Spent the weekend lining the garage roof and the difference is night and day

I went thru a similar process last summer with my 22x22 attached garage. I actualy started with those Owens Corning R-13 fiberglass rolls because they were on sale at the local supply shop, but I ended up switching to the Rockwool Comfortbatt for the ceiling joists. The density makes a huge difference in blocking that radiant heat from the asphalt shingles. I also noticed that sealing the gaps around the top plates with Great Stuff foam did more for the actual temperture than the insulation itself did at first. Since you mentioned those moisture stains, you might want to double-check your soffit vents before you seal everything up with drywall. I realized halfway through my project that my soffits were mostly painted over, wich was choking the airflow even with a fan running. I ended up installing some AccuVent plastic baffles to keep the Rockwool from blocking the air path. It kept the plywood sheathing much drier once the humidity spiked in August.

If you're still planning on R-15, that leaves a nice 2-inch gap for air to move behind the insulation, which is great for that gable fan. I tried the thicker R-23 in my 2x6 setup but it felt too cramped against the roof deck, so sticking with the thinner batts might actually be the better call to prevent that moisture from coming back. It keeps my space about 15 degrees cooler than the driveway now, which makes the morning workouts way more bearable.

I took a diffrent route after realizing my South-facing metal door was essentially a giant radiator. Even with the attic fan running, that metal panel was hitting 130 degrees by noon. I picked up a Matador garage door insulation kit from the hardware store last July. It’s just polystyrene panels that snap into the tracks, but the difference was night and day for the workbench area right next to the door.

Before I did the ceiling like you guys, I acutally used a laser thermometer to check the floor. The slab was soaking up so much heat from the driveway. I ended up painting the exterior concrete with a lighter cooling stain and added a threshold seal to keep the hot air from sucking in at the bottom. It doesn't replace the Rockwool in the rafters, but stopping the heat at the door made the fan’s job alot easier. If you still have those thin metal panels exposed, you're fighting an uphill battle regardless of the R-15 in the ceiling.

I'd look at the windows if you've got 'em. My garage has two small ones on the west side and they'd let in so much heat it didn't matter what I did to the ceiling. Put some reflective ceramic tint on them last month and it's a total game changer. Also, don't forget the weather stripping on the bottom of the main door. If you can see light under there, you're just bleeding all that cool air out onto the driveway. It's a cheap fix and takes like ten minutes to swap out.