I have a finished room over half of my garage. The unfinished space is attic storage and not insulated.

The room is generally colder than the rest of our house and I think a big reason Is the open space between the floor joists. It runs the width of the room - about 17 feet.

What could I put here to insulate it better?

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  • 1
    In that photo - we're looking at the outside wall of the room, and the space is under the room floor and between the joists? And the joist cavities are partially insulated?
    – SteveSh
    Nov 16, 2022 at 0:26
  • Agree with @SteveSh here - it's really hard to tell what we're looking at. Where is the photographer standing. What part of the structure are we looking at? What's on the other side of the pink insulation? What's below the grey insulation?
    – FreeMan
    Nov 16, 2022 at 0:56
  • I am in the attic (not insulated) looking at the wall of the finished room. The other side of the insulation is the room. The space between the joists is the space between the floor f the room and the ceiling of the garage
    – Joe
    Nov 16, 2022 at 2:26

1 Answer 1


Cold air from the attic is freely circulating under the floor of the cold room. Plug those openings with insulation.

If you can stuff 12in deep in to the space, that would already be excellent. Make sure the insulation is stuffed tightly to keep itself in place and to choke off air circulation. No need to use spray foam, just rock wool or fibre glass batting is fine. (Don't use rags, old mattress foam or other shortcuts, house insulation is fire rated) They say "don't compress insulation", but that is fine here. Squeeze it or fold it over as needed to get it jam tight.

Additionally you can close those openings with strips of wood, like plywood. Floor joist space does not have to breathe. Leave gaps for the cables so they do not get pinched, and stuff the gaps with left over pieces of insulation. Frankly, this is extra work that won't add much after you have stuffed the openings.

This assumes there are no other openings on another side, but if that's the case, this would also stop or limit drafting.

  • Ah! So I can just stuff it. That was my original plan, but I thought it might have to breathe. Thanks!
    – Joe
    Nov 16, 2022 at 6:13
  • Oh, and does it matter if I use the paper backed batting? Would that cause moisture to build up?
    – Joe
    Nov 16, 2022 at 6:14
  • You don't need a vapor barrier here. You're just trying to keep cold air from freely circulating under the floor. If paper or foil insulation is what you have, just slice the paper/foil with a knife or peel the paper off the batt. This is one place where Rockwool has an advantage over fiberglass in that it is more forgiving of a bit of moisture buildup.
    – SteveSh
    Nov 16, 2022 at 11:33

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