Insulating Garage

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Kubstix

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Demoing garage right now and debating on some things. 2 stall attached garage with 2 bedrooms above. Down to studs right now and going to insulate. Walls and ceiling had R-11 and drop ceiling (failed code not fireproof) The only thing I made a decision on was #1. Not sure what to do with 2 and 3. All insulation will be covered with 5/8 Purple Board Fire/Mold/Mildew resistant since we sometimes wash the cars in the garage, or pull in with snow, rain, ect. Help Please, Northeastern Pennsylvania.

1. R-21 Faced on 2x6 walls one exterior and one interior. Exterior wall will have paper outside, interior wall will put paper inside (heated room behind).

2. R-15 Roxul or R15 Faced Fiberglass for 2x4 interior walls from garage. Going with Roxul won't have a vapor barrier, fiberglass will? Can't make a decision here.

3. Ceiling. Either R-30 Faced Fiberglass with sound barrier, or Roxul R-30 without a vapor barrier? I bought 4 rubber insulators for the garage motors bolt up, replacing metal rollers with nylon. Going for sound proof so kids won't hear the doors opening and closing. Is the performance of the Roxul worth it for this?

Spray foam is out, we live in a rural area they would come out and quote me 4-5k. I have lots of water piping in here (PEX-A) as well as my water heater. Garage doors are insulated doors.
 

Dana

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Kraft facers are not true vapor barriers, but are "smart" vapor retarders that become vapor-open when there is sufficient moisture in the cavity to support mold. But they are not air barriers, and need something (like wallboard) to block air flow.

For the partition walls between the garage and fully heated space there is no need for a vapor barrier or vapor retarder as long as the garage door is normally closed. Even if the garage is left open it's fine to go without a vapor retarder on the partition wall in climate zones 4 & 5, but may incur a slight moisture risk in zone 6 if the garage-side wallboard is painted with something more vapor tight than standard interior latex.

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For the 2x4 exterior walls of the garage unfaced rock wool is fine. Unlike a fully heated conditioned space there are no breathing/cooking/bathing sources of moisture in the garage, and even the tightest garage doors leak enough outdoor are to sufficiently ventilated with outdoor air such that the dew point of the garage air isn't very different from the dew point of the outdoor air. With only small differences in dew point there isn't a significant vapor drive to move moisture through latex paint, which is a Class-III vapor retarder slowing down any moisture movement even when there is a vapor pressure difference.

R30 rock wool is usually denser & thinner than R30 fiberglass, dense enough that it doesn't really need an air barrier to hit it's thermal performance levels. R30 rock wool is usually 7.25" thick- the same depth as 2x8 framing. R30 is usually 9.5-10" thick, and performs at R28 when compressed into a 9.25" deep 2 x 10 joist. If the R30 fiberglass fully fills the joist bay that might be a better from a soundproofing point of view despite the slightly lower R value, but it's not nearly as fireproof as rock wool. As stated previously, if it's a kraft facer it doesn't matter which side the facer is on, so for ease of installation stapling it to the joist edges at the garage ceiling side is fine.

If it's 2 x 12 joists or I-joists / trusses it's better all the way around to fill it with mid-density (2-2.5lbs per cubic foot) cellulose. (If that's what you have I can expand on that topic a bit.)

The most critical aspect for soundproofing is to air seal any electrical & plumbing penetrations of the subfloor above, as well as any penetrations of the garage ceiling sheet rock.
 
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Kubstix

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Kraft facers are not true vapor barriers, but are "smart" vapor retarders that become vapor-open when there is sufficient moisture in the cavity to support mold. But they are not air barriers, and need something (like wallboard) to block air flow.

For the partition walls between the garage and fully heated space there is no need for a vapor barrier or vapor retarder as long as the garage door is normally closed. Even if the garage is left open it's fine to go without a vapor retarder on the partition wall in climate zones 4 & 5, but may incur a slight moisture risk in zone 6 if the garage-side wallboard is painted with something more vapor tight than standard interior latex.

images



For the 2x4 exterior walls of the garage unfaced rock wool is fine. Unlike a fully heated conditioned space there are no breathing/cooking/bathing sources of moisture in the garage, and even the tightest garage doors leak enough outdoor are to sufficiently ventilated with outdoor air such that the dew point of the garage air isn't very different from the dew point of the outdoor air. With only small differences in dew point there isn't a significant vapor drive to move moisture through latex paint, which is a Class-III vapor retarder slowing down any moisture movement even when there is a vapor pressure difference.

R30 rock wool is usually denser & thinner than R30 fiberglass, dense enough that it doesn't really need an air barrier to hit it's thermal performance levels. R30 rock wool is usually 7.25" thick- the same depth as 2x8 framing. R30 is usually 9.5-10" thick, and performs at R28 when compressed into a 9.25" deep 2 x 10 joist. If the R30 fiberglass fully fills the joist bay that might be a better from a soundproofing point of view despite the slightly lower R value, but it's not nearly as fireproof as rock wool. As stated previously, if it's a kraft facer it doesn't matter which side the facer is on, so for ease of installation stapling it to the joist edges at the garage ceiling side is fine.

If it's 2 x 12 joists or I-joists / trusses it's better all the way around to fill it with mid-density (2-2.5lbs per cubic foot) cellulose. (If that's what you have I can expand on that topic a bit.)

The most critical aspect for soundproofing is to air seal any electrical & plumbing penetrations of the subfloor above, as well as any penetrations of the garage ceiling sheet rock.


I opted for R-21 in the 2x6 Interior and Exterior walls. Roxul R-15 in the 2x4 interior walls. And R-21 for the ceiling with R-15 Roxul as well. Is this OK you think? My Joists are 2x10.
 

Dana

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By "...R-21 for the ceiling with R-15 Roxul as well" do you mean you're stacking R21 + R15 batts on each other?

That's not a moisture problem, but the stackup is only 9" into a 9.25' deep cavity so it's not a compression fit. A less compressed fit improves the sound abatement slightly, but any gaps are potential thermal bypasses, so fit them very carefully, fluffing them out a bit so that it's at least a modest compression when putting up the ceiling gypsum.

Be sure to air seal the band joists and all penetrations before installing the batts.
 
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