it costs very little extra, but the extra R-value makes the assembly more effective, and an extra half inch of height won't make a noticeable difference.

  3. alex9999999 | | #3

    Sorry, good questions: I’m in Western Massachusetts, and if I do insulate the slab I will install flooring of some kind, (maybe just plywood); if I don’t insulate the slab I would paint it.

  4. alex9999999 | | #4

    Thank you Andy for that very intelligent answer. The headroom issue, the possibility of water intrusion and the construction costs all have me inclined to go with paint and area rugs.

    But my worry is I'll still have a freezing cold basement. So I'm left with the question: How much of my basement's coldness is a result of the (mostly below grade) walls being uninsulated, and how much is a result of the floor being uninsulated? In other words, Is insulating the walls but not the floor a pretty good way to address the coldness of the basement, or a totally useless way, or somewhere in between? Does anyone know of a building-science study on the thermal benefits of slab insulation vs. basement wall insulation?

  5. ohioandy | | #5

    Alexander, your question can be answered by various methods of modeling the heat loss through any type of wall, and through the floor. It's discussed on this site at length, with various DIY tools, or hire someone with expertise. But that's all academic.

    Your basement floor is not a significant source of heat loss, especially when compared to basement walls. If you're taking the time to properly insulate your basement walls, including the requisite airsealing of the rim joist area, and upgrading any windows, you will find this space VERY easy to heat, whether you add insulation to the floor or not.

    Is you heat source already in place? If not, go ahead and do the energy modeling of the space; it will give valuable guidance in getting that part right.

  6. GBA Editor
    Martin Holladay | | #6

    You've gotten good advice. You'll never see enough savings due to reduced energy consumption to justify the investment in floor insulation.

    The only reasons to consider the insulation are (a) comfort, (b) reduction in summertime condensation, and (c) a lower likelihood of having a musty smell due to slab dampness.

  7. alex9999999 | | #7

    Thank you, thank you Martin and Andy. Any idea if you can put something like luxury vinyl tile DIRECTLY over 1/2" or 1" of rigid foam? I would like a softer, warmer floor but I just don't have the space or the wherewithal for a plywood subfloor, especially two layers.

  8. GBA Editor
    Martin Holladay | | #8

    If you want to install floor tile directly to the rigid foam, you will need to install a material called polystyrene tile backerboard (usually used for showers). Brands include FinPan ProPanel Lightweight Waterproof Backer Board, Schlüter Kerdi-Board, and Wedi Building Board.

    The product isn't cheap.

    If you need more information on flooring substrates, check the manufacturer's installation instructions for the brand of flooring you want to install. These installation instructions are usually available online.

  9. alex9999999 | | #9

    Oh wait, the No Mold Finished Basement article says vinyl is a bad choice because it traps moisture. And I was starting to think it would be a good choice because it does not support mold growth.

    The floor issue is a struggle. I want something that will be ok, or easily replaced, if exposed to some moisture, because I think it will be inevitably, whether from seepage or condensation or a plumbing leak. Any suggestions?

  10. Expert Member
    Dana Dorsett | | #10

    If you're putting (moisture suceptible) wood on the slab the case for insulating the slab goes WAY up. Summertime outdoor dew points (and your ventilation air) in MA average in the mid-60s F, whereas your subsoil temperatures in western MA are in the ~50F-ish range. Even the R0.5 of a sheet of plywood is enough for the bottom of the plywood to dwell at low enough temperatures to accumulate moisture.

    And even R2 of half-inch EPS is enough to keep it warm enough, as long as you aren't putting a thick carpet with underlayment above the wood.

    Vinyl applied to sealed concrete is OK. It's fine if the concrete get's saturated wet- it can take it (otherwise most bridge and house foundations would have failed by now.) But ceramic tile would be better- thinset works just fine when wet and doesn't outgas VOCs, unlike the adhesives used with vinyl flooring.

  11. alex9999999 | | #11

    Thanks Dana; I was thinking about interlocking, floating vinyl tiles....decent choice?

  12. Expert Member
    Dana Dorsett | | #12

    I don't have any experience with the interlocking floating vinyl tile products. I would think it would require an extremely flat slab to work without adhesives, but don't really know.

  13. Expert Member
    MALCOLM TAYLOR | | #13

    Alexander and Dana,

    I put down (the annoyingly named) Luxury Vinyl planks on a very poorly finished slab about two months ago. They initially showed some lipage, but it seems to have almost completely gone away as they settled. I did put dabs of adhesive on the perimeter rows, but just enough to keep them in place until the baseboards went on. They strike me as a good choice for basement flooring - and the installation is idiot-proof.

  14. alex9999999 | | #14

    Thanks Malcolm....where are you? Does the floor feel hard? Cold?

  15. Expert Member
    MALCOLM TAYLOR | | #15

    I'm on Vancouver Island. The floor is in the staffroom of a resort, so no-one is ever barefoot. It does feel warmer than nearby wood floors, and softer too.

  16. FourCats2 | | #16

    Alexander, I am curious to know what you finally decided to do for your basement floor. Your questions and issues sound exactly what I am concerned about in my basement in metro Boston and am curious to know what you did and how it felt this past year. Please let me know.

  17. PAUL KUENN | | #17

    For two homes I've installed 3/4" EPS with 1/2" plywood then floating vinyl. We're zone 6 and the homes are all from early 60s with nothing under the slab but wet clay. So far they both love it. Ceiling height was an issue but they got used to it. I'm 6'2" and had no problem. We all have cramped basements here.

  18. kalcium | | #18

    A note on this - per Table 4A, pp 379 of the manual J book, the effective U value of a basement floor more than 2 feet below grade for 32 foot slab (shortest side) is 0.020.

    If you insulate at R3 or more, the value is 0.014. (manual J doesn't bump up the value as you add R-value for some reason)

    If we assume a basement that is 1000 square feet and 58 degree heating difference, that gives heat loss =
    1000 x 0.020 x 58 = 1160 BTU (uninsulated)
    1000 x 0.014 x 58 = 812 BTU (insulated)

    So you save 348 BTUs.

    This probably translates to very little savings.

    So at least according to manual J, one might argue that insulating a below-grade basement floor isn't worth it for cost savings. there may be other reasons to do so however as pointed out above.

    Note I'm not a manual J expert so the above could be wrong...

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