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Insulating a stepped foundation from the interior

user-901114 | Posted in General Questions on

I have a basement room that I’m planning to insulate. It’s new 2×6 construction but the foundation is 15 years old. It’s in zone 6. It’s stepped with the west wall being half 8ft and half 4 ft of poured concrete, a south wall being 2ft and an east wall being a 1/2 ft off the floor. I insulated the exterior of west wall with 2in of XPS from the outside while digging it out for a drain. That wall had been getting wet, but is now very dry due to the drain work. I’ll also be building a wall to seperate the garage from the room and insulating above the garage.

There is a wood stove and a wall of south glazing in the room. I wasn’t planning on insulating the floor.

I am considering framing a 24oc inner wall from ceiling to floor which means the outer wall will be partially concrete. I’d use PT on the floor plate with sill seal under that. Making a 12in space to insulate with roxul or perhaps cellulose.

I don’t feel really good about burying the sill behind insulation. I like being able to see the sills in house, so my alternative is to roxul the 2×6 framed sections and puting an inner layer of 1in polyiso over the studs. Leaving the concrete heat sink and sill exposed for me to look at..

Any watchouts with my inner wall idea? Any feedback in general? A better way?


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  1. GBA Editor
    Martin Holladay | | #1

    My own rule for insulating basement walls (or walls that are partially below grade) is to avoid the use of air-permeable insulation like fiberglass batts, Roxul mineral wool, or cellulose. The danger is that some of the concrete will be cold, and the warm interior air will reach the concrete, leading to condensation.

    It's always safer to use rigid foam or spray foam for this application. There are a variety of ways to do it. You can insulate the concrete separately from the framed wall if you want, or you can frame up a new stud wall as your propose. Just pay attention to air sealing and the continuity of your thermal barrier, and try not to leave any air space between your concrete wall and your insulation layer.

  2. user-901114 | | #2

    Thanks Martin. Is it a big penalty in heating dollars if I were to not insulate the concrete and sill at all? Keeping in mind that the biggest wall has insulation on the outside and the south wall is baking in the sun all day (when it's sunny)... Do the concrete walls serve as a thermal mass for the wood stove, which is the primary heat source for the house? Or are they just a heat sink that will be sucking away BTU all day?

  3. GBA Editor
    Martin Holladay | | #3

    If you have installed R-10 of rigid foam on the exterior of your basement wall, that meets the minimum requirement of the building code.

    There's a problem, however: if you only insulate one out of four concrete walls, you haven't really provided continuous insulation. You end up with a big thermal break at the corners. Heat will just find away around your insulation, because concrete is a conductor.

    Because you weren't able to insulate all of the exterior of your concrete walls (on all 4 sides), I urge you to insulate your concrete walls on the interior.

    In your climate, there really isn't any thermal mass benefit to keeping your basement walls inside of your thermal envelope, so don't let thermal mass considerations lead you astray.

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