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I've been wading through lots of post on flooring and wall insulation and haven't seen a lot on vapor barriers. I would assume human generated humidity in a small dwelling would make this more neceesary so as to not trap moisture in the walls or under the floor. Do people do this? If so, in what order do you install? Insulation, vapor barrier vs vapor barrier then insulation?
for all practical purposes the outer wall of your van/trailer/rv is a vapor barrier. so you would do insulation then vapor barrier. highdesertranger
Ahhh, thanks. I did run into some other posts with mixed comments on the vapor barrier use. Seems like there is an idea that a vapor barrier can trap moisture and it might be better to insulate with air flow and ventilation being your priority. That way moisture/condensation that does accumulate will have the chance to dry out. Any thoughts on those thoughts?
I run the following: vehicle skin-insulation-vapor barrier-sheeting, with fresh air intake and an exhaust port, the whole thing run on a powered ventilation system that runs numerous times per day.

Sealing up tight and providing proper airflow are, IMO, the basic requirements of vehicular dwelling, at least in my neck of the woods where we get lots of cold, and the transition from hot air to cold surfaces causes the condensation.

In warmer climes I can't speak to that for it will most likely be different.

I don't recall you saying where you're at, or from, so this may not apply! Big Grin
(12-13-2015, 11:46 PM)ryguy Wrote: [ -> ]Seems like there is an idea that a vapor barrier can trap moisture and it might be better to insulate with air flow and ventilation being your priority. That way moisture/condensation that does accumulate will have the chance to dry out. Any thoughts on those thoughts?

hi RVguy, insulating vehicles is a great topic, because there are few actual "facts" and lots of opinions. You'd think that after 50 years of conversion vans and RVs there would be a solid database of best practices, but there really isn't. Gary (I forget if its GaryGary, Gary123, etc Wink ) really does a great job consolidating vehicle insulation information here:
http://www.buildagreenrv.com/design-and-...nsulation/
Very worth reading.
On the vapor issue, I like the idea of using 2-part spray foam as a moisture barrier because I know it doesn't let vapor through. Great Stuff foam is in fact a closed cell foam as well, but you will come across many negative opinions of it on forums, including pictures of wall rust behind it. My best guess as to what's happening is that as a result of both application method and trying to cover spots that can't be reached, that it never made good 100% contact with the metal, leaving air pockets and channels where vapor or condensation was able to occur...and then trapped it there to cause corrosion. Which, of course, lends support to your idea of planning air flow into a design.

Keep in mind that when people say "this is what I did and there's no problems", that what that really means is based on their method, their dwelling climate, their vehicle usage and ventilation, and how much they may cook inside, that they have not seen evidence of moisture problems against the van skin...which is darn difficult to see once its insulated Wink . This certainly doesn't mean these opinions are incorrect, only that there isn't a science about it and what works for one, may not apply to another situation.

I guess that's enough muddied water for today, good luck!
A vapor barrier is fine in any climate as long as you have a way to remove the collected moisture from the surface on the warm side, before the growth of the first spore.
The other issue is if the moisture gets trapped next to unprotected metal. Then it's a bit of rust your dealing with ..... before the spores.
Venting rather than a sealed space is a good option, all things considered.
I think in a small space it's best to plan for ventilation. 1 or 2 roof vents and windows to let moisture out. We tend to be to efficient and want to seal everything to keep the cold out and the heat in. The end result of this is bad indoor air and mold in our homes. Open windows and vents to prevent this problem and you will be healthier. This is one reason people who live in tents use canvas because it breaths and let's the vapor out.
I always insulate with styrofoam or with polyiso and it always has a plastic liner on one side and aluminum barrier on the other. That's all the vapor barrier I think I could need. You could go a step further by using aluminum tape on the seams, but I've never bothered.
Bob
Thanks everybody! I'll be living in salt lake city and around southern Utah most of the time so it will be drier but also quite cold and really hot at times. I'm putting a fantastic vent in and will have two side windows to help ventilate. I'll probably be insulating with polyiso undecided on a further barrier, more reading required hahaha