I am working on insulating my new garage and I have finished the ceiling. I am starting on the walls. I am wanting to use Rockwool R15 on the 3 2x4 studded walls but all the R15 for 2x4 walls I can find locally is non faced. Is this OK? I thought I needed a vapor Barrier. Is there a way to add this after putting the batting in? Or do I just suck it up and use R13?
The difference between R13 and R15 is not huge, so don't agonize over it.
However, yes, you can buy roll vapor barrier that you apply over the face of the studs before you drywall the inside. Tape the joints and edges.
The guys who did my garage did all unfaced insulation and then a separate vapor barrier that was plastic sheeting and could be taped at the seams. Seemed real fast to install and looked awesome.
Duke said:The difference between R13 and R15 is not huge, so don't agonize over it.
However, yes, you can buy roll vapor barrier that you apply over the face of the studs before you drywall the inside. Tape the joints and edges.
It would be cheaper and seems like it would be faster to do R13 so I will probably go that way.
In reply to 93EXCivic :
For a garage I think you will be fine with R13, unless you have a nosy building inspector. You're not trying to maintain a steady 72 degrees year round.
In reply to Duke :
Well it isn't just a garage. There is an office and gym in there. But from what I have seen locally since it is a detached building no insulation is required and I am pretty sure the requirement for walls is just R13 anyway at least down here in Alabama. How much would R13 vs R15 really effect energy cost in a 44x22 building?
I did R30 in the ceiling and I am doing R19 in the one 2x6 wall.
Well, the 2021 International Energy Conservation Code is insane.
Alabama is in Climate Zone 3 unless you're right near the coast. Along the coast is Zone 2.
For Zone 3 wood framed residential buildings are supposed to have R49 ceiling and R20 walls by the 2021 IECC. Zone 2 is R49 ceiling and R13 walls.
Up here in Zone 4 it's R60 ceiling and R30 walls.
I'm all for energy efficiency but that's nuts.
If you're just doing this yourself and don't have a permit pulled, I think you're fine.
Duke: For giggles (and a threadjack) what's the requirements for ceilings and walls in Zone 5?
In reply to Duke :
I do have permit pulled. But the inspection for the building is complete and the only remaining inspection would be final electrical. Talking to the permit people, since it is a detached building there where no requirements for wall coverings (ie I could use plywood, drywall or leave them completely uncovered) so I'd assume that means there is no requirement for insulation in a detached building?
Where would one even get R20 wall insulation? The highest batting I have seen is R15 that fits 2x4 walls.
In reply to pres589 (djronnebaum) :
Actually, walls and ceilings top out at R30 and R60. What happens is that floors go from R19 for Zone 4 to R30 for Zone 5 and 6, then R38 for Zone 7 and 8 (Alaska).
That's for batt insulation in wood framed wall cavities only. You can decrease the required values by putting continuous insulation over the wall sheathing:
R30 wall cavity insulation is equivalent to R20 batts + R5 continuous, or R13 + R10 continuous, or no batts and R20 continuous. For whatever reason there doesn't seem to be a similar provision for ceiling insulation in the residential energy code, though there is in the commercial energy code.
In reply to 93EXCivic :
I'm sure they're considering it a low-energy building because it is pretty much utility space and it is separated from the main house. It doesn't actually have to be detached as long as the separation wall of the house met the proper insulation requirements.
You can get R20 batts in a 2x6 wall. 2x6s at 24" on center are the same structural capacity and board-feet of lumber as 2x4s at 16" on center. Or you can put board insulation over the sheathing under the siding in addition to R13 batts.
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