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Community and Q&A

Cellulose insulation in basement?

Jessie Pratt | Posted in GBA Pro Help on

We are putting a granny suite in our walk-out basement (newly constructed house) and are thinking of insulating the knee walls (wood framed walls between the top of the stepped foundation and first floor deck) and the ceiling with cellulose. The cellulose in the ceiling would butt up against closed cell spray foam at the rim joist. The floor above will be concrete (radiant heating).

Would anyone recommend against using cellulose in this situation? Are there issues with using cellulose in the basement knee walls? I have read recommendations against insulating a basement ceiling, but we need the insulation for noise reduction and it would also help the first floor slab from radiating heat downward. The basement slab is also radiant.
The house is located in zone 7 (cold winters, warm humid summers)

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  1. GBA Editor
    Martin Holladay | | #1

    I don't foresee any problems, as long as:
    (a) you pay attention to airtightness when insulating the framed walls, and
    (b) you remember to insulate the concrete walls with rigid foam or spray foam.

    If there is a ledge at the top of the concrete walls, don't forget to install horizontal foam insulation to cover the ledge.

  2. Jessie Pratt | | #2

    If the concrete walls are insulated on the exterior with 6" of xps (which they are) would we still need to insulate the interior? My concern with the cellulose is the moisture it would take on in the summer, but it may be able to handle the moisture load.

  3. GBA Editor
    Martin Holladay | | #3

    If your concrete walls are already insulated on the exterior, you are all set.

    If your house was designed well, there shouldn't be any reason for your basement to be damp, so you shouldn't have any reason to worry about the cellulose.

  4. Dick Russell | | #4

    My only concern about the foundation walls would be conduction downward through the footer. If the surface of the concrete feels decidedly cool in winter (a thermometer taped to it and covered with a patch of insulation for a brief time would tell), then for the sake of comfort it might make sense to apply a 1" layer of foam, protected by drywall. But I'd like to hear other opinion on this.

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