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Old 04-01-2014, 04:59 PM   #1
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Polyiso or XPS for Garage ceiling under drywall?


We're demoing the garage of our 1953 Ranch in New Jersey and need some advice on insulation. I plan on doing lots of air sealing and 5/8" fire rated drywall throughout but I have a few questions after reading. The house faces south and the garage is on the east side of the house.

The question revolves around whether it would be beneficial to use either XPS or polyiso on any of the portion of the ceiling or walls under the drywall. The space isn't going to be conditioned per se, but I might overwinter delicate plants with an electrical space heater to keep it above freezing. Basically the house was leaking warm air into it (or vice versa) so it mostly stayed above freezing. But now that I'm air sealing and insulating the part that touches the house, would there be any benefit to putting a layer of polyiso up against the walls under the drywall?

The joists that touch the house felt damp, presumably from the warm house air condensing on the cold garage drywall/wood etc... Theoretically if I put a layer of polyiso against this wall it would be a thermal break and technically on the "cold" side.

The back and east side touch unheated areas, so I don't see any benefit there, but above the garage is a family room/bedroom which is conditioned. This technically would be putting the XPS or polyiso on the cold side correct?

So my game plan is to put polyiso (1"?) on the ceiling and side that touches the house, and OSB on the sides that are touching unheated areas (so we can attach whatever we want at a later date), and 2x4 sized fiberglass batts in all walls, and 9" batts in the ceiling.


Thanks, Bill

The basic plan is this for the ceiling, but also for the wall touching the house.



http://www.buildingscience.com/docum...n-crawlspaces/








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Last edited by Ocelaris; 04-01-2014 at 06:20 PM.
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Old 04-03-2014, 02:57 AM   #2
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Polyiso or XPS for Garage ceiling under drywall?


Looks good.

Either will work. Just make sure they have the proper thermal barriers on them and air seal the backsides of the wall and ceiling prior to insulating.

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Old 04-03-2014, 09:34 PM   #3
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Polyiso or XPS for Garage ceiling under drywall?


No, only the foil-faced has a perm rating low enough to stop the moisture drive- if ever vinyl flooring above- "Figure 6: Vapor Barrier—Installing impermeable rigid insulation keeps the wood framing warm and provides a low perm layer that addresses the upward vapor drive. How impermeable? Less than 0.1 perms. Foil faced rigid insulations are the ticket here. This approach allows any type of flooring to be used above. Even better—exposing the shiny side of the foil facing (face it down into the crawlspace) almost eliminates radiative coupling and means that the surface of the insulation approaches the temperature of the ventilation air reducing condensation."

Unless you use 8" of XPS to get 0.14 perms, use the foiled-faced. You both missed that point; " Unless you stick to carpet, and ventilated furniture and ventilated cabinetry you are going to have to use high-density foam—at least 2 lb/ft3—due to its lower perm value. And at least 3 inches thick or thicker (gives you less than 1 perm at this thickness)." Bold is mine, from: http://www.buildingscience.com/docum...n-crawlspaces/

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Old 04-06-2014, 11:28 AM   #4
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Polyiso or XPS for Garage ceiling under drywall?


Never seen a garage with a lesser vapor retarder level ever have a condensation or moisture issue regardless of floor covering.

He also didn't mention what type of insulation he/she was using.

If its a poly skinned XPS, perm rates are the same (basically) as foil faced ISO.
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Old 04-06-2014, 09:03 PM   #5
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Polyiso or XPS for Garage ceiling under drywall?


The floor surface above the garage is plywood covered with oak. We ordered the 1/2" foil backed polyiso for walls and ceiling that touch the house, and the 5/8" fire rated drywall to keep the fire proof envelope. R30 inbetween the joists and R13 in the 2x4 studs. OSB and then drywall will be put up on the walls that are touching exterior walls, and R13 inside. The main problem I'm trying to solve is the air sealing, previously we had a ton of air leak through the cracked drywall and this hopefully will fix that, any extra insulation is just a bonus.

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Old 04-07-2014, 08:34 PM   #6
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Polyiso or XPS for Garage ceiling under drywall?


Yes, as WoW said, any faced foam board will work. Be sure to caulk under the drywall behind the baseboard (if accessible) and canned foam any wiring/plumbing holes/chases in the attic to stop contributing room air to the stack effect, natural or mechanical; http://www.wag-aic.org/1999/WAG_99_baker.pdf

http://www.homeenergy.org/show/artic...page/2/id/1360

Why OSB under drywall?

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Old 04-07-2014, 09:21 PM   #7
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Polyiso or XPS for Garage ceiling under drywall?


The wife insists on something under the drywall for hanging stuff off the walls, shelving etc... it's just 1/2" OSB which is really cheap, nothing fancy.

I've got a lot of caulking (air sealing) to do, and that was my reason for taking all the drywall out. There are so many leaks into the house from the garage, the only way to get at them was to pull all the drywall off. I'll take more pics once we get some power back in the room, right now I disconnected the circuits so documenting is hard to do.

Thanks again, I've read so many of the articles linked, now when I talk to friends and family I feel like a pro in comparison! I've been reading on this forum about insulation and air sealing for going on a year at least, and finally am able to insulate the attic (in 2 weeks) after air sealing it, the garage in comparison seems like a small job!

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