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Community and Q&A

Daylight basement insulation – exposed XPS and other questions

user-6454437 | Posted in General Questions on


I’m about to insulate the floor and 2 walls of my walkout/daylight basement, and I was hoping to get a little advice on a couple of points I’m not clear on. I’ve researched quite a lot, and learned a lot on this site, but I’m still not sure on a few things. Any help is much appreciated.

It’s a poured concrete foundation with no existing exterior or interior insulation. East wall is concrete up to 36” and the rest stud framing. South wall is concrete up to 9” and the rest stud framing. So it’s a sloping grade with part of that east wall below grade and the south wall pretty much above grade. West and north walls of this room are existing interior walls. No signs of major leaks or water damage in the year and a half we’ve been in the house (knock on wood!), but a lot of moisture in the air and a terrible musty basement smell.

My plan is to put 1” XPS on the floor (I already have it and since found out that EPS is more environmentally friendly. I’ll source and use EPS or PolyIso on the walls though) seams caulked and taped and leaving ¼” expansion around the edges which I will fill with caulk or foam.

1” EPS or PolyIso glued/caulked to the east and south walls will meet the XPS on the floor and be caulked at that corner seam

2×4 stud walls tight to the EPS on those walls. PT bottom plate with Tapcons through the XPS into concrete. Fill cavity with Roxul?

Then ¾” T&G plywood on top of the XPS on the floor, and Tapcons into the concrete using caulk in the holes before the Tapcons go into concrete. ¼” expansion gap between plywood and walls

Finish walls with drywall.


1) If I leave that expansion gap between the ¾” ply and my framed stud walls, that will leave ¼” of the XPS on the floor exposed. I think that code says that the XPS cannot be left exposed. How can I get around that? Will the baseboard covering that small gap suffice, and shouldn’t I be leaving a small air gap under baseboard anyway to allow moisture to escape? Or is it better do the subfloor first and frame the walls on top of the plywood?

2) Should I put the rigid foam and the framing all the way up to the ceiling on those two walls, being that the height of the concrete walls are only 36” and 9”, or should I just cover the concrete part with the foam insulation, and leave the existing walls as they are? They already have existing drwall. I could frame up to 36” on the east wall and make a ledge/shelf and up to 9” on the south wall which would make a sort of weird step I guess. The R value is minimal, but I guess it all helps, and going up to the ceiling will look a lot cleaner. Can I glue the EPS to the existing drywall with no worries?

3) Should I fill the2 ½”cavity in the new framing between the EPS and the drywall?
Is this air gap a fire hazard? Would Roxul in that cavity be a good call, giving me extra R value and solving the fire/code problem?

Sorry for the long post. Hope it all makes sense. Thanks for taking the time to read it.

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  1. Expert Member
    Dana Dorsett | | #1

    Run the subflooring under the stud plates, with 1/4" of clearance to the wall foam. That way you'll have the full 1-1/2" of stud plate for nailing the gypsum & baseboard rather than having to toenail it into 3/4" of lumber edge exposure.

    Leaving empty vertical spaces deep within walls is a fire code violation. You are probably better off running the wall foam all the way up to the ceiling, but for clarity:

    Are the existing stud bays empty?

    If not, are there polyethylene sheeting in there, or just insulation?

    What is your climate zone?

  2. user-6454437 | | #2

    Thanks Dana. In Oregon. Climate zone 4. I'll need to pop a hole through and have a quick look in the existing wall, but based on what I've seen in another part of the house, I think there's no poly sheeting and some old fiberglass insulation.

    Good point on having a nailer for drywall and baseboard.Think I will do the sub flooring first. I guess I was trying to make it easier to take sub flooring out in case of basement flooding.

    In those cavities in my new framing between EPS and drywall, is Roxul a good way to go?

    Thanks again

  3. GBA Editor
    Martin Holladay | | #3

    Dana gave you good advice. In one of your questions, you wondered, "Shouldn't I be leaving a small air gap under the baseboard anyway to allow moisture to escape?"

    The answer is no. You don't want to design your floor assembly or wall assembly for vapor transport. Stopping all vapor transport is best. That's why thicker rigid foam for your walls and floor would not only be acceptable -- it would be preferable.

    The polyethylene in your framed walls is unfortunate but not fatal. I would extend the 2x4 walls all the way to the ceiling if I were you. Install the required fire blocking at the top of the wall, and fill the air spaces behind the drywall with the insulation of your choice.

    With all walk-out basements, it's a good idea to verify that your builder included vertical rigid foam insulation on the exterior side of the concrete stemwall under the door threshold. The soil near the walkout door gets cold -- in some climates, it freezes -- so you definitely want exterior vertical insulation at this location.

