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Does a vapor barrier belong above my steel ceiling in heated shop?

Having been nicely schooled on reasons not to put vapor barrier on my walls with exterior foam in zone 6, SE MN, I now wonder about the proper way to insulate the ceiling. This is a heated shop with a vented roof and scissors trusses. I would like to use ribbed steel panels for the ceiling, and have been given rolls of R38 unfaced insulation that I would place on top of the steel panels.

I would like to make this airtight, however, and I see two options. First, I could caulk the overlapping panel seams and use great stuff around the perimeter of the ceiling. Or, if could use polyethylene sheeting applied to the bottom chord of the trusses before putting up the steel.

The polyethylene sheeting seems like the easier option, and the one I would like to go with unless I'm overlooking some major design flaw in this approach. Thoughts?

Asked by Kent Jeffery
Posted Sun, 11/11/2012 - 23:12

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3 Answers

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1.
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Kent,
In your climate zone, the polyethylene probably won't cause any problems. However, it's not a particularly effective air barrier.

Polyethylene air barriers can work, but you have to be very careful not to put any holes in it, and all seams have to be located over a framing member and carefully sealed with Tremco acoustical sealant, which is messy.

The hardest part of the job will be installing those steel panels without poking any holes in the air barrier.

Plan A is probably better, although I don't know if canned spray foam at the perimeter of the steel panels will work in light of all the thermal expansion and contraction that the ceiling will experience.

Here's the bottom line: since you want an air barrier, just install a drywall ceiling.

Answered by Martin Holladay, GBA Advisor
Posted Mon, 11/12/2012 - 07:45

2.
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I keep waffling with regard to the ceiling. I like the look and the ease of installation of the steel panels, and over on garagejournal.com, some folk have complained of drywall sag, even with 5/8" Type X. Have also considered OSB, but air sealing this would be harder, I would think.

Third option: What about attaching XPS rigid foam insulation to bottom chords of trusses, joints taped and gaps spray foamed, with steel panels over the top?

Thanks for your insights, all...

Answered by Kent Jeffery
Posted Mon, 11/12/2012 - 07:56
Edited Mon, 11/12/2012 - 07:57.

3.
Helpful? 0

Kent,
Your suggestion of a layer of rigid foam is a good one. If you go that route, I suggest that you use foil-faced polyisocyanurate. It is environmentally friendly, easy to tape, and has a high R-value per inch.

Answered by Martin Holladay, GBA Advisor
Posted Mon, 11/12/2012 - 08:17

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