Hi r/DIY,

The living space of my house sits entirely above the basement/garage. This basement/garage area is not heated, and stays cool/cold all the time. At the moment, there is no insulation in the ceiling of this area (just joists and open to the subfloor of the living area), so our hardwood floors are always very cold. I live in coastal California, so the temperatures are not extreme, but on the cool side for a large part of the year.

I'm going to start by air sealing all the plumbing/wiring holes, but I was also thinking of adding some insulation to keep the living space a bit warmer. I am wondering what type is best practice to use. Is stuffing the space between the joists with rock wool batts an acceptable approach? And is it recommended to use a vapor barrier?

I've searched a lot online to see what's recommended, but all the information I can find assumes that there is already insulation and just suggests how to improve it (e.g. spray foam).

Thanks!

Comments

You must log in or register to comment.

all-ur-booby-R-2-me t1_iybzdae wrote on November 30, 2022 at 6:54 AM

Check insulation ratings for your area. Coastal cali is alot of region north and south.

ultra2fast OP t1_iyc0g5n wrote on November 30, 2022 at 7:08 AM

I'm in Zone 3, in San Francisco.

DragonsBane80 t1_iyc1t7r wrote on November 30, 2022 at 7:26 AM

Yes, you can insulate it. Rockwool is ideal, as it's flame resistant, while fiberglass/ cellulose bats are not so much.

I wouldn't vapor barrier. Ideally you'd drywall after the insulation, but that's also not necessary. Drywalling will however act as a blocker to airflow as well as adding sound deadening.

5degreenegativerake t1_iyccwyn wrote on November 30, 2022 at 10:07 AM

Drywall would also give you a better chance of surviving a house fire that starts in the garage.

Hypotheticall t1_iycteqj wrote on November 30, 2022 at 1:21 PM

Fire-rated drywall with fire taping

oneeye3040 t1_iyczdqz wrote on November 30, 2022 at 2:13 PM

I'd just use a r-38 faced fiberglass insulation, assuming you have 2x12 floor joists and not I-joists or floor trusses. then use a 6mil polyethylene for the vapor barrier.

MisterIntentionality t1_iyd3c1c wrote on November 30, 2022 at 2:43 PM

You need to put insulation in the ceiling. Any other attempts to insulate without insulation above is futile since the majority of heat is going to be lost up there.

I would just hire a company to come out an blow insulation up there. I just had my one car garage done for $650 and they had to build an air vent along the soffit on one side. It was really worth the pros doing it and me not having to take hours or days to learn how to lay insulation properly on exterior walls.

In Cali I'm sure its probably pricier but when I priced it out to pay someone to do it was about as much as it would be for me to buy materials.

ultra2fast OP t1_iyd9gg0 wrote on November 30, 2022 at 3:27 PM

The ceiling is the one area that is actually insulated! So they did do the most important area, but walls and floor are not insulated at all.