Insulation and HVAC for a Detached Garage – GWC Mag

In this Q&A thread, car enthusiast 17whippoorwill tells us he has just closed in his new garage, a 28-ft. sq. structure in Climate Zone 6A. It’s built on an insulated slab with 2×6 walls; He used Zip System sheathing on walls and roof, which is vented at the soffits and ridge. He writes that he’s “been through the wringer” with the spray foam used on his house, so he’s looking for less expensive insulation alternatives. He’s also considering something to heat and cool the space so he can work on his cars all year round.

Here’s what he wants to know:

Because he doesn’t want to use drywall, can he use faced mineral wool batts in the walls and leave them exposed? Are faced fiberglass batts any better or worse?

Can he cover the ceiling with 2-in. thick foil-faced polyiso rigid panels and (again) and not use drywall? He writes that the ceiling joists are not spec’ed to carry weight, and it’s his understanding that because the garage isn’t a living space, code doesn’t require drywall on the ceiling.

Will he need a vapor barrier of poly or similar between the insulation and the drywall?

Finally, he’s thinking about installing a wall-mounted, single-zone mini-split for his HVAC. He figures an owner-installed unit would be less expensive, but he may have trouble finding a contractor to service it.

So, today’s Spotlight topic: How can this car-loving homeowner insulate and heat his new garage without spending a bucket of money?

The crowd weighs in

DCcontrarian suggests 17whippoorwill place blown-in or batts in the ceiling, but that means having some sort of air barrier on the ceiling, such as drywall. He thinks faced fiberglass batts are the easiest and most…