Why is the room over my garage so hot?

Rooms over garages are the usual suspect. The floor sits over a garage that isn’t cooled, the knee walls often have nothing but paper facing behind them, the sloped ceilings may have no baffles, and the ductwork frequently runs through the same space.

On one job, the blower door couldn’t pull the house to its test pressure with the bonus room door open. At full speed it was moving 6,022 CFM and still couldn’t get there. Close that one door and the house tested normally. As far as the air was concerned, that room was closer to outdoors than indoors.