  4. user-6454437 | | #4

    Thanks Martin,

    I'm glad you mentioned putting rigid foam on the exterior side under the sliding door threshold. I had seen one of your previous posts about that and was going to post a couple of questions.

    I'll be replacing the nasty old sliding door anyway, so can I extend the xps I'll be using on the floor to go under the new door? (actually under the sill pan that I will install under the door) And then do I run my vertical exterior piece of foam up to meet that piece of foam that will be under the door, and glue and caulk that seam?

    Also, how far should I extend the width of that exterior piece beyond the door edges. I'm somewhat limited on one side because of a concrete driveway. But I could extend all the way to the edge of the house on the other side if needed. Am I correct in thinking though that I shouldn't have exterior foam sandwiching the concrete wall where I'm using the foam on the interior walls, because that would be creating a vapor barrier on both sides? I'm guessing in this instance only a few inches (vertically) would have foam on both sides.

    Thanks again for the advice. Its a huge help to me!

    ps. I don't have the headroom to go thicker with the foam on the floor, but I can bump it up to 2" EPS on the walls based on your suggestion that thicker foam would be preferable.

  5. GBA Editor
    Martin Holladay | | #5

    If you plan to install rigid foam and plywood above your slab, then your new door will be installed on top of the new plywood subfloor. You'll need to install a sill pan above the plywood before you install the door.

    Since you are planning to install vertical rigid foam on the exterior of your foundation, you need to consider where your door will sit. If you cantilever the door somewhat, the door threshold can cover the exterior rigid foam. If you want thick exterior rigid foam, you may want to plan ahead by installing a cantilevered 2x10 or 2x12 pressure-treated rough sill under the door threshold (instead of the rigid foam and plywood).

    However, this "cantilevered door" approach may not work with the framed wall -- it all depends on the existing siding. If you need to keep the door aligned with the existing framing, and you probably do, then you won't be cantilevering the threshold.

    If you are installing vertical rigid foam on the exterior of your foundation, you will need (a) a plan to protect the above-grade portion of the rigid foam from abuse and UV light, and (b) a plan to protect the rigid foam at the door threshold -- stiff aluminum or copper would work.

  6. user-6454437 | | #6

    Thanks again for the informative answers! I feel much happier about my plan now.

    The cantilevered door makes sense to me, and would work for my application. I'm going to think on that and see if it works cosmetically. My original plan was to protect the foam with angle aluminum or galvanized roof flashing.There will be a deck going up to that sliding door one of these days. so I can figure out a transition that keeps the rigid foam safe.

    Should that exterior rigid foam be only under the door threshold, or should it extend as far as I can go along the exterior perimeter?

    Thanks once again

  7. GBA Editor
    Martin Holladay | | #7

    If you have a walk-out basement, the vertical exterior foam is usually installed on the entire walk-out wall, from corner to corner.

  8. user-6454437 | | #8

    Hi again and thanks again. I'm back to the basement project after a short layoff for health reasons. I was hoping I might get help with a couple of other related questions regarding insulating the below grade portion of my basement walls.

    I looked at quite a few posts by both Dana and Martin and others, and think my plan is good based on what I learned. I know you've covered a lot of this stuff on this site before, but my situation seems a bit custom and I want to be sure that I get it right before I seal it all up with sheetrock.

    I glued 1" XPS to the foundation wall, (which is 36" high), and extended it to the ceiling (gluing it to existing drywall on the above grade portion of the wall). Caulked and taped seams.Great Stuff foam around perimeter. I plan to frame a stud wall up next to that foam (built on top of plywood subfloor as Dana had suggested), with 2x2 top and bottom plates and 2x4s turned sideways. This is to conserve space. I don't need to run electrical or anything. I'm using a box extender to extend one existing outlet.

    I bought 1.5" polyiso foam, foil face both sides, to cut into my new stud bays and glue to the 1" xps. I thought that this would help by giving me a thicker layer of foam against the foundation wall (2.5"), and getting me up to the required R15 - 1" xps = R5 + 1.5" polyiso = R9.6 + 1/2" sheetrock = R .45
    It would hopefully do those things while making sure there is no air gap in the cavities and also conserving space. I could use Roxul or similar higher up in the bays above the foundation wall. I would have a little more room to get the batts in higher up because due to the existing wall not being plum/level, there will be about 3/4" gap at the top between my new 2x2 top plate and the existing wall. This means I wouldn't have to compress the batts too much.

