Cooling a bonus room over a garage

Ask a builder which room in a typical house is hardest to condition and this is usually the answer.

A normal bedroom has one exterior wall and neighbors on the other five sides — conditioned rooms above, below and alongside. Those neighbors do a lot of work.

A bonus room over a garage usually has:

• Three exterior walls, because it sits over a projecting garage rather than inside the main building footprint.
• A roof directly above, often with sloped ceilings and knee walls, exactly like a loft conversion.
• A floor over unconditioned space — which is the one nobody accounts for.

Five of six surfaces exposed. No other room in a typical house comes close, and that is before the ductwork question.

The floor is the part people underestimate, because floors elsewhere in a house sit over conditioned rooms and are effectively neutral.

A garage is not neutral. It is uninsulated, it usually has a large uninsulated door facing the weather, and on a sunny afternoon it can be hotter than outdoors. In winter it is close to outdoor temperature.

Two specific things worth checking:

• Is there insulation in the floor joists at all? Look up from inside the garage. Many are simply drywalled with nothing behind.
• Is it still in place? Batt insulation installed upward into joist bays sags and falls out over time, particularly where it was friction-fitted rather than supported. Gaps are common and they are exactly where the heat moves.

Also check the door. If there is a door from the bonus room into the garage attic or the garage stairwell, it is an exterior door in every sense that matters and is usually built as an interior one.

Bonus rooms are frequently finished after the house was designed, or sized as an afterthought. Two consequences follow.

• The system was not sized for it. Adding conditioned area to a system already at capacity means the whole house works harder and this room still loses.
• The duct run is long and often unconditioned. Getting air here usually means crossing an attic or the garage ceiling, and every foot of that run loses capacity and leaks.

Which is why the instinct to close registers elsewhere and push air here is so common, and why it does not work — closing vents raises duct pressure and increases leakage, so more of your air ends up in the attic rather than in this room.

Start from the published square-footage figure, then apply the adjustments that genuinely apply. This room legitimately triggers more than one.

A bonus room is usually top floor, frequently very sunny through dormers or gable glazing, and often occupied by more than one person if it is a playroom or media room. Apply what applies, and no more.

Do the envelope work first. Floor insulation and shading both reduce the capacity you need, and a capacity step is roughly $34 a year, every year.

1. Look up from inside the garage. Confirm the floor above is insulated and that the insulation is still where it was put.
2. Check the door into the garage or its stairwell. Treat it as exterior: weatherstrip it, and never modify a rated assembly.
3. Shade the glass. Dormers and gable windows admit sun at the worst angles, and shading has no running cost at all.
4. Add air movement. A ceiling fan at 1.5 W on low makes a warm room usable for nothing.
5. Give the room its own source. Rather than trying to force the ducted system to serve it, at 404.8 to 425.5 kWh a year.
6. Prefer a reverse-cycle unit. The same five exposed surfaces that make it hot in August make it cold in January.