· by Michael Blair, Nashville's Insulation
The finished room over the garage, the bonus room, is the most uncomfortable room in a large share of Nashville houses built since the nineties. It sits over an unconditioned garage, under a roof, with knee walls backing onto attic space on two or more sides. Every surface but the floor to the hall is exposed to outdoor conditions, and the way those surfaces were built explains why blowing more insulation into the attic changed nothing.
Six sides, five of them exposed
A normal bedroom shares most of its surfaces with other conditioned rooms. The bonus room shares almost none. Its floor is over a garage that is at outdoor temperature. Its ceiling is under the roof deck, often with a sloped section where the insulation is thin. Its short knee walls back onto the attic, and the attic in a Middle Tennessee July can be well over a hundred degrees. Above the flat part of the ceiling is more attic.
The floor is the surprise. The garage ceiling was insulated with batts pushed up between the joists, but the joist cavities are open at the ends to the attic and the rim, and the batts often do not touch the subfloor above. Attic air moves through those cavities under the floor all summer. A room can have a floor that is warm to the touch in August because the space under it is a duct for attic air.
The knee walls
Knee walls are the short vertical walls where the sloped ceiling meets the floor, and behind them is triangular attic space. They were typically insulated with a batt on the attic side, unfaced, with nothing covering it. Wind-washed by attic air, sagging, and sometimes never fastened, that batt does a small fraction of what it was rated for. The wall behind the bonus room couch is often barely insulated at all.
Fixing a knee wall means either air sealing and insulating the wall itself and covering the insulation with a rigid air barrier, or moving the thermal boundary to the roof deck so the triangular attic behind the wall becomes conditioned space. Both work. Neither is what blowing more cellulose onto the flat attic floor does.
- Floor: air seal the joist cavities at the ends and insulate in contact with the subfloor
- Knee walls: air barrier and insulation, or bring the space inside the envelope
- Sloped ceiling: dense-pack or foam where the batts are thin
- Ductwork: seal and insulate, or reroute out of the attic
The ductwork and the return
The bonus room is usually the last register on the longest duct run, through the hottest attic, and it often has no return of its own. Supply air arrives warmed by the attic, and with the door closed the room cannot push air back to the system, so it gets even less. On a single-zone system the thermostat is downstairs and satisfied long before the bonus room is.
Sealing and insulating the duct run helps. Adding a return helps more. A ductless mini split serving the bonus room alone is the fix that most reliably makes the room usable, because it takes the room off the end of a system that was never sized to reach it and gives it its own thermostat.
The order to do it in
Air sealing first, everywhere the room meets the attic and the garage: the top plates of the knee walls, the ends of the floor joists, the penetrations for wiring and ducts. Then insulation in contact with the surfaces it is meant to protect, with the knee wall insulation covered on the attic side. Then the ducts. Then, if the room is still not right, the mini split. Doing them in the other order pays for equipment to fight a leak.
The garage side matters for safety as well as comfort. The ceiling between the garage and the room is a fire separation and should be intact drywall, and air sealing the floor also keeps car exhaust and fumes out of the room above, which is a real issue in bonus rooms that are used as bedrooms.