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Insulation approach for an above ground sandstone wall

beth_rogers | Posted in General Questions on

I am renovating a 1905 wood frame house in Pittsburgh, we have cold winters and hot summers.  The basement for the house is sandstone and is above ground.  Only the basement floor is below grade.  The basement walls are probably 2′ thick.  I want to insulate the basement, it will be heated as it will be living space.  But I want to take advantage of the thermal mass of the walls so my thought is to add 2″ external rigid foam with some kind of siding on top of to keep water out.  I’m wondering if anyone has experience with this approach, or a better idea?  It’s weird not having an underground foundation.

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  1. user-2310254 | | #1

    Hi Beth,

    Have you read Martin's article on insulating basements? See

    Here is my layman's take based on reading various GBA articles. (Maybe an expert contractor will chime in in subsequent posts.)

    Is your stone wall made with uniform cut block? If so, applying reclaimed rigid foam to the interior would probably be a lot easier. If the surface is uneven (more like a rubble wall), GBS articles typically advise installing closed-cell spray foam to the interior wall surface.

    The one thing I wonder about is whether the moisture reservoir nature of the stone raises any additional issues.

  2. beth_rogers | | #2

    Hi Steve - Thank you. I have read Martin's article, probably a few times. But I have yet to be convinced that it is worth losing the thermal mass of the wall by insulating on the interior. The stone wall is definitely not uniform, I expect it to be hard to hang anything on it. I could possibly see creating an exterior wall and then filling the gap between it and the stone wall with spray foam. And I know the transition from the wood frame siding to the whatever happens on the exterior stone is going to be key.

  3. user-2310254 | | #3

    Hi Beth,

    GBA has articles on thermal mass such as this:

    The article notes the following in regard to how thermal mass works in colder climates:

    "If daytime highs are 50°F or less for months at a time — as they are in colder areas of the U.S. during the winter — thermal mass won’t help much. After all, heat is flowing through your walls in just one direction: from the interior to the exterior. Under these condition of steady-state heat flow, you need insulation more than you need thermal mass."

  4. beth_rogers | | #4

    Good article, thanks. My point though is that the mass already exists, and insulation can be added. It's not a question of one or the other. And the space within will be heated and cooled as part of the central heating/AC system. I believe the R value of the thermal mass is insignificant in this case. I believe (hope?) the effect of adding insulation "should" stop heat and cold flowing through the massive wall. It's just a matter of does the insulation help more on the inside or the outside of the mass wall. If it's on the outside, there will be a lag time between getting heat stored into the stone, or, getting the stone cooled during the summer, but I think the dissipation rate is very much slowed because of the insulation. Or conversely, if the insulation is on the inside, won't it be fighting the exterior temperatures stored within the stone walls? I also haven't gotten to thinking through any condensation problems. But I'd rather have that be outside between the stone and the insulation then inside behind the insulation.

  5. beth_rogers | | #5

    Also, there is this: I think it is analogous to what I am trying to do but am looking for some input.

  6. user-2310254 | | #6

    Hi Beth,

    This site ( indicates that the R-value of limestone is between .067 and .114 per inch. If this is accurate, a 24 inch thick wall would be between R-1.61 and R-2.74.

    Martin's article suggests it makes more sense in a heat-dominated climate to put the insulation on the inside.

    Of course, you may be right that the limestone is analogous to the brick in the "Ugly Brick" article. (Advice on insulating brick walls often includes a lot of "it depends" statements.) I would note that the contractor was installing rigid insulation continuously from the footer all the way to the roof line (along with putting in new windows and bucks).

    Maybe we can get some input from someone who works in your region.

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