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Community and Q&A

Upgrade insulation of floor of room over garage

user-968787 | Posted in General Questions on

I have a 300 sq-ft room over an unheated attached garage. As is common the floor is cold. The engineered joists have been sprayed with 3-4 inches of 2lb spray foam.

I want to supplement the insulation in the floor (garage ceiling). I’m not really sure what the best option is.

Climate zone 7a 8500DD.

Any advice appreciated.

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  1. user-577475 | | #1

    I've had some success with blown in cellulose in a similar retrofit situation. You can generally have it done, or do it yourself, with a 1" or larger hole in either the ceiling or floor drilled at each end of the floor span in each truss space. Depending on how deep your floor trusses are, it may take a lot of cellulose but it is still one of the lowest cost per cubic foot of insulation and is recycled material.

  2. BobHr | | #2

    Was the foam applied to the underside of the floor? If there did it correctly a 2lb foam will be an air barrier and a vapor barrier. Were the ducts sprayed too?

    Does that room have an air return. Air returns will suck a lot of air from adjoining spaces. If they are not well sealed you run the risk of sucking any fiberglass or cellulose insualtion in to your ducts.

  3. GBA Editor
    Martin Holladay | | #3

    Three inches of closed-cell spray foam gives you an R-value of about R-20. That's OK, but not great. Still, it should be enough to avoid cold floor problems. If you have cold floor problems, it may be because of a defective heating system, not a problem with your insulation.

    If you want to upgrade to R-40, you can always have more spray foam installed.

  4. user-968787 | | #4

    Yes the foam is on the underside of the floor. The underside is still exposed. Drywall is going up this summer so this is the opportunity to improve the situation.

    Another 3" of spray foam is pretty expensive so i'm looking for an alternative.

  5. GBA Editor
    Martin Holladay | | #5

    Either fiberglass batts (as long as they are thick enough to completely fill the joist bay -- or, better yet, be compressed) or blown-in cellulose will add R-value to your floor assembly at lower cost than spray foam.

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