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Should I insulate the interior side of the basement wall if there is already XPS on the outside?

user-244918 | Posted in General Questions on

I have an unfinished basement in N.E. Ohio. Uninsulated slab floor with poly vapor barrier underneath. 13 course block walls with 2″ XPS foam on the exterior side. We are looking into finishing the space for a guest room/suite with a bathroom, excercise area, office and family room/play space. Being that there is already r-10 on the outside of the walls will adding a layer of 2″ XPS (r-10) create any problems? I have been reading everything Ican find on GBA and Building Science but everything seems to refer to either or, not both.


  1. GBA Editor
    Martin Holladay | | #1

    Insulation on both sides of your wall is fine. You'll end up with a foundation wall that is similar to an insulated concrete form (ICF).

  2. Expert Member
    Dana Dorsett | | #2

    Don't use XPS on the interior- use unfaced EPS instead, or use only 1" of XPS, which would bring you up to IRC 2012 code min for climate zone 5 (N.E. OH): (R15 continuous/R19 if thermally bridged by studs.)

    CMU won't wick as much moisture as poured concrete, but it'll wick some. At 1" of XPS or 1.5-3" of Type-II (1.5lb per cubic foot density) EPS you would be in the ~1-1.5 perm range, which is sufficient vapor retardency to protect the studs & studwall from ground moisture. With 3-5 perm latex paint on the wall board the drying toward the interior would be able to always keep up.

    Anything more than another R5 on the interior may be overkill unless you're going for much higher than code-min performance, or have expensive energy. But if you wanted to, ~1.5 perm foam (that would be 1" of XPS or 1.5" of EPS) would allow you to insulate the studwall with unfaced fiber insulation, bringing the total R-value of the exterior R10 + ~R1.5 CMU + R5-ish foam + R10-ish studwall ( after factoring in the thermal bridging of the framing through the fiber insulation) to interior-foam R up to the mid-20s for whole-wall R value. That's not PassiveHouse levels, but it's higher than Pretty Good House levels.

    Whether you insulate the stud wall with fiber or not, it's worth putting an incy of XPS or EPS under the bottom plate of the studs as a capillary break against ground moisture wicking up through the slab, and as a thermal break to keep the bottom plate above the dew point of the summertime air.

  3. Expert Member
    Dana Dorsett | | #3

    Martin: With ICF they typically use 2.5" of Type-II EPS on the interior, which runs a bit over 1 perm, which is enough for the foundation to dry toward the interior.

    With 2" XPS it would be half-that, about 0.6 perms. Unless there is a good capillary break at either the footing or the foundation sill there is some risk of ground moisture getting to the sill plate, since there is effectively zero drying toward the exterior, and weak drying capacity toward the interior. Since we don't really know if those capillary breaks exist here, it's safer to keep it over 1-perm toward the interior.

    The attached diagram specifies a pressure treated sill plate though, so it may be just fine either way, if indeed that's what was built.

  4. GBA Editor
    Martin Holladay | | #4

    I disagree with Dana. There is no reason to encourage a foundation wall to dry to the interior. This just assures that you will have a new source of moisture for your basement.

  5. user-244918 | | #5

    Thank you for the response, it makes sense and I had not thought of it that way. I talked to the local building inspector yesterday about this project and was told they like to see at least an r-10 on the basement walls and he didn't care how I achieved it. I should already have that with the exterior XPS. I have also read that the ideal r value for a basement wall in a cold climate is r-20.

    Would 1" XPS ($12.00/sht vs 2" @ $35/sht x 50 sheets) be enough to insulate the space better than it is now, meet code, and and not create a moisture condensation problem inside the stud walls I would build on top of the foam? I am trying not to be penny wise and pound foolish but it's $600.00 vs $1890 for the whole basement.

  6. user-244918 | | #6

    You guys responded in between when I typed my latest and when I actually posted it before leaving the office, sorry about that.

    Dana: The 1st floor sill plate is actually pressure treated. Does this matter so if the block wall below it becomes and stays wet the sill plate will not rot? I can't say for sure if there is gravel under the slab or not but I assume there is. I do know there is at least a sheet of plastic. It is actually visible sticking out in some areas between the "construction joint" material and the concrete slab. There is unfaced "r-19" around the rim joist but I have been removing it, shoving 2" XPS in there with spray foam and then replacing the r-19. This winter there was quite a bit of frost on the rim joist under the existing fiberglass.

    Martin: In your opinion would 1" of XPS be too likely to promote drying toward the interior?

    I am not going for PassiveHouse performance but also not trying to get by only on code minimum. I am trying to get a better understanding of this and trying to spend my money where it matters most. I don't want to try and save 1200 here only end up having to spend many times that redoing it down the road.

    I appreciate everyone's input.

  7. Dana1 | | #7

    The reason for letting the foundation dry toward the interior is to guarantee the moisture can't wick high enough to affect the foundation sill. The very modest amounts that end up in your basement through ~1.5 perm foam is more easily managed in the basement than when concentrated in the foundation sill.

    But as I stated in the first post, if the foundation sill is pressure treated it doesn't matter. The foundation sill itself has a vapor retardency of about 1.5 perms, so even if the top of the foundation has high moisture content, it's not going to be putting moisture into the stud bay above at a rate faster than it can handle. But if the foundation sill were standard lumber, without at capillary break there would be high risk if the foundation were high-moisture.

    Whether or not your CMU wall will wick much moisture has a lot of factors that we can't know without more investigation- overall CMU walls don't wick as heavily as poured concrete, and barely at all if the cores are hollow, but if the cores are filled with concrete it would raise the capillary moisture transport potential by quite a bit.

    The frost under the R19s is all about air leakage from the basement, not moisture wicking up the CMU, which has HUGE drying capacity into the basement when there is no vapor retardent foam covering the inside.

    At 1" XPS is almost a Class-II vapor retarder, which is pretty tight, just not ultra tight. At 2" it's well into the class-II vapor retardency range. You can't really move a ton of moisture through 1.5 perm foam through a whole basement wall's worth (we're probably talking a coupla quarts per year, unless the CMU is totally saturated) but it should be enough to protect a foundation sill (if it needed protecting.) It only takes 1" to meet code when you have R10 on the exterior. But if you have a source for 1.5" EPS (R6-ish) it's usually cheaper than 1" XPS, and unlike XPS it will still have the same R-value in 50 years. As XPS leaks out it's climate damaging HFC blowing agent over a handful of decades it loses performance, eventually settling in at around R4.2 per inch, just like Type-II EPS was on both day 1, and still is on day 20,000. It's not worth paying more for unless you actually NEED the lower vapor retardency (and even then there are often better options.)

    It's pretty common to have vapor barriers under the slab, but no capillary breaks at the wall footing, which is probably the case here. But if you have pressure treated foundation sills, don't sweat it- put up any type of foam you like, just make sure there's something between the CMU & framing. Even with the vapor retarder under the slab it's worth putting an inch of foam under the bottom plate, since the temp of the bottom of the stud plate may be below your indoor dew points and end up with relatively high moisture content. This is a common issue with uninsulated slabs in cool climates, and a reason (beyond the energy use issues) to insulate the slabs. Putting a rug on an uninsulated slab will often result in mold even when groundwater is blocked by vapor barriers.

  8. user-244918 | | #8

    Thank you both for all the info. I really appreciate it.

  9. charlie_sullivan | | #9

    In addition to the other advantages Dana notes, EPS is cheaper than XPS.

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