My garage gets really cold in the winter and I'd like to insulate my ceiling and/or roof.

Here is my ceiling

enter image description here

I store quite a bit of things in the ceiling, so the idea is to insulate the inside of the roof and then build out the "ceiling" - put in a ceiling and ceiling door with drywall. If I have insulation in the inside of the roof, would I need to insulate the ceiling also, or would this be redundant..?

I have also seen aluminum sheets and foam insulation - are these used in combination or separately? What is the purpose of each and what should I use for my application?

  • Though it is common practice, the bottom chords of plate connected wood trusses typically are not designed for to support 'attic live loads'. Typically, trusses such as those pictured are designed to support the ceiling and nothing more.
    – user23752
    Nov 20, 2014 at 22:29

1 Answer 1


I just did closed-cell spray foam in my attached garage.

spray foam garage

There is a bedroom above (to the left of the I-beam in the photo), while the front sloping part is a hot roof (see Do I need to add roof vents if I close off a small attic space in the garage? for detail/pics).

In your case, you should decide if you want the attic space to be 'conditioned' space or not -- You should not have insulation in both the ceiling and on the roof. That will just create different thermal zones, which can mean you have different places for condensation to form, which means you can get mold. Without actually conditioning (heating or AC) the garage itself, which one is hotter and/or more humid at any time will change throughout the year, and I think having two layers of insulation is just going to cause problems.

I'm not sure what you mean by 'aluminum sheets' -- perhaps you are referring to foil-faced Polyisocyanurate? The foil in that case forms a radiant barrier -- in the case of attic space, I'm not clear on if this would actually make a noticeable benefit. Polyiso does have a very high R-value per inch (near that of closed-cell foam) so it may make a good choice anyway regardless of the foil -- but cost and value are at issue (I'm not sure how it compares cost-wise to regular XPS).

Keep in mind that if you're using any type of foam boards between rafters, that the rafter itself will not be insulated, so you only get the insulation value of the wood itself. This is one of the benefits of spray foam (and the reason I went with it) -- you get a complete thermal break, and it fills all the crevices. Expensive, but they did my entire garage in about 4 hours (including setting up and cleaning up, and encasing some duct work that hangs below the ceiling joists).

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