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Community and Q&A

Insulation on exterior and interior

Meep | Posted in Building Code Questions on

My house was built in 2009 in climate zone 6. It has a poured concrete foundation with R-5 XPS exterior insulation. Currently there is no insulation on the inside.

I am planning on finishing my basement and am curious on what the best way to insulate the interior would be. I spoke with my city inspection department and got some conflicting information on energy code compliance. One inspector advised not to add any additional insulation and the other advised 1″XPS with a 2×4 uninsulated studwall was sufficient but the XPS was not totally necessary.

As i understand it i have to me R-15 ci? This would imply 2″ XPS on the inside? Would it not be a good idea to have some additional rigid foam on the interior surface?


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  1. Expert Member
    Dana Dorsett | | #1

    On the above grade section, from a wintertime dew-point control perspective R5 isn't quite sufficient for an R13 studwall without interior vapor retarders, but it's not particularly risky. The below grade section of wall will have warm enough temperatures that you will not get dramatic moisture accumulation in the wall.

    But then there is the issue of moisture migration into the concrete wicking up from the footing- you want at least SOME vapor retardency and a capillary break between the studs and concrete.

    From an IRC 2015 code-minimum performance point of view R5 continuous insulation + 2x4/R13 studwall would get you to code min thermal performance, but if you want to avoid the potential mold-farm put a continuous layer of at least 3/4" of cheap Type-I EPS (R2.9, low permeance)) with facers, or 1" of unfaced Type-II EPS (R4.2, ~2.5-3 perms) against the concrete and build your studwall snug up against it on the interior, with R11-R15 batts. Unfaced or kraft facers are OK, but no foil faced batts, and no interior polyethylene.

    While you're at it put 3/4" -1" EPS under the bottom plate of the studwall as a thermal and capillary break from the slab. You won't need to use pressure treated lumber if you do that, and it won't accumulate summertime moisture.

  2. GBA Editor
    Martin Holladay | | #2

    I agree with Dana. You should install at least 1 inch of continuous rigid foam on the interior of your basement wall before you install studs (or any insulation between the studs).

    While Dana is a fan of EPS, and so am I, it's worth pointing out that from a building science perspective, XPS works fine in this location. (However, green builders try to avoid the use of XPS because it is manufactured with a blowing agent that has a high global warming potential.) Polyisocyanurate foam will also work fine in this location, as long as you maintain a 1-inch gap between the bottom of the polyiso and the slab.

    For more information, see How to Insulate a Basement Wall.

  3. Meep | | #3

    Thanks for the helpful responses, I will go with EPS and fiberglass batts.

  4. Expert Member
    Dana Dorsett | | #4

    If going with low density EPS be sure to buy the stuff with facers, which makes air-sealing the seams with tape a lot easier. Unfaced Type-I EPS is dirt-cheap, but it's easy to break in handling, and harder to air seal.

    Foil faced polyiso is easy to seal too, but usually a bit more expensive per R than Type-I EPS with facers. XPS is usually even more expensive still (and less-green than polyiso or EPS.)

    Of course, no virgin-stock foam is greener than using reclaimed foam, which is usually quite a bit cheaper too. I can often find reclaimed 2" roofing polyiso for about the same money per square foot as virgin-stock 1" Type-I EPS at box-store pricing, sometimes less, eg:

  5. Meep | | #5

    I have poly faced eps available near me, looks like it is semi-impermeable, is this ok from a moisture control standpoint? Would it still allow drying to the inside of the wall in the event of condensation?

  6. user-2310254 | | #6

    EPS is fine. You need r-15 for below grade portion according the 2012 IRC. That is 3 to 4 inches depending on the foams density. This article outlines all the key you need to consider:

  7. GBA Editor
    Martin Holladay | | #7

    Steve's thickness recommendation assumes that all of the R-value will come from the EPS.
    Dana's thickness recommendation (in Comment #1) assumes that some of the R-value will come from the EPS, and the rest of the R-value will come from fluffy insulation between the studs.

    In any case, you don't need to let the concrete dry to the interior. You want to cover all of the concrete wall with a continuous layer of EPS. Once this is done, the surface you're concerned about (in terms of condensation risk) becomes the interior surface of the EPS.

    One inch of EPS takes you (just barely) out of the risky area where you need to worry about condensation on that surface, but more EPS wouldn't hurt.

  8. user-2310254 | | #8

    Martin, thanks for clarifying. I guess we should also point out that the insulation requirements may be different if parts of the basement are above grade. If more than 50 percent of a wall section is above grade, then above-grade wall insulation requirements apply. Correct?

  9. Dana1 | | #9

    The IRC does not distinguish between above & below grade portions of basement walls- the minimum performance prescription doesn't change if only 1/4 of the wall is below grade or 90% of the wall is below grade.

    From a moisture control point of view having the IRC prescriptive R value for Class-III vapor retarders on the exterior side of the fiber insulation works. You can get away with slightly less, but there's no need to push your luck. For zone 6 that would be R7.5 minimum on the exterior of a 2x4 fiber insulated wall, per Table R702.7.1 in Chapter 7 (about 1/4 the way down the page):

  10. user-2310254 | | #10

    Oops. I was looking at the IECC. In this case, I guess they are not identical.

  11. Dana1 | | #11

    The IECC does not distinguish the R-value between the above & below grade portions of a basement wall either. See Table N1102.1.2 (about half way down the page):

    The prescriptive values for basements don't describe the most moisture-safe methods of dealing with basements though. A 2x6/R19 studwall up against the foundation can easily become a mold farming operation.

  12. user-2310254 | | #12

    Dana. I'm sure you are correct. I was going off the explanation in this Department of Energy PPT on IECC residential requirements, specifically the notes in slide 51. ( Do you think this is "bad" information that should go in the trash folder. It made sense to me since homes in my area often include daylight basements with framed cripple walls that have a lot of daylight exposure.

  13. Meep | | #13

    Thanks for all the helpful responses. Im doing 1" or 1.5" of EPS with R13 batts for the foundation walls.

    I did run into another issue, on closer inspection i think i may have a mold issue already with the above grade exterior framed walls in the lookout section of my basement. These are 2x6 with R19 min fiberglass and a poly vapor barrier on the interior of the wall. There is no drywall up. When take a real close look at the poly i can see sporadic black spots all over the place. I know this wall construction is probably not the best for a basement. Im guessing hot humid summer air penetrates the wall cavity and condenses on the outer side of the vapor barrier?

    Does this sound like mold, if i tore all of the insulation out and replaced it, what is the best mold resistant retrofit?

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