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How to best insulate a garage roof with storage truss?

Griffin728 | Posted in Green Building Techniques on

Hello, I’m building a 16x44ft detatched garage and workshop that will be heated with a single 240v ceiling mount electric resistance heater periodically to 65 degrees and maybe kept at 40+ degrees through the winter. I’d like to insulate it as much as I reasonably can to cut down on the heating cost and limit the carbon footprint that comes with semi conditioning it in the first place. The garage will contain at least one high voltage electric car charger, but that’s another topic.

The walls are 2×6, so Im thinking standard fiberglass batting covered with drywall will be good there. The roof is half storage truss and half vaulted scissor truss. If you’re unfamiliar, the storage truss is framed using 2×4 with another interior triangle made of 2x3s and about 5ft headroom. I would like to insulate against the roof deck with vent chutes. It looks like that only leaves about 6in of insulation space at the peak, however, so the r38 Batts I was considering may be tough to fit. Is there any better option I should consider here, such as drywall and blown cell? Should I fir down the rafters to get the cathedral batting to fit, just go with 2×6 batting all around, or something else? Cost is a big condideration, but I’m willing to wait 10-20 years for any financial payback.

Last question, how much can I realistically air seal if the ceiling were left unfinished, and is that an argument to finish it with sheetrock? Should I consider a vapor barrier? I thought about running 1″ rigid foam on the truss and blowing cell behind it, but now I’m probably going overboard, and exposed foam might be a fire hazard with the ceiling heater, right? I know, it’s just a garage, but I want to do it right, and I enjoy the comfort factor too in the summer.

Minneapolis, MN

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  1. GBA Editor
    Martin Holladay | | #1

    The best way to get an interior air barrier is with drywall. It's cheap. In a garage, you don't even have to get fancy with the taping -- one quick coat of tape is fine.

    The other decisions are pretty much up to you. Obviously, if you plan to install R-38 batts in bays where the framing isn't deep enough, you will need to fur down the framing in some way to secure the batts well and to provide a surface for your interior air barrier.

    Whether or not you want to invest a lot in garage insulation depends on your budget and your goals. For more information on insulating sloped roof assemblies, see How to Build an Insulated Cathedral Ceiling.

  2. Griffin728 | | #2

    Thanks Martin. That makes sense intuitively. I'm wondering if you have any data that compares batt insulation hung without a drywall air barrier to the same batt insulation with the air barrier? Being its a garage, I don't care much about what it looks like, but this would be useful to help me justify the cost/value of the drywall to the building system.


  3. GBA Editor
    Martin Holladay | | #3

    You're right that the performance difference may not matter much to you in this type of garage application.

    Fire safety is an issue. Don't use kraft faced batts unless you cover the kraft facing with drywall -- kraft flashing is asphalt-impregnated paper, so it's flammable.

    If you use unfaced fiberglass batts, you have a new challenge: how will they be secured? You can depend on friction, but gravity tends to win out over friction in the long run.

  4. Griffin728 | | #4

    Perfect, thanks. You sold me on the fire hazard aspect. Drywalling may also allow me to do more blown cell instead of batts as well.

    Speaking of which, what is considered best practice to dense pack the wall of a new build? Do you leave the top foot of drywall off, cut holes in the wall after it's hung or use some sort of plastic membrane before the sheetrock is hung. Are any of these better/more cost effective, or is batting for the walls still the best bet?

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