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Community and Q&A

Minimalist on top of slab insulation options

Keith H | Posted in Green Building Techniques on

Hi all,

I have an existing 1970s structure with a 42″ below grade basement slab. I’m re-doing the flooring in the basement (settled on cheap laminate over expensive stained and polished concrete) and am wondering about getting a tiny bit of insulation on top of the slab before I install the laminate.

I have a 91″ ceiling height. so I don’t have the head height to consider any of the lovely options described in the lovely GBA q&a here such as 2″ of XPS and an inch of plywood.

Additionally, because my water table is quite high, I’m very reluctant to put down plywood/OSB etc. anywhere near the floor. Yes, I’ll likely have to chuck the laminate if it gets wet but flooring choices are tough in this scenario. I’m less willing to chuck the rough system.

So I was wondering (here’s the bad idea):
Why can’t I just put the laminate on top of xps?

So my system would be something like:
laminate (3/8″)
polyE or taped laminate underlayment?
1″ XPS (or should I use a different foam)
4″ concrete slab (could epoxy seal it for vapor control)

That’s a finished floor height of ~1.5″ and an R of ~5.5. That’s about half of the desired R-10 but a lot better than the R-0.5 to 1 the floor and underlayment alone would provide.

Opinions? Ideas?


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  1. GBA Editor
    Martin Holladay | | #1

    You wrote, "Because my water table is quite high, I'm very reluctant to put down plywood/OSB etc. anywhere near the floor."

    If you have reason to believe that your basement slab sometimes gets wet, it hardly matters what your details are. You are evidently planning to install sacrificial flooring. Go right ahead and do what you want; you are apparently ready to rip it all out and do it again when it gets wet.

    If, on the other hand, you want to address the water entry issues, here is some advice: Fixing a Wet Basement.

  2. Keith H | | #2

    Cost is part of every building decision, especially for existing structures where items that are affordable and simple during construction become enormous projects after.

    Consider these costs:
    Sacrificial flooring which might last 2 years or 10, without labor (because installing quick clic is 'weekendable') with 1" XPS insulation and polyE or Delta-FL, perhaps $3 or $4/sf.
    New slab, insulate it, interior french drain, polye vb: ~$20/sf
    Excavate exterior, install french drain, drain mat, insulation: ~$20/sf

    Which is greener? R-10 under the slab at $20/sf or 5 kw solar array? Of course, it's really A or B or C or but presumably you see my point that putting R-10 under the still functional slab is not likely to save enough energy to re-capture the carbon cost of the new concrete?

    So I'm looking for an economical, low finished height option to insulate an existing basement slab. Unfortunately, a 3" rough system (2" of XPS and 1" of plywood) is too great a loss of height. The stairs would need to be redone ($2k minimum) and the finished height would be closer to 7' than 8'.

    So my original question, which I guess no one has any suggestions on:
    Could you get away with a floating laminate floor installation directly on top of XPS? If you are installing a floating floor, what is the purpose of all that plywood?


  3. GBA Editor
    Martin Holladay | | #3

    Q. "Could you get away with a floating laminate floor installation directly on top of XPS?"

    A. First of all, most brands of floating laminate flooring are designed to be installed over a proprietary foam underlayment. So there will be a little bit of foam under the flooring. But to get an answer to your question about XPS, you'll need to call the flooring manufacturer. The question came up on another Q&A forum -- read about it here -- and the manufacturer of one brand of flooring said, no, you can't do that.

  4. Keith H | | #4

    Thank you very much for that great old thread. Good info.

    The one other thought I had was to install 1/4" durarock over the XPS. I could certainly live with another 1/4" loss of height. Hardibacker et all look to have more than 1000 psi compressive strength in that thickness based on the pdf linked below. A 1000 psi is much more in-line with osb than XPS. And a tile backer type product will be a lot more durable in the event of wetting than osb or plywood.

    Thanks for the replies. I'll report back if I actually conduct this experiment.

  5. Dan Kolbert | | #5

    That sounds like a bad idea. If there's enough give to the XPS to feel it in the floor, it will crack the cement board and then you'll have a big mess. I agree w/ Martin but if you're prepared to experiment, I'd look for flooring with the biggest individual sections you can find to distribute the load better, and make sure they click together reasonably firmly so the whole floor stays together.

