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tdncz 05-26-2010 04:19 PM

Moisture Barrier Help
I just built a two story, 24'x24' garage. The upstairs will be a 370 sq ft - climate controlled -- apartment for guests.

OK, I live in New Orleans and it is hot and humid 8 months of the year and gets below freezing, maybe 30 total days (really nights) a year.
The apartment's ceiling is the building's roof (NO attic) and is 10x12 pitch. The apartment's floor is the garage's ceiling.

1. I have vents on the eaves and a long roof vent (on the roof's peak) and plan to insulate between the roof and ceiling with no vapor barrier (I will have those styrofoam "W" shaped things between the roof and insulation to insure the space between the drywall and roof "breathes"). So I plan (from the outside): roof shingles, felt, plywood, styrofoam "w', insulation, drywall. (again no vapor barrier)

2. On the garage ceiling (which is the apartment's floor), I plan to have (from the uninsulated garage below): the drywall (or plywood) ceiling, MOISTURE BARRIER, insulation, plywood, wood floor.

I do not plan to insulate the garage yet -- it will be open to weather for most of the day. I may insulate it later, but not now.

So, what I really have is sort of an apartment on stilts..... with a garage underneath

Any thoughts to either or both # 1 or # 2 above regarding moisture/vapor barriers???


the carpenter 05-26-2010 09:18 PM

Moisture barrier (Poly) always goes on the WARM SIDE. Your entire appartment should have poly on it. It is to stop the warm moist air from inside the building infiltrating the insulation and walls. The poly stops it before it gets to the insulation. I'd be putting it as close to the inside of the appt. as possible. The floor of the appt. will be fun, either vapour barrier or closed cell styrofoam up tight against the floor.
Now, I am in Canada, so when I refer to the warm side, I am refering to the inside of the building. I can't imagine that you guys put the plastic on the outside of buildings down there. Always the inside. Am I right guys?

the carpenter 05-26-2010 09:47 PM

Apparently I am wrong. New Orleans is not Canada. This article gives a great take on vapour barrier placement.

tdncz 05-27-2010 08:21 AM

Thanks for the link.
You are right. The gulf coast is VERY humid and the rules for northern climes regarding moisture barriers do not apply. I have asked a few insulation "experts" and get a variety of answers. For instance, the link you provided indicates that an option is to elliminate the moisture barrier all together. I have NEVER heard that one.

One other option for me is to elliminate the mouisture barrier underneath (i.e. the garage ceiling/ apartment floor), and just use roofing felt between the apartment floor's hardwood flooring and the plywood subflooring. AND elliminate the moisture barrier all together from the apartment ceiling (building roof) since it too has a shingle/roof felt barrier, and I have vents to allow it to breath. Just trying to get a solid answer....

Thanks again.

the carpenter 05-29-2010 10:08 PM

the only way I see of covering your bases is to get that closed cell sprayfoam installed everywhere. It acts as insulation and a vapour barrier, it's a very high r-value, and it won't get wet or have moisture issues. But it is ,unfortunately, very pricey. There must be a norm around your area. Go to a residential construction site and ask the boys what they nomally do.

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