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Community and Q&A

Insulating my slab

mattseab | Posted in GBA Pro Help on

Climate zone 4a. I am in the process of insulating my basement walls with 4″ of xps and had been planning on insulating my slab with 1/2″ or 1″ xps but as I read more and more on GBA I am now wondering if it’s even worth it. I will definitely insulate the exterior of my slab with 2-3″ of xps depending on how much I can get away with before it extends beyond my siding but after reading some of Martin’s comments to other people’s questions I am leaning towards forgoing the slab insulation. To one person he wrote that 1/2″ of xps is “fairly insignificant” and 1″ of xps “isn’t much”. If I went with 1″ of xps and two 1/2″ layers of plywood, plus 3/4″ flooring I would be 2 3/4″ higher which would cause problems at my exterior door and at the bottom of my stairs. I am on year 5 of building my house (owner/builder paying out of pocket) and am totally burnt out. Removing my exterior door that is glued with heavy duty liquid nails to a homemade sill pan which is glued to the slab with heavy duty liquid nails and having to modify my header and exterior trim is a no-go for me right now not to mention redoing my stairs so the height of my steps would be consistent. I have been racking my brain trying to figure out a way to rig something up so I can leave the door in place but have no feasible ideas. If I knew it was worth it I would take a break from working on the basement now and come back at a later date when I’m not burnt out and remove the door and redo my steps in order to insulate the slab with 1″ or even 2″ xps but if the efficiency gained from all this is minimal then I will forgo all slab insulation. So what say the experts?

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  1. Expert Member
    Dana Dorsett | | #1

    An inch (or more) of slab insulation is "worth it" (and a mere half inch isn't quite enough) even if it never "pays off" on in terms of total energy use. It's not about energy use- it's about something else:

    A major function in a zone 4A climate is to keep the slab warm enough in SUMMER, to keep the slab above the summertime ventilation air's dew point, which limits the amount of summertime moisture accumulation in the basement, and the associated "basement smell".

    If a full inch is going to be too much of a height adjustment, compromise and go with 3/4", which for 1.5lb polystyrene would deliver a long term R3. That should be enough to protect the overlaid subflooring and finish floor, as long as you don't put a thick shag carpet over it.

    Using EPS rather than XPS is greener. Over a few decades time they both end up at the same performance level, but the HFC blowing agents used for XPS are massively worse for the environment than the pentane used for blowing EPS (most of which is recovered at the factory). The HFCs give XPS a higher initial performance, but the performance boost it's pretty temporary relative to the lifecycle of a house, and just the greenhouse gas impacts of the HFCs are measured in centuries.

  2. mattseab | | #2

    Thanks! Any thoughts on a solution at the door threshold? I thought about using two layers of 3/8" plywood in the room where the door is or going with 1/2" finished flooring rather than 3/4" but that still puts me at 2" which is larger than my threshold.

  3. GBA Editor
    Martin Holladay | | #3

    I'm sorry if my comments (to the effect that 3/4 inch or 1 inch of rigid foam "isn't much") gave you the wrong impression. Context is everything.

    All other factors being equal, thicker foam (say, 2 inches or more) under or on a basement slab is preferable to thinner foam. But in a remodeling situation, you have to take all factors into account, including inward-swinging doors and basement stairs.

    Dana gave you good advice. When insulation is installed on the interior side of a basement slab, it raises the temperature of the first condensing surface (the finish flooring, plywood, or upper surface of the rigid foam). That's useful.

    There are several possible solutions to your problem with the exterior door. A skilled finish carpenter can cut the bottom off your door and install a modified threshold and weatherstripping. It's also possible to order a new door (in any custom size). Finally, it's possible to make your in-swinging door into an out-swinging door. Each solution has its own challenges, but these challenges are solvable.

  4. Expert Member
    Dana Dorsett | | #4

    JonR: My bad! I need an editor! :-)

    Insulating above the slab willl keep the moisture & mold susceptible SUBFLOOR warmer (and drier)...

    ... but the not-at-all susceptible SLAB will be cooler (and wetter.)

  5. mattseab | | #5

    Thanks to all of you for the good advice. Martin, I normally find that when I have a question about a topic someone else has already asked the same question so instead of getting you guys to answer the same stuff over and over, I try to find my answer via other people's answers. This was a good reminder that sometimes it's best to ask my question in order to get a specific answer for my situation. Thanks again to all of you.

  6. Jon_R | | #6

    It would be nice to see a definitive answer (including why) to the issue of mold growing on damp concrete behind interior side insulation. Lstiburek says it's an issue with a non fully adhered vapor barrier. And it isn't hard to find photos of mold growing on concrete behind foam. But perhaps it's so rare that it can reasonably be ignored. Or the mold grows but there isn't enough air exchange to notice the odor.

  7. Jon_R | | #7

    Insulating above your basement slab will make it cooler in summer not warmer. But it will make the upper floor surface warmer. Without insulation, there may or may not be a condensation issue, depending on just how humid you allow the basement to get (a dehumidifier will control this). With the insulation, there had better not be any air exchange down to the cool slab - that's more likely to condense. If condensation or other trapped water sources cause mold, the odor is likely to make it into the house.

  8. GBA Editor
    Martin Holladay | | #8

    As I have noted before, I imagine the problem is quite rare -- because lots of people have successfully installed continuous rigid foam above a concrete slab without reporting odor problems.

    If good air-sealing methods are used (including tape at the rigid foam seams, and canned spray foam at the perimeter of the room), I can't imagine there would be much vapor exchange -- especially in light of the additional layers installed above the foam (generally plywood or OSB, followed by finish flooring).

  9. mattseab | | #9

    Martin, do you think floating the plywood layers, laid perpendicular to each other and attached to each other is sufficient to prevent curling at the edges or should I use tapcons around the perimeter just to be sure?

  10. GBA Editor
    Martin Holladay | | #10

    The answer depends on the thickness of the plywood you specify. If the lower layer of plywood is 3/4 inch thick, and the upper layer is 1/2 inch thick, I imagine that your suggested method will work.

    But my guess is that you will know when the work is completed whether you have a satisfying subfloor. If you are uncertain, add some TapCons.

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