    This leads me to my main question/concern. Although it seems like a good practical solution, I realize that the thermal bridging of the framing will effect the insulation performance of that polyiso, and decrease its R value (so I'm not really getting my total of R 15). Should I seal all the edges of each piece of polyiso between each bay? If I did that, would that create a problem because the foil facing of the polyiso is not permeable and it could trap moisture? If this is a bad idea, should I just compress batts down to 1.5" thickness into those bays? Maybe that would help the R value versus the cut pieces of polyiso, but then I would be down to just 1" of foam against the concrete wall.

    One more quickie - I'm leaving a 1/4" expansion gap between the foam on the walls and (so floor foam butting up to wall foam leaving 1/4" gap the foam on the floor). Should I caulk that seam with the foam board adhesive or fill with expanding foam? Should I also tape that joint?

    Thanks once again for your very helpful replies.

  9. GBA Editor
    Martin Holladay | | #9

    Q. "Should I seal all the edges of each piece of polyiso between each bay? If I did that, would that create a problem because the foil facing of the polyiso is not permeable and it could trap moisture? If this is a bad idea, should I just compress batts down to 1.5" thickness into those bays?"

    A. Yes, you should seal the edges of the polyiso.

    (By the way, I already answered this question before, in Comment #3. You asked, "Shouldn't I be leaving a small air gap under the baseboard anyway to allow moisture to escape?" I answered, "The answer is no. You don't want to design your floor assembly or wall assembly for vapor transport. Stopping all vapor transport is best.")

    The type of wall insulation you plan to install is called cut-and-cobble insulation. Here is a link to an article with more information on this method: Cut-and-Cobble Insulation.

    Q. "I'm leaving a 1/4 inch expansion gap between the foam on the walls and [the floor] (so floor foam butting up to wall foam leaving 1/4 inch gap the foam on the floor). Should I caulk that seam with the foam board adhesive or fill with expanding foam?"

    A. Either product will work fine.

    Q. "Should I also tape that joint?"

    A. You can if you want, but you don't have to.

  10. user-6454437 | | #10

    Thanks Martin,

    I can't believe I missed the article on "Cut and Cobble". That helped a lot. It confirms what I had thought, which was that my plan to cut and cobble the polyiso on top of the xps is acceptable, but not necessarily recommended. As an alternative, I'm trying to find batts that are thin enough to compress into those1.5" bays, that will still give me the R 9 or so that I need. I read that the Roxul batts would be difficult to split, and I don't particularly want to use fiberglass batts.

    Much obliged for the answers, and how fast for how fast they come.

  11. user-6454437 | | #11


    Do you see any benefit to me painting the underside of my t&g plywood before laying it on top of 1" xps as extra protection from vapor transport? If so, can you recommend a paint for this application please?

    Just to recap my situation:

    1969 daylight basement.Slab has no foam or vapor barrier under it.
    I added foam to basement walls (xps on walls, and polyiso will be cut and cobbled between two new 2x3 walls)
    I added epoxy to the largest cracks in the slab. All were less than 1/8" wide. The slab has a black coating on it which I'm guessing is old mastic from asbestos tiles (which we had removed), or an old sealer coating of some kind.
    I added 1" xps on top of slab
    I'm about to screw down (tapcons) 3/4" t&g plywood over the xps.
    I will be installing vertical rigid foam insulation on the exterior side of the concrete stem wall.

    Thanks once again for all of your informative replies.

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  12. GBA Editor
    Martin Holladay | | #12

    Q. "Do you see any benefit to me painting the underside of my tongue-and-groove plywood before laying it on top of 1" XPS as extra protection from vapor transport?"

    A. No. The XPS layer has a vapor permeance that is already fairly low (0.4 perm to 1.6 perm, depending on the brand). If you worry about vapor transport, you can install a layer of polyethylene (rated at 0.06 perm) between the XPS and the plywood if you want.

  13. Expert Member
    Dana Dorsett | | #13

    Cut'n'cobbled 2.5" polyiso between 2x3s 16" o.c. lowers the thermal performance to less than R10 whole-wall. If it's 1.5" polyiso it cuts it to about R6. It's less work and measurably higher performance to install the polyiso with foam board construction adhesive to your XPS layer, strapped in place by 1x 4s through-screwed to the foundation 16" o.c. with masonry screws.

    A 6-mil poly vapor barrier between the 1" floor XPS and the t & g subfloor (or between the XPS and slab) is quicker and far better at protecting the wood from ground moisture than any paint. In most floor stackups and most homes the vapor retardency of the foam would be sufficient. Don't sweat the annular ring gaps in the vapor barrier at the TapCons. The combined area of gap for the entire floor will be at most a few square inches, and of no real consequence.

  14. Jon_R | | #14

    Somewhat permeable is typically best for basement wall insulation. See here for:

    "The research discussed above showed that allowing some moisture vapor to move between the foundation wall and the basement generally leads to dryer systems."

