Two of the core questions an auditor asks during a home inspection are, “Where is the thermal boundary?” and “Where is the nearest Dunkin’ Donuts?” (Those two questions run neck and neck.) The thermal boundary is the demarcation line for energy movement between the unconditioned exterior and conditioned interior air. This can be a bit fuzzy sometimes and one part of that fuzziness can be the garage door on an attached garage.
To insulate or not to insulate?
It’s odd when I find myself advising clients not to insulate, but that’s the case here. Normally, I can’t pile enough of the stuff in your house. Given my druthers, I’d probably spray 10 inches of closed-cell spray foam over your new dome house, add ventilation, and call it a day. But most folks are sticklers for windows and doors, so that won’t work.
The question of garage doors comes up mostly in relation to attached garages with a finished room overhead. The arrangement integrates living and utility space in a way that needs some attention.
Like a window, a garage door is a bad wall
A building enclosure should be well insulated and nearly airtight. And you may have just thought to yourself, hell, garage doors are neither well insulated nor airtight. Yes, they make lousy components of the building enclosure.
The door panels and contact edges are very leaky, and if you install foam board or fiberglass batts on the interior side of the panels, the insulation tends to chafe and fail over a not terribly long period of time. One can purchase a tight, well insulated garage door, but you know what is better insulated than any door? A wall.
Even if the garage door were insulated to some spectacular level, odds are that the concrete slab it is built on is not. So walls it is.
The ceiling of the garage and the interior connecting walls need to be treated as the edge of your home’s building enclosure. All too often, an attached garage is an intermediate space which is insulated and finished very haphazardly.
Check out the photo at the bottom of this page — the one of a garage ceiling. Umm… That doesn’t look good.
The photo shows a garage ceiling under the finished room. If the camera’s flash had lit inside the hole, you would have been able to see the underside of the plywood subfloor. Not good news.
The walls between your home’s heated interior and your garage should be viewed as the building’s thermal and air boundary. All of the insulation should be well installed, and all of the drywall should be intact and taped. Treat it as carefully as you would any finished wall.
Attached garages are all too often a weak point in the building enclosure. But as a friend is fond of saying, it doesn’t have to be that way. Insulate, air seal, and finish the garage ceiling and interior walls, as these assemblies are part of your home’s building enclosure. If you do that, those cold floor problems and cold wall problems will become less of an issue.
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