Fiberglass insulation thickness question, and others

[Squankum]

Just noticed last week that the first eight feet of the ceiling of my basement garage is the floor of the screened-in porch above. House is about 60 years old and in a region where people don't take winter seriously enough when it comes to insulation (or putting a hat on.)

IR temp gun shows wooden floorboards of the house, from underneath, throughout the house 62F, that 10 foot stretch, 52F.

My gut reaction: staple some faced fiberglass up in thar! Long rolls of 15" wide stuff at 9.5" depth are available, but...

I measured, the floor joists are definitely 9" deep. No more.

I think I read somewhere recently that you want to leave an inch of airspace in a situation like this. (Moisture?) I got the wacky idea of buying some faced 3.5" rolls and unfaced 3.5" rolls or batts and doubling up, but that would cost quite a bit more.

Also, this faced surface, having it exposed, technically, really a fire hazard? It needs to be covered with OSB or sheetrock? Am I history's greatest monster if I just kinda skip that step? (It's a rental...) I don't have any '32 Ford coupes blowing backfires through their carbs, let's say...

Thanks in advance!

 

[tylernt]

Insulation's effectiveness is reduced when compressed so I don't advise the 9.5" stuff. If this were a house I owned and planned to own for a long time, I'd use 2" of EPS or XPS foam followed by 6"/R19 fiberglass. But since this is a rental I'd just throw the 6"/R19 fiberglass up there and call it done.

5/8" fire-rated drywall is usually required for garage walls shared with the house and for attached garage ceilings. I don't think it would be a good idea to leave it off or use OSB there... drywall buys occupants time to evacuate the house should a fire start in the garage.