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Community and Q&A

Is foam under the basement always recommended?

Pascalli2 | Posted in Green Building Techniques on

I have been thinking about the benefit of putting XPS/EPS foam under the basement slab, and it has gotten me thinking.

I am building in zone 6, where it is quite hot for a couple of months in the summer, getting up into the 90s, and cold in the winter, getting down to the 10’s (Fahrenheit) on and off for a couple of months.

What I am thinking is – during the hot summer months, wouldn’t I want as little foam as possible between my basement floor and the cool earth that far down? During the winter, I can see how insulating the slab makes sense, since the ground will probably be 50 degrees or so, and the inside temperature will be around 70, but how does one balance the cooling benefit in summer against the energy savings in the winter?


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  1. GBA Editor
    Martin Holladay | | #1

    There are several reasons to include insulation under a basement slab.

    One is energy savings. In your climate, the insulation will save energy.

    Another reason is even more important: a reduction in condensation and mold on the concrete slab. This problem is more acute during the summer than during the winter. If your slab has foam under it, it is more likely to stay dry, so your basement will smell better. Moreover, you will be able to install carpeting on your slab -- something that you can't do if the slab is uninsulated, because of the mold risk.

  2. Expert Member
    Dana Dorsett | | #2

    Dead-right! In a zone 6 climate there's a pretty good energy use rationale for at least R8 sub-slab foam, but in zone 6 deep subsoil temps often south of 45Fand at least R4-R5 is critical for managing summertime mold potential if you want to use any type of mold-susceptible flooring (wood, carpets, even throw rugs.)

    I live in zone 5 in a location with deep subsoil temps in the low 50s. Even here I can't leave a cardboard box on the uninsulated slab over the summer, even with a dehumidifier keeping the basement at under 60% RH, or it WILL develop mold/mildew. In the winters when the basement RH drops into the 30 it's not a problem, but to keep it sufficiently dry in summer would require quite a bit more mechanical dehumidification.

    The amount of sensible cooling you get out of an uninsulated slab will never compare to the heating season savings in a zone 6 climate, even if you live in the most arid portion of zone 6 with little summertime humidity.

  3. Pascalli2 | | #3

    Thanks guys - I had not taken condensation into account in my musings. That makes sense!

  4. wjrobinson | | #4

    A basement here with ICF walls, 2" rigid under the slab, and radiant tube heat in the slab is the most liveable basement. Every time I go in that basement I am amazed at how nice the space is. And this home has no stratification that I can measure. Temperature from basement floor to cathedral ceiling measures similarly.

  5. Richard Beyer | | #5

    Guy's condensation and water problems can be controlled in new construction by installing under and over slab waterproofing barriers and a simple dehumidifier. Foam is not always the answer and is not a waterproofing barrier. Foam is also susceptible to insect and termite infestation below and above grade. Another reason for under slab membranes is gas and soil contamination control from entering the living space. Don't forget about radon mitigation in your design to. Below are some quick links I found to help you...

  6. GBA Editor
    Martin Holladay | | #6

    Let's see...

    Option 1. Install a layer of rigid foam under the concrete slab to make sure that it is never cold enough to encourage condensation.

    Option 2. Skip the rigid foam, let the slab get cold, and run a dehumidifier forever.

    Which one makes the most sense?

  7. RZR | | #7

    Richard is correct foam is not an anti-fungi and good long lasting moisture inhibitor. The best way to take moisture out is take down the level of clay in the sub-soil with large sand and rock like natural limestone, lava rock if available, as a moisture rubble trench permeable drain path that does not freeze too, you get from a locally quarry cheap! There are easy jar and site soil test on the internet to determine the amount and type of clay to less than 10% I would say, or clean rock for drainage. Borax spray is an anti-fungi you add to the soil with high moisture content many use in earth floors for centuries now.

    Some think foam is the answer to everything. It's not!

  8. Expert Member

    I don't think anyone is suggesting using the foam to inhibit moisture coming from below, that's what poly is for, along with a drainage layer used as a capillary break. The foam is there to keep the slab warm, which removing clay and adding rubble doesn't do.
    I don't really like foam either and where there are viable alternatives try and use those, but under a slab, or on the interior of concrete walls, there aren't a lot of other choices.

  9. RZR | | #9

    Malcom, read above, it was being suggested that foam be used for no other reason than to keep condensation off the slab. As the OP stated certain mass like high density portland cement used in concrete needs a thermal break in the winter, not summer. Fungi needs heat, moisture, food, and although there have studies to show EPS is not a food, it really depends and what surface treatments are being used and what manufacture is being tested. For all we know, the foam itself is a food source. I agree, I don't like foam either or putting the quality of my homes in the hands of manufactures.

    If we want to control fungi set up a capillary break as we suggested, a good one. Without moisture collecting on mass it will be hard to grow fungi. Use a vapor barrier. If you want to control your slab temp use another mass that has a higher mass effect, look at clay and pozzolans. If you do not know how to design effective mass research it. AJ pointed one method of solar thermal radiant floors, use the sun and night time radiation off a metal roof vs the ground geo by running inexpensive pex. You could go geo here but it is expensive. Walls, high clay plaster that regulated temperature and moisture naturally better than exposed concrete and are healthier. Another choice are Permanent Wood Foundations in combination with certain non-structural interior mass (plaster) you can run pex in.

  10. GBA Editor
    Martin Holladay | | #10

    If you have a concrete slab in contact with soil or crushed stone, it's often going to be cool enough during the summer to encourage condensation -- unless (a) you use a dehumidifier in your basement, or (b) you install a layer of insulation under the slab to isolate the slab from the cool soil and rock below.

  11. RZR | | #11

    Martin, that will depend on the design of the concrete. There are some that use pozzolans vs dense portland cement that evaporate better. Clay and earth floors have an excellent ability to do this. I'm not a fan of commercial grade concrete slabs with junk in them, so I'll bug out of this one. Good luck!

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