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Old 04-27-2012, 09:08 PM   #1
3 Rivet Member
Anchorage , Alaska
Join Date: Oct 2010
Posts: 168
Vapor Barrier Worth the Trouble?

Getting ready to bolt a new marine plywood floor, painted with marine enamel, onto a POR 15'ed frame.

Plan to fill the gaps in the frame with Styro board insulation.

Seems in normal situations, I should put down a plastic sheet under the plywood floor, before I bolt it down, for a vapor barrier.

Don't recall seeing any discussion in the forum around putting down a vapor barrier, and wondering if there are condensation issues, or flex/abrasion issues in a coach, that make it impractical.
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Old 04-29-2012, 06:52 PM   #2
4 Rivet Member
1963 28' Ambassador
Vintage Kin Owner
Northern VT , Vermont
Join Date: Aug 2007
Posts: 360
There should be a lot of discussion in the archives on this. I was where you are years back and had the same questions. Our Vermont climate is similar to yours. Humidity and temp extreems. There is a lot of condensation between the skins on these rigs and it runs down inside the skins to the floor, hence the airstream problem of perimeter floor rot. Subfloor plastic sheeting will hold this moisture in as well as water out. I opted to use no insulation sub floor and added 2 vents in the belly pan up front for air circulation to help dry when towing. We've used it in temps near 0 and notice no problems. There have been a number of negative post on styro, mostly pooling water & rusting crossmembers, and crumbling due to vibration. Guess if we move to Texas it would be a moot point!
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Old 04-29-2012, 07:05 PM   #3
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1977 31' Sovereign
1963 26' Overlander
1989 34' Excella
Johnsburg , Illinois
Join Date: Jul 2005
Posts: 2,943
Just as in conventional houses, the vapor barrier should be on the warm side of the insulation next to the heated area. You need to stop the moist air from entering into the insulation or plywood and then condensing out to liquid water when the immediate local area falls below the dew point temperature. Just like water forming out side of a ice cooled drink on a warm summer day. If that is allowed to happen, the insulation becomes wet and ineffective or the plywood in the floor becomes damp and rots.
Additionally, there needs to be a splash barrier between the road and the bottom sized of the floor. If water does get in the under aluminum panels there needs to be a way to allow it out and vent the area to allow it to dry, just like you have roof vents in the attic of you house..
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Old 04-29-2012, 08:15 PM   #4
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1993 21' Sovereign
Colfax , North Carolina
Join Date: Jun 2003
Posts: 21,252
Putting anything under the floor that will not allow water to drain out has "bad plan" written all over it. If any water does get under the floor, it will just sit between the wood and whatever you put down under it, and rot the floor.
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