    How flat is the slab now? May want to fill in any low spots if they're significant.

  6. Chris Wiegand | | #6

    Depending on what you mean by minimalist insulation, you could use 1/2" thick Homasote brand ComfortBase, and following the installation instructions use 3/8" plywood on top for your subfloor. R-value is minimal at R-1.2, but it's better than nothing?

  7. Expert Member
    Dana Dorsett | | #7

    Rather than XPS (which is blown with HFC134a which has a global warming potential ~1400x that of CO2), use EPS (blown with pentane at ~7x CO2).

    Moisture drives are in both directions. While ground moisture can be blocked with a vapor barrier under the subfloor, room air moisture can end up concentrating at the bottom of the subflooring in summer if it's not warmer than the summertime room air dewpoint.

    Climate/deep-subsoil temps etc matter, but anything over R3 is usually sufficient for limiting mold growth under your subfloor & finish floor for most of the US lower-48. There is longer term financial argument for more than that in heated basements in US climate zone 4 or higher. The dew point of 75F 60% RH summertime air is about 60F, so it takes at least SOME amount of R under the subfloor to keep it that warm in areas with subsoil temps colder than 55F when you have R1-2 of subfloor + finish floor on top of it.

  8. Keith H | | #8


    Thanks for the in-depth answer. A lower profile system that achieves R-4ish in ~1" rough would have a lot of value in the retrofit market. At least then the basement floor would be as good (bad) as the fenestrations. Hence my question.

    As for XPS, I'm sensitive to the CO2 potential of XPS. We recently remodeled my father's basement and used a combination of EPS/PolyIso to insulate the walls rather than XPS. However, the superior compression strength of XPS over EPS probably means I'm not going to go with XPS for this project unless I figure out a 'subfloor' that will provide the necessary strength.

    I wonder if it is possible to model surface temperatures of the flooring assembly layers in WUFI...

    In my climate zone of 5b with a slab depth of only ~36" (somewhere near the frost line), I'd argue that more than R-3 has some benefit. From an effort perspective, however, insulating the slab is most about comfort and possibly protecting a laminate floor from condensation or minimal moisture intrusion damage. Major moisture intrusion will likely be destructive of the flooring, though probably not much else.

    As for vapor drive, in my particular climate (semi-hot dry summers, cold but not extremely cold dry winters), it's vapor drive from the ground/slab up I'm concerned about.

  9. Expert Member
    Dana Dorsett | | #9

    Type IX EPS (2lb per cubic foot nominal density) has the same compressive ratings to 2lb XPS, and is usually cheaper.

    Even half-inch OSB is sufficient subfloor for distributing the weight using Type -II (1.5lb nominal density) EPS under a laminate floor. The amount of "give" you have with half-inch OSB fully supported by foam & slab is less than what you get with 3/4" OSB decking between joists.16" o.c.

    With any psi-rating of the foam you will still need to stagger the seams of the foam with the seams of the subfloor to avoid the potential shear-compression issues at the foam edges.

    You are correct that in most zone 5B locations the summertime dew points are low enough to not matter much in terms of moisture accumulation in the sub-flooring. And yes, R3 will have at least some comfort benefit during the coldest months. If your slab is actually shallow enough that it would actually get below freezing with R3 foam above the slab it would be unusual. Some frost-protection insulation on the exterior of the foundation might be in order if you had high moisture content to the soil, but in arid zones frost heaves are rare (and frost heaves propagate in the direction of the heat loss too, making it even less likely as long as the building is heated.) If you've never seen a frost heaved slab in your area even in an unheated barn you shouldn't sweat that one- it''s a bigger issue up in colder wetter climates than in yours.

  10. GreenGreenie | | #10

    Hi Dana,
    Very helpful info. I'm re-doing my 1970's walkout basement and want to insulate the floors before laminate and tile goes down. Do I need to shorten existing walls and insulate under them or can I just do the exposed floor?

  11. Expert Member
    Dana Dorsett | | #11

    It's useful to have some bottom plate exposure on the pre-existing partition walls above the finish floor for nailing on kick-boards, etc, but not absolutely necessary. If the bottom plates are pressure treated there's not much risk of mold/rot getting started on the (now cooler, damper) bottom plates after insulating, but with untreated lumber there is at least SOME risk, but usually not a very high risk.

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