Helpful? 0

Insulating a garage ceiling

I am designing a conditioned space over a garage in Chicago (Climate zone 5). The floor/garage ceiling structure will be 18" deep wood floor trusses, with 3/4" plywood floor sheathing and 5/8" gyp on the underside of the garage ceiling.

Code (IECC 2012) says R-30 insulation is needed here, which would be a 9 1/2" thick fiberglass batt. Typically this would get installed from below with the Kraft paper facing on the bottom (so that it would be in contact with the gypsum board). However, the kraft paper should be on the "warm-in-winter" side, which it's not, and the floor trusses end up uninsulated (and the 2x4 members have the 3-1/2" side against the ceiling, not the 1-1/2" side).

Does anyone have recommendations of a better approach to insulating a system like this?

Asked by David Wytmar
Posted Fri, 01/10/2014 - 14:47
Edited Fri, 01/10/2014 - 15:49


7 Answers

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Drywall installed leave a row of subfloor unfastened.
Fill from top with cellulose.

Answered by aj builder, Upstate NY Zone 6a
Posted Fri, 01/10/2014 - 15:36

Helpful? 0

A.J. is certainly correct that cellulose is a much better solution than fiberglass batts, especially for this kind of floor assembly.

Most experts suggest that the insulation should be in continuous contact with the subfloor. Joe Lstiburek is an outlier -- he says that it's OK to let the insulation sit on the drywall, with an air space between the top of the insulation and the subfloor. But the Lstiburek approach only works if you install spray foam at the rim joist, and if you are meticulous about sealing penetrations at the drywall ceiling and subfloor. Even then, the approach won't work as well with batts as with cellulose.

Don't worry about the kraft facing. It isn't important. Your subfloor is already a vapor retarder.

For more information on a variety of solutions to this problem, see How to Insulate a Cold Floor.

Answered by Martin Holladay, GBA Advisor
Posted Fri, 01/10/2014 - 15:45

Helpful? 0

AJ, Martin, thanks for the feedback. I recognize the article now and it didn't come up when I searched for it.

Part of the problem is that if I spec cellose, it won't get installed, because it means a different trade has to come in. Batts and caulk and rigid board can be installed by guys who are already on site. The 1-1/2" of rigid on the underside helps too.

Answered by David Wytmar
Posted Fri, 01/10/2014 - 17:32

Helpful? 0

Dense packing cellulose around the perimeter at the beginning of the install solves the air leaks. Cellulose also will fill in truss webs. You can buy it material and labor included for a set fee.

Fiberglass is going to let air move and make it less effective. One air leak in rigid foam and rigid foam's R value is 0.

Good luck.

Answered by aj builder, Upstate NY Zone 6a
Posted Sat, 01/11/2014 - 13:54
Edited Sat, 01/11/2014 - 13:55.

Helpful? 0

I would spray open cell to the underside of plywood. Foam is an air barrier, foam will fill in around web trussing.
Insulation will be in contact with underside of floor.
We do spray foam if you need a contractor in Chicago area.

Answered by Mike McNish
Posted Sun, 01/12/2014 - 12:26

Helpful? 0

Mike, what would R-30 cost for a 24'x24' area?


1-open cell
2-closed cell
3-cellulose (seems you would know the other trades prices)
4-fiberglass with the rigid foam layer

Bang for the buck to me is cellulose. I could even set up a laborer to do the install. Not against open cell at all, but am trying use much less of it in the future.

Answered by aj builder, Upstate NY Zone 6a
Posted Sun, 01/12/2014 - 17:49

Helpful? 0

Hey David,
Closed Cell Foam will work really well for your project. You can get the insulation closest to the conditioned area, it adheres easily to the surface and it will give you an air-tight seal so that the cold Chicago air can’t travel up into the room above.

I would say about half of our customers insulate with Foam It Green only and the other half does a combination method. You will get great results with either approach.

Foam It Green’s closed cell foam has an R-Value of 7 per inch so if you local inspector is strict you would need 5 inches to get an R-30.

If you choose the combination method I recommend first adhering R20 rigid foam board to the underside of the subfloor. Once the foam board is fully adhered you can spray 1 ½” to 2” of closed cell Foam It Green over the board to get the extra R-Value and air-tight seal.

And a few extra bonuses with going the Foam It Green route- you can quickly and easily do the job yourself, our formula resists mold growth on the face of the foam and we’re right in your neighborhood. We have a pickup location just outside of Chicago.

Our website is if you would like to take a look at our kits.

When are you planning on doing the project?

Answered by Sprayfoam Ashley
Posted Tue, 01/14/2014 - 12:44

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