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Community and Q&A

Insulating below grade basement

TWslide | Posted in General Questions on

Afternoon – 

I am beginning a DIY basement finish and I wanted to check my method of insulating. 

I am in climate zone 4, with a fully underground foundation, with the majority of the rim joist below grade as well. To add complication, the house also does not have a sill plate, but the joist and rim joist resting on the block with no capillary break.i understand this is not ideal, but after 60 years the joists and rim joist show no rot/mold, only maybe a 1/4 inch of efflorescence where the joist touch. the block in some spots. The exterior of the house is brick veneer, with a 1inch air gap all the way to the block, in front of the rim joist.

Code requires at least r10, so I have purchased r 10 rigid xps, glued to the walls. The 2×4 wall will be framed in front of this with Sheetrock. 

after reading this article,
will the interior rigid insulation cause a moisture issue with my joists at the top of the foundation? I am planning to also treat the rim joist and joist ends with bora-care. 

thank you for your time and thoughts. 

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  1. GBA Editor
    Brian Pontolilo | | #1

    Hi TW.

    I'm curious how your rim joist is below grade. Does the birck veneer extend below grade?

    One positive is that there is an air gap between the walls and the brick. I would still proceed cautiously here. For example, you could install just a 1 inch piece of EPS in the rim which will get you minimal R-value, but will air seal the rim and still allow some inward drying.

    I'm curious to hear what others have done in this situation and how it was worked out.

  2. TWslide | | #2

    Hey Brian -

    The veneer does continue all the way down and sit directly on the block, maintaining the 1” gap down to the openings in the block. The rim board feels to be a rough cut 1x10, again with no signs of rot.


  3. Expert Member
    Dana Dorsett | | #3

    >"Code requires at least r10, so I have purchased r 10 rigid xps, glued to the walls. The 2×4 wall will be framed in front of this with Sheetrock. "

    R10 XPS is only warranteed to R9 at 20 years, and will eventually hit the R8.4 range (the same as EPS of simlar density) at full depletion of the climate-damaging HFC blowing agents, the part that makes XPS the LEAST green insulation material in common use in the US (by far- it's not even close!):

    That said, the additional air films of the empty studwall cavities will probably get you to code compliance on a U-factor basis, even if you're R1- R1.5 shy for making it on an R-value basis.

    EPS or polyiso would have been a much greener choice, being blown with low-impact hydrocarbon blowing agents. In the EPS case most of the isopentane blowing agent leaves the foam while it's still at the factory, where it recaptured, not vented to the atmosphere.

    >"...only maybe a 1/4 inch of efflorescence where the joist touch. the block in some spots."

    Efflorescence indicates moisture moving through the masonry. It's worth taking a 2-prong wood moisture meter and testing in multiple spots. Digging out the brick veneer to a foot or more below grade and installing weeps every third vertical joint a few inches above grade, and corresponding vents at the upper course of brick to vent the veneer to the exterior would likely lower the moisture content of the band joist & joist ends.

    Once sealed up with low permeance, insulating foam it will be even cooler/wetter with less drying capacity, with a correspondingly a higher risk to the joist ends. Before installing the (non structural) studwall it may be worth installing a 2x8 or 2x10 ledger board through-screwed to the foundation through the foam with stout masonry screws to support the joists in the event that the joist-ends begin to rot.

    1. TWslide | | #5

      Dana -

      Thanks for the reply. I purchased the XPs as recommended by a local contractor. Wishing I knew of its green house issues earlier.

      I had tested the wood after this post, which the highest level was 17%. I know that is not ideal but what I am thinking is that this area is drying towards the interior and that has kept it rot free for 60 years. With this, I am planning to coat with bora care to add some insurance.

      Since I’ve already glued the XPS on the wall, is it ok to leave? I am hoping it will not cause more moisture to rise to the rim area through the cmu block. Block has no indication of moisture to the inside.

      Also, given the potential moisture issues with the air seal, is it worth using roxul at the rim and letting the air move? I have yet to insulate the rim area.


  4. Deleted | | #4


  5. Expert Member
    Dana Dorsett | | #6

    >"I had tested the wood after this post, which the highest level was 17%. I know that is not ideal but what I am thinking is that this area is drying towards the interior and that has kept it rot free for 60 years."

    The wood moisture content is almost certainly going to rise after insulating, since it's average temp will be lower, and the moisture in the proximate masonry will go up due to both lower temperature and lower vapor permeance of 2" of XPS (about 0.5 - 0.7 perms, a Class-II vapor retarder that would qualify as a "vapour barrier" under Canadian building codes.) For the past 60 years it has been able to dry freely, with warmer average temps, and it's STILL above 15% moisture content. That isn't a good indication that it will be good for another 60 after insulating, unless other measures are taken.

    The XPS is already manufactured and installed, so removing it isn't going to do the planet any favors. It will work just fine. But even with the Bora Care I'd personally be inclined to install a ledger board and lower the grade level to a foot below the top of the foundation.

    If you're going to leave the exterior grade level where it is, a least make a 1-2' wide trench down to a foot (or more) below the top of the foundation, line the trench with landscaping fabric, then back fill with 3/4" or larger washed stone (no fines), wrapping the fabric over the top of the stone, with a top layer of stone as ballast to keep the fabric in place. That will give the foundation at least some capacity for drying toward the exterior, with minimal capillary draw for any surface or ground water toward the brick veneer & foundation.

    1. Expert Member
      Peter Engle | | #7

      If you do the trench thing that Dana mentions above, make sure that there is no roof water or surface water running into the trench. If this happens, the trench turns into a moat and that can lead to basement flooding and foundation damage.

      Insulating your band joist is risky, as Dana mentions above. If you use a vapor permeable insulation (rockwool, fiberglass, even low-density EPS), you will get some drying to the interior, at least when it is warmer outside than inside. You will need to maintain the basement at low humidity, summer and winter (<55% summer and about 35% winter). With low interior humidity, there is less risk of interior moisture condensing on the band joist and more capability for drying. Venting the back of the brick veneer, as Dana mentions will also reduce the risk somewhat.

      1. TWslide | | #8

        Dana and Peter. thanks for the reply and information. I will consult a excavation company on if regrading is possible, but it doesn't appear likely. I was wondering if the trench idea would hold water rather than allow it to run off as it currently sits, but i do have 18" overhangs and functioning gutters.

        If i were unable to regrade, would the best bet here be to keep the insulation on the walls and not insulate the rim joist? The rim does have a 1" gap to the veneer and CMU blocks beneath. Would it be a good idea to install weep holes to allow some exterior air in the 1" air gap that runs from foundation to soffit?


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