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Community and Q&A

Insulating basement wall

leonardotmnt | Posted in General Questions on

When we bought our house the owner had an interior french drain installed because of some water intrusion.  The company that installed the drain cut the finished drywall and studs to remove the bottom 2-2.5 ft of the wall.  We’d like to refinish the basement and have the wall rebuilt.  The remainder of the wall has batt insulation.

Ideally we’d like to tear the remaining studs and drywall out and put XPS up on the wall behind new studs but that’s cost prohibitive so we’re just going to rebuild the bottom section of the wall.  Should we use XPS behind the rebuilt bottom section of the studs or just use batt for that section since everything above the 2-2.5 ft mark is already batt?  One wall is below grade and the other is completely above.  We live in PA if that matters. 


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  1. Expert Member
    Dana Dorsett | | #1

    Use EPS, not XPS (and not polyiso) for the insulation below the high tide mark. XPS is the least green insulating mateial in commin use, with a severe environmental impact due to it's HFC blowing agents, whereas EPS (using the same polymer) is blown with low impact pentane. As the HFCs diffuse out over time XPS performance falls to that of EPS of similar density.

    Batts should not be in contact with a damp foundation wall. Batts can be reasonably combined with sufficient foam board against the wall for dew point control at the foam/batt interface on the above grade section. The amount of foam-R needed to make that work depends on the climate. PA spans US climate zones 4A, 5A, and 6A, so to make a specific requirement requires knowing where in PA you are located.

  2. Expert Member
    Peter Yost | | #2

    Hi Leonard -

    The insulation and air sealing of below-grades walls is most important at the very top of the wall (the transition from the concrete wall to the wood frame above) and the above-grade portion of the foundation wall (because the temperature difference inside to out is greatest here; the difference drops as you move further below grade).

    You will only finish your basement once so I would do it right and spend the time and money to redo the wall for both energy and moisture reasons.


  3. leonardotmnt | | #3

    We have 2 walls that aren't below grade. Would those be fine to leave as is with the batt insulation? If we only have to redo 1 wall it might be more feasible to do. The 4th wall might not be possible to do because the furnace and some other things are up against that wall. Right now that wall isn't finished at all.

    Another question as far as building the wall goes. I've seen on this site that it was recommended to use a dimple mat behind the rigid foam insulation in situations where there's a sump pump in the basement. Is this something you should always do? Most installations I've seen just put the foam right against the block. Is it the same type of dimple mat as people would use on a floor? Should the order be: wall-->dimple mat-->rigid foam-->studs-->drywall?

    I feel like the more I try to figure this out the more confusing it gets.


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