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Old 09-14-2011, 02:51 PM   #1
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Vapor Barrier on elevated 4 season room


House is a two-story with walkout basement in Davenport, Iowa. The 4 season room is on the first floor, hanging off the back of the house. Windows on 2 sides, door (to outside deck) on 1 side, open to interior of house on the other. This room is about 8 feet off the ground. Underneath the room is a poured patio. The configuration in the room was: (carpet, pad, plywood, joist cavity, plywood, aluminum soffit). The soffit serves as the 'finished' ceiling above the poured patio.

The room is pretty cold in the winter. On average, about 8 degrees colder than the rest of the rooms on the 1st floor. I did some quick investigation and realized there was no insulation in the joist cavities in the floor. I contacted an insulation company and they drilled holes into each end of the joist and filled the cavities with (blown-in) insulation. Instantly, the room felt warmer. Although this helped, the room is not quite as warm as the rest of house. The configuration in the room is now: (carpet, pad, plywood, blown-in insulation, plywood, aluminim soffit). Now to my question.

I plan on installing new carpet in a few weeks and wondered if I should take this opportunity to install a poly vapor barrier on the plywood since there doesn't appear to be any VB in the configuration. Would this help with the temperature? The VB would be between the plywood and the carpet pad. The configuration would be: (carpet, pad, VB, plywood, blown-in insulation, plywood, aluminim soffit). Or, should I add another layer of underlayment on top of the VB to prevent the VB from tearing during carpet installation? Like this: (carpet, pad, 1/8 underlayment, VB, plywood, insulation, plywood, aluminim soffit). Should I be concerned about water/moisture accumulating in either of these configurations? What if my son spills his drink on the carpet. Will the water remain trapped on the poly and cause problems later on?

Ideally, I would like to tear off the aluminum soffit ceiling (from underneath) and spray foam the underside of the room, but I'm looking for alternatives at this point.

Thanks for any advice you can offer.

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Old 09-15-2011, 10:20 AM   #2
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Vapor Barrier on elevated 4 season room


Vapor barriers provide no R-value (insulative) gains. They strictly control the migration of moisture vapors through a system. I wouldn't worry about adding one in, if temperature is your concern, then the soffit demo and spray foam is your best bet.

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Old 09-15-2011, 10:49 AM   #3
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Vapor Barrier on elevated 4 season room


I agree with AGW, above. Remove the plywood and aluminum as they are vapor retarders. You already have one vapor retarder (plywood) next to the heated side. The joist bottoms are cold giving you a heat sink and cold floors; http://www.buildingscience.com/docum...n-crawlspaces/

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Old 09-15-2011, 01:41 PM   #4
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Vapor Barrier on elevated 4 season room


Great info. Thanks
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