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Making the best of a misplaced vapor barrier

user-6110711 | Posted in General Questions on

The project: an aluminum-bodied step van to be converted to a low-energy tinyhouse. It will move around once in a while, and needs to handle a range of climates.

The materials:
– heavy aluminum skin, built fairly air-tight (much tighter than corrugated RV siding)
– 4″ thick fiber/paper-faced polyiso
– plywood interior panels, most likely
– a 100W heater/vapor generator that runs on potatoes and wine
– additionally heated by a woodstove when it’s feasible, catalytic propane when it’s not. I’m also looking into air-air heat exchangers.
– floor and ceilng vents, possibly extractor fans

The intended climates:
– Priority 1 is very cold/dry (Montana in the winter)
– Priority 2 is cold/wet (Pacific northwest in the winter)
– Priority 3 is hot/dry (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.

My understanding:
For my most-prioritized climates, there seems to be consensus that the vapor barrier should be placed between the interior space and the insulation, allowing the latter to breathe to the outside but not get liquid water on it.

The worry:
My biggest fear is spending a winter in a rainy urban area where I can’t burn wood, moisture (including from propane heat) infiltrating through the plywood and polyiso, condensing on the (flat) alu roof, and not only making the insulation much less effective, but eventually pooling so much water up there that it one day brings the ceiling down on me in a rotten yucky soaking-wet disaster.

The problem:
Heavy aluminum sheeting is already a very effective vapor barrier, and it appears to be in exactly the wrong place. Furthermore, the plywood will be a partial vapor barrier as well; I’ve read many a warning to avoid double barriers. The “proper” solution that I can think of – making the alu outer skin breathable, and installing an air gap between it and the insulation – seems impractical.

The questions:
1. How realistic is my disaster scenario?
2. Short of the above solution involving an air gap, what is the best I can make of this situation?


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  1. GBA Editor
    Martin Holladay | | #1

    You have several misunderstandings.

    You wrote, "There seems to be consensus that the vapor barrier should be placed between the interior space and the insulation, allowing the latter to breathe to the outside but not get liquid water on it."

    In fact, what you describe as a "consensus" is a 40-year-old idea that is best described as a generalization based on flawed ideas.

    In fact, vapor diffusion is rarely responsible for the type of problem you're worried about. Most of the condensation that occurs in these situations is due to water vapor that is transported by air leaks, not vapor diffusion. If you pay attention to airtightness, you won't have these problems.

    For more information on vapor diffusion and vapor barriers, see these articles:

    Vapor Retarders and Vapor Barriers

    Do I Need a Vapor Retarder?

    All About Vapor Diffusion

    In your case, you won't have significant diffusion through the rigid foam, so don't worry about vapor diffusion. Pay attention to sealing the seams of the rigid foam when you install it. Do your best to ensure that the rigid foam is tight to the aluminum skin of your van, with as few air pockets as possible.

    If you are worried about vapor diffusion, you can always buy foil-faced polyiso instead of paper-faced polyiso. Foil-faced polyiso is more widely available and easier to tape -- and the foil facing assures that you won't have any vapor diffusion to worry about.

    Your biggest problem is likely to be keeping your indoor relative humidity low enough to be comfortable. You're likely to see condensation during the winter on the interior of the van's windshield, since the windshield will be cold.

  2. user-6110711 | | #2

    The consensus I refer to includes your article on Vapor Retarders; answering the question of why someone would want one, you replied, "During the winter, a vapor retarder on the interior of a wall will slow down the transfer of water vapor from the humid interior of the home into the cool stud bays."

    There won't be significant vapor diffision through the foam? The chart in the Diffusion article lists 1" of unfaced polyiso at 26 perms; at 4" that's a Class III vapor retarder.

    The plywood will be a lot tighter than that, if I build it right (and the aluminum already is). Should I be worried about it being too tight - two vapor barriers (alu and plywood) sandwiching insulation?

    What I do get is that airtightness is going to be more of a factor. To promote that, should I bother covering the polyiso with some kind of smoother material that's easier to tape (plastic sheeting of some sort)?

  3. GBA Editor
    Martin Holladay | | #3

    Your misunderstanding is due in part to your confusion between "vapor barriers" (for example, polyethylene) and "vapor retarders" (for example, vapor retarder paint or kraft facing).

