I have the classic problem of two bedrooms above a garage that are too hot in the summer. In the winter it can be cooler upstairs than the rest of the house but not as much as a temperature difference as in the summer. There is an attic above the entire second floor that I'm not sure whether or not could be a factor too. The attic has blown insulation that is deep enough to cover the ceiling joist plus a couple inches but has settled/shifted some over the years. If it helps, this house was built in 1998.
Since I'm in the process of replacing the carpets, that gave me the opportunity to see what is between the subfloor and garage ceiling. After cutting a small hole, what I found is a void of about 9" filled with fiberglass batt insulation. The insulation looks to be kind of bunched up so it fills the void vertically for the most part. I can't see the rim joists.
Would pulling up the subfloor, removing the fiberglass, and replacing it with rigid foam be effective at blocking the heat from the garage? Or am I looking in the wrong direction and should be looking at the attic instead? Or is it time to call in someone who knows what they're doing?