Garage Insulation

A room over the garage has cold garage air under its floor, rim joists on the edges, and sometimes a cantilever hanging past the wall. Batts that only half-fill the joists leave an air gap right under the subfloor. The rim and cantilever bays get sealed first, then duct boots and penetrations, then the joist bays get filled so insulation touches the subfloor, often by dense-packing through the ceiling drywall.

ENERGY STAR's floor target for our climate zone is R-30. Rochester and Olmsted County code calls for 5/8 inch Type X gypsum on a garage ceiling under habitable rooms, so any ceiling that gets opened goes back to that standard.

Garage walls are usually open studs on the garage side, so fiberglass batts, mineral wool, or spray foam go in easily. In an unheated garage, the wall shared with the house matters most.

Outside walls are worth insulating when the garage is heated or will be. Without heat and a tight overhead door, insulating them only buys a few degrees.

This wall keeps heat in the house, and under local building code it is also the fire separation, with at least 1/2 inch gypsum board on the garage side. The door into the house has to be solid wood or steel at least 1 3/8 inches thick, or 20-minute rated, and self-closing.

We fill the cavity, seal outlets, pipe holes, and the bottom plate against car exhaust, and make sure the gypsum goes back with no open holes. If your house door is hollow-core or does not close on its own, the estimate will say so.

A garage with its own attic over a flat ceiling is insulated like a house attic: seal the ceiling, set baffles at the eaves, and blow cellulose or fiberglass across the joists. It pays off when the garage is heated or the room beside it is living space.

Where the garage attic opens into the house attic, the separation between them (1/2 inch gypsum under local building code) has to be continuous before insulation goes in.

Closed-cell foam fits garage rim joists, the underside of a cantilever, and the walls of a heated shop, where it seals and insulates in one pass. Exposed foam in a garage has covering rules of its own, so the product label and Rochester and Olmsted County code get checked first. Foam types are covered on our spray foam insulation page.

A heated garage needs the same treatment as the house: insulated walls and ceiling, air sealing at the top plates and outlets, and weatherstripping at the overhead door.

The overhead door and the slab edge are often the biggest losses left after that, and the estimate notes where each one stands.