    You wrote, "There seems to be consensus that the vapor barrier should be placed between the interior space and the insulation, allowing the latter to breathe to the outside but not get liquid water on it." There is no such consensus about vapor barriers.

    You quoted one of my statements about vapor retarders -- but again, you failed to note that a vapor retarder is different from a vapor barrier. Moreover, while it's true that, during the winter, vapor retarders slow the flow of water vapor from the interior to cold sheathing, the same materials can cause problems during the summer, as I wrote elsewhere.

    If you're still worried about outward vapor diffusion in winter -- and I don't think you need to be -- then by all means use foil-faced polyiso instead of paper-faced polyiso.

    In the type of assembly you are talking about, there is nothing wrong with having more than one vapor barrier or vapor retarder.

  4. GBA Editor
    Martin Holladay | | #4

    Closed-cell spray foam is definitely the preferred insulation material for your project, because it assures that you won't have any air pockets between the insulation and the metal skin of your vehicle. As long as you don't have any philosophical problems with the use of spray foam, and as long as you can afford it, that's the way to go. You can buy two-component spray foam kits at most building supply stores.

    Thanks for your feedback on my reminiscences about hippie building projects in the 1970s. Every generation seems to re-learn a few basic lessons, and the need for these lessons to be obtained through experience rather than by listening to older generations is a universal law that is unlikely to change.

  5. user-6110711 | | #5

    Vapor barrier vs. vapor retarder, got it. Either way, my original concern was that the aluminum (definitely a Barrier) was in the wrong place, and you're saying it won't matter as long as air pockets are minimized.

    About air pockets - the walls and floor are quite flat; no problem there. The roof, however, has 1.5"-thick, 1" wide aluminum C-channel braces on its underside. The opening of the "C" points sideways, not down. I could relocate them outside but that might lead to structural problems (the roof skin would be held up by rivets under tension). What should I do about this - wrap the braces in plastic, stuff them with insulation (perhaps spray foam), or just ignore them?

    Off-topic, but I really enjoyed your article on the hippie building mentality reinventing several thousand years of technological progress. I recently ended five years in a quasi-eco-village community in California, and wish we'd had that kind of wisdom from the start. The folks who learned it all the hard way back in the 70's are by now mostly too burned-out/jaded to impart much of it themselves (not that many of us listened anyway!); thanks for being an excellent exception.

  6. user-6110711 | | #6

    I can't afford to use spray foam exclusively (and the 4" paper-faced polyiso was reclaimed at a great price, so replacing with foil-faced is unlikely for the same reason), but perhaps it'd suffice to fill in the C-channels, trim it carefully, and wedge the polyiso around it?

    Spray foam is lumpy and there seems to be no way around that except trimming it. Suppose I use it to eliminate air pockets at the aluminum, at the tradeoff of pockets between say, 1.5" of spray foam and another 2-4" of polyiso? Should I be worried about these partly-insulated pockets?

    For the walls and possibly the floor, I was going to use light construction adhesive, mostly to minimize air gaps, with actual structural strength coming from paneling, shelving, cabinetry, and other interior bracing pushing out against the aluminum. Essentially it'd be a plywood box "floating" inside a shell of insulation, with no fasteners going through the entire thickness, to limit thermal bridging. There'll be a fully-insulated bulkhead between the cab and the back.

  7. user-6110711 | | #7

    Now that I have the van gutted again and am about to start construction, I realize the air pockets may be significant on the walls and floor, and hard to completely eliminate. Also, there will have to be cutouts in the insulation panels to accommodate door latches and sundry other fittings, which cannot be perfectly sealed with spray foam as they have to move.

    Should I put plastic sheeting or another vapor barrier tight against the polyiso (paper-faced) where there must be a pocket? Or just not worry about it?

  8. GBA Editor
    Martin Holladay | | #8

    Don't worry about it. The polyethylene wouldn't help (because it's almost impossible to make it airtight).

    Anywhere you end up with bare metal that extends to the exterior of the van, you're going to end up with a little condensation when the weather is cold. But in most cases the moisture will eventually evaporate harmlessly.

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