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I'm doing a lot of research on how to layer my materials, and getting conflicting information, much of which makes assumptions that aren't true in my case. I'd like to hear from people who have experience with similar circumstances (materials and location).

My materials:
 * Aluminum-bodied stepvan. Built fairly well, not quite airtight but nowhere near as airy as corrugated metal RV siding. It won't rust, so if I design a system that gets it wet, not a problem per se.
 * Fiber/paper-faced (not foil-faced) 4" polyisocyanurate boards. This stuff is apparently much less effective when wet, unlike XPS.
 * Some kind of heavy plastic sheeting.
 * Adequate ventilation: floor vents, opening skylights, and probably extractor fans.
 * Heat will initially be catalytic propane, to be mostly replaced by a woodstove in the not-too-distant future. I'm also looking into air-air heat exchangers.

Intended climates:
 * Priority one is very cold/dry (Rocky Mountains in the winter).
 * Priority two is cold/wet (PNW in the winter).
 * Priority three is hot/dry (central California or Montana in the summer).
 * Hot/wet will be rarely encountered; I'm willing to sacrifice suitability for this climate to optimize for the other three.

Do I place sheeting between the insulation and the alu skin? Then won't moisture from the inside condense on the sheeting and soak the insulation?

Or do I place it between interior plywood and the insulation? Then won't any moisture that does get to the insulation be trapped forever? Would drain holes help? I suspect not; that it has to breathe to get dry. An air space between the alu and the polyiso is not a good option because it takes up even more space, but I'll consider it if it works really well.

Huh
Or Tyvek?

Or should I not bother with one at all, given that the alu skin is basically already a vapor barrier about as tight as I can make any other, and the way to dry a wet interior is to heat it, during which I'll be comfortably warm (and relatively dry) anyway?
(05-21-2016, 01:54 PM)BlackNBlue Wrote: [ -> ]Or Tyvek?

Or should I not bother with one at all, given that the alu skin is basically already a vapor barrier about as tight as I can make any other, and the way to dry a wet interior is to heat it, during which I'll be comfortably warm (and relatively dry) anyway?


I've been in construction for a very long time, and one thing I've always noticed is that old, late-1800's farmhouses always had solid, dry wood in the walls whenever you open them up. They also didn't have any attempt at vapor barriers...heating those drafty old homes is always a challenge.

Compare that to newer homes with vapor wraps and you find all sorts of moisture problems in the walls. I'm not saying that vapor barriers are bad per-se, but perhaps as you've seen in researching them, climate, insulation and ventilation all are key factors in determining how and where to place the barrier. In an aluminum walled van that will visit a variety of climates, I'd be very tempted to just skip a barrier/wrap.
Yes, those old farmhouses were very well ventilated, whether one liked it or not! Back when the human population was more reasonable, wood was in greater abundance, and the occupants of such buildings were typically numerous enough that there was a constant supply of firewood, this worked ok. We've got quite a different scenario here.

Research (ongoing) so far points to using no additional barriers. In fact, it's revealed a potential new problem: the plywood with which I plan to panel the interior is also a vapor barrier, depending on how tight I build it. So unless I switch exterior materials (impossible), interior (switch to what?), or add ventilation holes to either (neither sounds appealing), I've got two vapor barriers to deal with. Oy vey...

I just started ripping out the ceiling panels (sheet alu) to discover about 1" of yellow fiberglass batting stuffed up there, in between alu C-channel roof braces. Useless due to the massive thermal bridges caused by the C-channels, but there appears to be no condensation problem up there even though it's pouring rain. On the other hand, the space is about as airtight as a jungle gym at the moment...
I'm just learning about insulation myself and had been wondering about vapor barriers. So, the answer to this question may be obvious, but not to me. Since polyiso is a closed cell foam, why wouldn't it also be considered a vapor barrier?
PolyIso, plywood, and in fact many common building materials are indeed vapor barriers of a sort. Part of the problem is that we struggle with names and terms. We have vapor retarders, we have vapor barriers, we have vapor permeable we have vapor impermeable, etc.

A true contiguous vapor-proof membrane would also be airtight, since water in vapor form is carried by air...and that is not really realistic in most any building scenario. So we're left with the goal of preventing moisture in AND if it gets in, letting it out so things dry. Not so simple.

In a vehicle, I think its more important to understand moisture Drive than it is to worry about vapor barriers. No matter how tight you cut your plywood or ISO, you really can't come close to creating an airtight cavity. If you provide paths for hot air to vent, you will also control moisture...this is seen in residential housing in ridge venting. You can still heat the house, and allow control of moisture drive for the most part.
Tyvek is not a vapor barrier.  It goes on the OUTSIDE of buildings to serve as a wind barrier and it is also waterproof against rain, but, like Gore-Tex, it is allows water VAPOR to pass through.
Brad, you just introduced me to the term "moisture drive", and now I'm back to attempting to crash Google's servers.

At the moment (subject to much change as I read and digest more), the true optimal solution would be to have a well-ventilated air gap between the exterior walls and the insulation. That would involve drilling a few holes and perhaps installing additional vents. I'm hoping to find a solution that's nearly as effective yet involving less work and less space sacrifice.

(05-21-2016, 05:00 PM)Suanne Wrote: [ -> ]I'm just learning about insulation myself and had been wondering about vapor barriers. So, the answer to this question may be obvious, but not to me. Since polyiso is a closed cell foam, why wouldn't it also be considered a vapor barrier?

I'm a bit confused myself, but it seems that polyiso (when not foil-faced) is considered semi-permeable, and therefore a vapor retarder not a barrier. Unlike other rigid foams, it can absorb water - according to one source, 5% by volume that results in a 40% loss of thermal resistance. It is however enough of an air barrier that it won't let hot air escape.
Thanks Brad. That makes sense to me.

If I may ask another question, hopefully in line with the BlackNBlue's OP, how do you provide paths for hot air to vent. Are you talking about ceiling vents/fans? Since there's no way to make polyiso or plywood airtight, does that mean you don't need to create vents in the upper (interior) walls to drive moisture and hot air to vent out from between the steel/aluminum of the vehicle and the polyiso?

Would the answer be different for BlackNBlue's location in dry/cold Colorado, vs. humid/moderate PNW?
So here's the thing. Misunderstanding and misuse of vapor barriers in all their forms has caused an incredible amount of damage to homes throughout the country. Some of it is due to really smart people over-thinking and engineering solutions that fail real world applications, and some of it is due to well meaning people applying an square peg to a round hole. Then there are good solutions that fail simply because of minor details like utility penetrations. There is an abundance of scientific studies done on all sides of this.

I've not yet built an RV, but when I do my plan is to simply provide for adequate ventilation with roof and floor venting, and not try to seal things up so tight that condensation that does occur on the inside walls will find its way back out when the sun heats the metal again. Cooking and propane use generate a lot of water vapor, but even just breathing puts out something like a gallon of water per day.

I've learned much of what I know about these things through diagnosing failures and problems that take some sleuthing to trace back to moisture drive. I don't design systems and have lost trust in many that claim to know what they're talking about, so I don't want to give specific "answers" when I myself will just be winging it based on the situation and what "feels right"...
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