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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?

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Rigid foam, no, not unless you airseal it. Spray foam, yes. Assuming you define "worth it" as valuing comfort over money. ROI is 10-20 years for just about anything in HVAC, if ever.

I'd pull the drywall off the ceiling in the garage, not pull up the subfloor, unless it was being replaced.

Ridged foam in the attic might be easy to install and do something.

Someone who knows what they're doing might say no matter what the insulation, you have insufficient airflow. I'm going to guess all three: above a garage, below an attic, and longest run from the furnace. Bonus points for unideal sun exposure (get better drapes).

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  • Comfort is the goal, but the budget doesn't have much room either. Yes, I would use spray foam to seal the rigid foam around the edges. Spray foam is tempting - and would probably be easier to apply - but not sure worth the extra expense. Air flow makes sense too. Both bedrooms do face south.
    – TheOtherTimDuncan
    Commented Aug 18, 2018 at 23:27
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There are a couple of options. Remove the drywall in the garage and get it spray foamed. This insulation stays in place and kinda acts like a vapour barrier next to the sub floor. Also can provide coverage for the ducts that run in the cavity. Or at least you will have access to insulate them. The other option is finish the garage. The problem with this is it doesn't address the exterior walls in the dropped area as they are not insulated with vapour barrier. Very few builders deal with these areas properly in the original build.

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