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ClimberKev
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# Posted: 8 Oct 2014 09:41am
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I'm going to begin building a guest room inside an existing pole building. I believe I need to install a plastic vapor barrier between the outside steel sheeting and the insulation. Should I also add one between the insulation and the interior of the room? Also, I'm adding a ceiling on the bottom of the trusses. Will I need a plastic vapor barrier over the top of the room? Any input is appreciated.
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Wilbour
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# Posted: 8 Oct 2014 10:16am
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To my knowledge the Vapour Barrier is designed to keep the humidity from the living space from reaching the insulation or framing (wood or metal). The ceiling should also be sealed the same way. I would not want to create a "Sandwich" of vapour barrier and insulation, where the insulation is covered on both sides. In my opinion you want the insulation to breathe so if some humidity seeps through it can be evaporated to the outside.
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bldginsp
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# Posted: 8 Oct 2014 10:29am
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Vapor barriers seem to be a real mixed bag of tricks. For every 'theoretical' explanation of how they 'should' work in certain applications, there are stories of how the vapor barrier failed in that situation.
If moist air against the vapor barrier is cooled to the dew point, you'll get condensation and the problems that come with it. I suggest no vapor barriers, as much insulation as you can, and good ventilation outside the insulation at the roof.
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Wilbour
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# Posted: 8 Oct 2014 10:53am - Edited by: Wilbour
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Quoting: bldginsp Vapor barriers seem to be a real mixed bag of tricks
bldginsp is correct. Cabin buildings are more likely to be leaky with regards to cold spots, low insulation levels and lack of direct ventilation. Some cabin owners do not cook or shower in the cabin so the only humidity would be evaporated from the few people spending time there. Others cook but always leave the window open. We all remember when "R2000" ideology took over and people got sick because we made our homes so air tight the molds grew like crazy. Now we have incredibly tight homes with tones of fresh air venation via costly HR units.
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ClimberKev
Member
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# Posted: 8 Oct 2014 11:02am
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Thanks everyone. I guess my real concern is condensation on the steel siding soaking into the fiberglass insulation. Is there a 24" insulation that comes pre-wrapped in plastic?
Thanks again!
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bldginsp
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# Posted: 8 Oct 2014 03:25pm
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there probably is such an insulation, but then all you are doing is keeping the moisture between the siding and the plastic wrap. Best thing to do is ventilate it so it can get out.
they make plastic baffles for vaulted ceiling insulation, which are just the 1" deep pans that go on top of the insulation and ensure that there is an air space above it and below the roof deck. Might work well for metal siding as well.
I really don't know what is recommended for insulation directly against metal siding in a wall. Seems like it's ripe for condensation.
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OwenChristensen
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# Posted: 8 Oct 2014 04:08pm
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You shouldn't have much condensation on the metal if you insulate. Condensation is caused by the difference between the inside and outside temp. The addition of insulation should let the metal inside and outside come to a more uniform temperature. Just do it the normal way with the vapor barrier on the inside.
Owen
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DwellerofCabin
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# Posted: 11 Oct 2014 04:02pm
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Vapor barriers are a good thing, air exchange is a good thing. You need, just like in a bathroom a fan to remove moisture from the bathroom, kitchen, etc. The vapor barrier is used in every home built in America but only on the inside. the barrier is to keep moisture out of the house, don't put in on the outside and inside. Also, they sell insulation, like for barns, attics etc in bags.
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