    The best for a floor that needs an interior side vapor barrier is something fully adhered - to the concrete. Already past that stage - I wouldn't add any additional vapor barrier.

  15. GBA Editor
    Martin Holladay | | #15

    The information on the web site you linked to was influenced by statements made by building scientist Joe Lstiburek.

    I challenged Lstiburek's statements on this point, arguing that there is no advantage to insulation systems that invite ground moisture into a basement. After several years, Lstiburek conceded. Here is a link to an article on the topic: Joe Lstiburek Discusses Basement Insulation and Vapor Retarders.

  16. Jon_R | | #16

    Of course "influenced by" isn't "completely based on". Lstiburek reportedly said "It’s OK for the concrete to be wet." "OK" isn't "best", so where does your "Stopping all vapor transport is best" come from?

    I'll buy "OK" - up until water leaks onto the floor. Or a little bit of air leakage happens and mold smell escapes. Not hard to find examples of both.

    My conclusion (from non-retracted research)- some inward drying results in the driest concrete with the least chance of bulk water or mold problems. But if you are sure the exterior drainage and interior air sealing are adequate and will always remain so, then it doesn't matter. But why not take advantage of cheap insurance?

  17. GBA Editor
    Martin Holladay | | #17

    I stand by my statement and recommendations. It's best if the rigid foam layer does not allow any water vapor to pass through from the damp concrete wall to the interior of the home.

    The scenario you are worried about -- water leaking onto the floor -- has nothing whatsoever to do with the vapor permeance of the foam layer. That's a sign of liquid water entry. Before a basement wall can be insulated, water entry problems need to be addressed, as I explained in one of my articles, Fixing a Wet Basement.

    For more information on this topic, see Preventing Water Entry Into a Home.

    (If you make the mistake of installing an air-permeable insulation like fiberglass on the interior side of a basement wall, it's possible that enough condensation might occur behind the fiberglass to get a puddle on the floor. But GBA has consistently warned readers about the dangers of installing fiberglass batts on the interior side of a basement wall. When condensation leads to puddles, you can't stop the puddles by increasing the vapor permeance of the insulation layer. The only way to stop that type of puddle is to choose a type of insulation that isn't air-permeable.)

    There are at least two reasons why you don't want your interior basement insulation to be vapor-permeable:

    1. The available moisture in the soil and the concrete is effectively infinite. You will never succeed in drying out a damp basement wall (or the damp soil it is touching) by inviting the moisture indoors.

    2. All of the moisture that you succeed in inviting indoors becomes a moisture load that the heating system, cooling system, and ventilation system has to deal with. You don't want that.

  18. GBA Editor
    Martin Holladay | | #18

    I certainly agree with you on one of your points: "Many people think they have addressed basement water entry problems. Then an unusual event occurs and they learn otherwise."

    But I stand by my previous statement. If you have a water entry problem, there is no way that switching from a low-permeance rigid foam to a high-permeance rigid foam is going to solve the problem.

    I think we'll just have to agree to disagree on this point.

  19. user-6454437 | | #19

    Thanks for all of your replies. I'm learning all the time, and this forum has been a great help.

    I haven't seen signs of water coming up through the slab, and taping plastic to the concrete to test it throughout the year hasn't shown me any moisture. There is though that constant musty smell in there , and the humidity meter shows the levels are consistently fairly high. To vapor barrier or not! I will decide tomorrow after re reading everyones posts. I'm hoping to finally finish the floor this weekend.

    Dana, I have been considering your suggested method for my walls of strapping the extra polyiso with 1x4 instead of cut and cobbling polyiso between 2x3 framing. The reason I didn't originally want to do that is that the walls are nowhere near level, and it could be a shimming nightmare to get it all straight and true. I'm now looking at getting some 2.5" mineral wool batts shipped to put those in the 2x3 wall bays. I had bought some polyiso already, so I will use the method you suggest in the next basement room I tackle, where one wall is straight and level. Is the 3/4" air gap ok between the polyiso and the drywall which would hang on the 1x4 strapping?

    Thanks once again

  20. Expert Member
    Dana Dorsett | | #20

    The 3/4" air gap established by the furring is fine, and the air films even give it a modestly higher R-value.

  21. Jon_R | | #21

    Aldrich found that inward drying and the resulting drier concrete (think large sponge) does mitigate bulk water. Foil faced polyiso needed 6" of exposed concrete near the floor to dry after wetting - that's what he recommends if you go that route.

    Many people think they have addressed basement water entry problems. Then an unusual event occurs and they learn otherwise.

    The amount of vapor flowing through rigid foam is negligible in terms of the HVAC dealing with it. No problems getting a basement dry with an inch of XPS on the walls.

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