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How to insulate basement interior (with 2″ XPS on the exterior)?

George M | Posted in Building Code Questions on

Hello. I am planning to finish my basement and I have a question regarding the interior insulation. The house was built in 2001 and is located in RI (Zone 5). The basement has poured cement walls (12″ thick), 8 feet tall. The below grade portion of the walls has 2″ XPS on the exterior and the above grade portion (about 2 feet) has no insulation.

Given that I have R-10 on the outside, my understanding is that in order to meet code

1. I only need continuous R-5 on the interior (1″ XPS or 1.5″ EPS) and then frame a 2×4 stud wall 0.5-1″ away from the rigid foam (no additional insulation in the stud bays).


2. Not use any rigid foam on the interior and only frame a 2×4 stud wall 0.5-1″ away from the bare concrete walls with R-15 Roxul in the stud bays.

I would prefer the second method for simplicity, however, I am a little concerned regarding possible condensation on the bare cement walls. Any thoughts?

A few other things:

If the bottom plate of the stud wall is PT, should I still include 1″ XPS or EPS as a capillary break between wall and floor? (There is 10 mil poly under the 4″ slab but no insulation). I am planning to finish the floor using glazed tile directly on the cement (probably without Ditra).

On the top of the foundation walls there is a double sill plate installed over the pink sill gasket (not sure if the sill plate wood is PT or not) if this helps.

It looks like HD and Lowes only carry faced EPS. Would this be a problem (trapping moisture in walls causing issues to sill plate)? I will need to check with the 2 reclaimed insulation places in MA and see if they carry unfaced EPS. My guess is the perm. rating of unfaced vs faced EPS is very different?

Thank you!

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  1. GBA Editor
    Martin Holladay | | #1

    The uninsulated part of your basement wall -- the above-grade portion, which is 2 feet high -- is by far the most important part of the wall when it comes to heat loss. That's where most of the heat loss occurs. So, from a code standpoint (and a building science standpoint), you should consider your basement wall uninsulated.

    If you need R-15 insulation according to local code requirements, then (according to my judgment) you need to install insulation rated at R-15 on the interior side of the wall.

    I prefer the all-foam approach (4 inches of EPS or 2.5 inches of polyiso), but you can combine a thinner layer of rigid foam with some fluffy insulation if you insist. For more details, see this article: How to Insulate a Basement Wall.

  2. D Dorsett | | #2

    With 2' of above grade exposure on the concrete there is no need for the concrete to dry toward the interior.

    Unless the slab is insulated and there is a capillary break between the footing and wall it's a good idea to have at least SOME vapor retardency between the concrete wall and any moisture susceptible materials such as furring, studs, or paper faced wallboard, etc. If building a non-structural studwall for the finish wall, in zone 5 use at least 1.5" of EPS between the concrete and a fiber-insulated studwall. That provides sufficient dew point control at the above grade section of wall to limit moisture accumulation at the cool side of the fiber insulation over the winter, and sufficient vapor retardency to limit moisture accumulation at the below grade section of the fiber during the summer. Put at least an inch of EPS under the bottom plate of the studwall as a capillary & thermal break, which will keep the wood dry enough that you need not use pressure treated lumber.

    Rock wool in contact with the concrete can wick moisture to the drywall. An air space between the rock wool and concrete would be a thermal bypass, and a fire-spread path. Fill that space with rigid foam (R5 minimum, R6 better if using R15 rock wool).

  3. George M | | #3

    Martin/Dana, thank you both for the quick responses. I checked with Insulation Depot and they do not have any EPS in stock. I am waiting to hear back from the other place. Hopefully they have 4" EPS per Martin's recommendation so I can avoid the Roxul in the studs.

    If I have trouble getting reclaimed EPS I will buy 1" XPS from HD or Lowes and fill the studs with R-15 Roxul (studs tight against the XPS).

    The other option would be to buy faced EPS. Home depot has different ones: R-Tech, TUFF-R, Super TUFF-R but they all seem to be faced on both sides. Is the faced EPS going to be an issue?

  4. Expert Member
    Dana Dorsett | | #4

    In RI you're driving distance away from two large reclaimers of foam board, where you can get the stuff at a fraction of the cost of virgin stock EPS at Home Depot:

    With the amount of exposed exterior of the foundation walls facers of any type are not a problem. I did my house (probably less than 50 miles from yours) with 3" of fiber-faced roofing polyiso (reclaimed, at ~$20 per 4' x 8' sheet, though I've seen it for as low as $12 or as high as $25 since.) With polyiso you'll have to keep the cut edges from resting on the slab to avoid potential wicking of ground moisture (insert an inch of EPS or XPS for the bottom inch) but it's fine to use in this application.

    Roofing polyiso is typically 2lbs density and rated R5.5/inch for roofing applications. At 3" it meets or beat the R15 code min performance in y(our) climate.

  5. George M | | #5

    Thanks. These are the places that I sent the request. Insulation Depot in Framingham (I believe it is part of Nationwide Foam now) did not have EPS or XPS in the New England Area.

    I am waiting to hear back from the Green Insulation Group (Worcester, MA)

  6. Expert Member
    Dana Dorsett | | #6

    Insulation Depot==Nationwide Foam (SFAIK it was just a name change, nothing more.)

    Nationwide doesn't have 3"-4" polyiso? Among several other standard thicknesses, roofing polyiso comes in 3", 3.25" , 3.5" and 4" , any of which would be fine. Even 2" polyiso and R11 batts in a studwall would be fine.

  7. George M | | #7

    I did not ask them about polyiso at the time, so they may have some in stock. Would polyiso trap any moisture from the walls? Also, I do not have a sump pump and in the event of any water in the basement polyiso would be a problem (even with an 1-2" of eps on the bottom) that is why I was more interested in EPS/XPS.

    Green Insulation Group has 3" XPS 4x8 for $28 and 3.25" EPS 4x8 for $18. With the 3" XPS I would be at R-15 and I would not need any batts in the studs. With the EPS I would probably need additional insulation.

    However, I found something interested in the RI building code as listed on their website (attached). Unless I am missing something, even though we are in Zone 5, basement walls only need 10/13. I will need to verify with the inspector. With an R-10 continuous rigid foam on the interior walls the 3.25" EPS would be sufficient.

    I have another question. Would you recommend wall to wall DITRA installed below the glazed tile? I currently have some efflorescence on some spots on the floor and bottom part of the wall (the wall/slab opening is sealed with NP-1 Polyurethane Caulk). No other insulation on the slab AFAIK.

  8. George M | | #8

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