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Community and Q&A

Insulating an Attached Garage

redhenranch | Posted in Energy Efficiency and Durability on

Hey Friends,
We are about to insulate an attached garage (no overhead building space, just a standing seam roof) and wanted to check-in on the best method. It’s 2″x6″ walls and 2″x10″ roof. We won’t probably heat it much at all except for occasionally, but we do want a conditioned and insulated space for working and storing root crops —- we’re in Vermont and our basement is heated (thus too warm for storing root crops). The exterior sheathing is ZIP. I was thinking to do blown-in cellulose, should I also try to put a rigid thermal barrier, also for better air-sealing? Any suggestions would be great, thank you!

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  1. tjanson | | #1

    Are you planning on cooling the garage? It's unclear what you mean by "but we do want a conditioned and insulated space".

    I would keep it simple for a mostly unconditioned space - mineral wool or fiberglass batts and drywall or OSB for for your finish wall and ceiling. Include a vapor retarder by either kraft faced batts, kraft paper or smart membrane vapor retarder installed over the studs, or using vapor retarding primer on your walls. I prefer 2x6 r23 mineral wool over 2x6 r19 fiberglass for my projects just because it's easier and more pleasant to install, but it's something like twice the cost.

    If you use OSB for the finish surface you are assuming a greater fire risk but gain durability and some practicality for screwing things into it. With OSB you should probably use a vapor retarder layer underneath so you don't have to rely on caulking the seams to prevent moist air intrusion into the stud bays.

    The garage door will be a weak point. You may want an insulated one if it's not already.

    You may want to verify that the windows are properly flashed to the Zip before insulating, sometimes those details are omitted on uninsulated structures because they have good inward drying.

    Edit: My reply was assuming you have 2x10 ceiling joists/ truss bottom chords running horizontally and you would be adding a conventional ceiling and making the "attic" vented.

  2. wayno_from_vt | | #2

    This is great. Thank you. I have a similar project in mind.

    We have an attached one car garage with a conditioned room overhead. The garage is unconditioned space with no windows and a single overhead door and a door to the main living space.

    I had leftover insulation from a basement project 2 years ago so I added a mixture of are R19 unfaced fiberglass batts and R23 rock wool batts to the stud bays and then installed 4 x 8 sheets of pegboard as interior surfaces.

    The previous owner used foam insulation around the room over the garage, leaving the areas behind the knee wall unconditioned. Then installed drywall on the garage ceiling. I cut a small hole in the drywall to take a few photos of the space. I can see that they left an air space between the foam board and the roof sheathing for ventilation, that’s good.

    I'd like to pull the drywall down, exposing the open area behind the knee wall for added storage. I'd also like to insulate the uninsulated area to make the garage more usable in the winter. That said, I don't expect it to be near the temperature of the main living space.

    I would use the same technique the previous owner used leaving a gap between the foam boards and roof sheathing running down to the soffit vent, using spray foam to plug the gaps as I go. I was planning to use unfaced fiberglass on the exterior 2 x 6 walls.

    While I'm at it, I'd like to add additional foam insulation around the room over the garage to make that room easier to condition in the winter and summer.

    I know the garage door will be a weakness.

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