Hey everyone! I need some advice on how to best insulate and condition my detached garage, which measures 16.5' x 22.5' (372 sqft) located in northern Michigan. It's built on a slab with 7' walls and a gable roof that peaks at 11'. The exterior has 1x10 tongue and groove pine siding. We use this space as a bar and entertaining area, so I want it to be comfortable all year round. I'm considering installing an 18,000 BTU mini-split system.
I'm planning to finish the interior walls and gable ends with more tongue and groove pine, but there's currently no ceiling installed—just exposed rafters—and I'd like to keep it that way. The roof isn't insulated; however, I might insulate the roof deck when I replace it in the future, even though the current roof is in good condition.
Since I want to maintain the look of the exterior siding, using house wrap on the outside isn't an option. Based on my research, I'm thinking about insulating the stud bays with R15 Thermafiber mineral wool, placing CertainTeed's MemBrain as a smart vapor retarder, sealing the sill plate, and then installing the interior tongue and groove over that, including for the gable ends.
Does this approach sound good? Any tips for improvements? There are no gable vents—should I add passive or humidity-sensing mechanical vents since I already have existing soffit vents? Thanks for your help!
3 Answers
I think your insulation plan is on the right track, especially considering how much heat will rise in a Michigan winter. Just remember that roof insulation usually needs to be thicker than wall insulation. While your wall insulation setup seems solid, if you don’t plan on adding a ceiling right away, you might be overdoing it. Insulation can help keep the garage cool in summer, but if you want that open feel, consider a wood-burning stove instead of relying entirely on cooling. Just keep in mind that exposed wood can be a fire risk, so putting some drywall behind that tongue and groove might not be a bad idea either!
Your insulation plan is looking solid! Just make sure your vapor barrier complies with local codes. The mini-split size seems appropriate for your space, but don’t forget you’ll likely need permits for this kind of work in Michigan. You can check what you’ll need either through your local building department or even use a handy app like PermitZen—it makes navigating permits so much easier! What’s your timeline like for getting this project off the ground? Permit processing can slow things down a bit.
You really should consider installing a ceiling to make the insulation worthwhile.
I really appreciate your incredibly reasonable and helpful comment!
Thanks for the feedback! Just to clarify, I plan on using the mini-split in ECO mode to keep the space just above freezing when not in use, then ramp it up before gatherings. I previously used a kerosene heater with no insulation, and it did a decent job. I’m hoping that with new insulated windows and doors, plus the planned in-wall insulation, it’ll perform much better!