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So, I thought I'd toss this one out here after reading all the insulation and vapor barrier posts, since I haven't really seen it mentioned. So, the idea of a vapor barrier is to keep moisture/condensation away from something, given. In a vehicle habitation that something would be the interior of the metal walls, given. So, wouldn't it make some sense to install a vapor barrier next to those walls instead of on the interior before the final wall covering? Depending on insulation needs it could be easy or challenging. I just pose this because I know part of my year will be in humid environs and I am possibility thinking. Really though, it just popped into my head last night. Tongue

Opinions? Smile
sorry, but I am Not the right gUy to ansWer the questioN. but Ventilation is also Key, yes?
I think it's easy for humidity to get inside anywhere, unless it's very very very neatly sealed. (E.g. no lugs / bolt holes) And it's hard for it to get out of a poorly sealed cavity. I fear that'd cause a counterproductive situation where it'd get trapped and promote rust.
My approach would b more letting some air inside every cranny and crevice( through small gaps among the insulation panels) , so that a drive with the windows down will make air circulate and alleviate the issue.
Newbie here sharing my thoughts.
Since the basic idea is to keep moisture off the steel inner walls, would it not work to essentially paint the inner walls with a water-proof paint, perhaps a roofing sealer? Making sure to NOT plug any drain/weep holes at the bottom of curse.
Insulation on vehicles is much more complex than many would have you believe. The insulation has to breath, and the moisture needs to be wicked to the interior of the vehicle and away from the skin. If this is not accomplished correctly it will lead to moisture, mold, and rust problems in the future, which is of course hazardous to both your health and your vehicle's health.

A metal skinned vehicle will sweat, meaning that moisture will appear on the inside of the skin. NOTHING will stop this from happening. This moisture has to be dealt with for a successful insulation installation.

It CAN be done, but it is usually not worth it. Insulation works best when there is a constant heating or cooling source in the insulated space. This is simply not true in most vehicles. In most vehicles, Mother Nature is the most common constant source of heating or cooling, and therefore she will rule temperature wise in the end.

At best this insulation will only keep Mother Nature at bay for a few hours, at a cost in the thousands of dollars range. In the summer, simple ventilation will accomplish the same thing, and in the winter by morning it will be just as cold if not colder on the inside than it is outside, because that same insulation is fighting to keep the now warmer outside air out. It won't succeed, but it will keep you cold and miserable in the meantime.

No matter how much insulation you have, you still need good heating and/or cooling sources. The insulation does not eliminate that need. Too many people credit their insulation for staying comfortable when the real credit goes to the moderate climates they're in. When they're heavily insulated rig radiates heat all night in the summer, they think it's because they need more insulation, when in fact it's the insulation that's radiating that trapped and unwanted heat.

One of the beauties of window vans is that the manufacturer has already done the work for you. You start out with finished floors, walls, and ceilings, that have been designed to prevent moisture build up from the start. In the winter, those windows can act like solar heaters, and in the summer they can provide you with ventilation. You still need your heating/cooling devices, but you will need to use them a lot less.

When I switched from a heavily insulated cargo van to a window van, my winter heating costs got cut by over 30% thanks to the free heat coming in through the windows. In the summer I can open the windows and bring the temps down to comfortable very quickly in the evening, while the cargo van remained a sweat box for hours, even with all the doors open.

We can treat Mother Nature as either a friend or a foe. I learned a lot about this with my off grid cabin, and the exact same principles hold true for my camper vans. My whole Southerly half of my cabin is mostly windows. In the summer they are shaded by trees, and in the winter when the leaves fall off, they get full sunshine. On a sunny but cold winter day, the sun provides all the heat needed until sunset. In the summer, at 120° in the shade, through modified convective cooling, the indoor temps remain in the 70's. Mother Nature CAN be your friend if you plan things correctly.
Offgrid, thanks for so much detail. So an insulated metal vehicle would suffer problems just like a sticks and bricks, though different problems, if the interior air is not kept conditioned to a relatively small range of temperatures and/or if the moisture isn't wicked into the interior if I'm understanding correctly, which makes sense. I actually wondered about the walls sweating when I asked since our uninsulated van years ago would have dampness on the walls in really cold or really humid conditions, tents too---one of the reasons I asked.

I have some understanding of how to set up convective cooling in a passive solar sticks and bricks situation. It sounds like, because there is a greater likelihood for me having to stay in an area with higher temperatures and humidity part of the year, that figuring out the best ways to manage convection/conditioning for cooling of the van would be higher on my list than insulation maybe? More challenging than being able to put in a solar chimney, convective roof, or vent type situation though.

Thanks. Smile
anewbiewannabe,

I just use a home made 12v A/C in my van, works like a charm, and unless it gets into the mid to upper 90's, the water only is sufficient.  Above that, and I add ice, which will last about 6 days, but I only need it once or twice a year normally.

[Image: 12v_AC_3.jpg]
(03-16-2015, 07:59 PM)Off Grid 24/7 Wrote: [ -> ]A metal skinned vehicle will sweat, meaning that moisture will appear on the inside of the skin.  NOTHING will stop this from happening.  This moisture has to be dealt with for a successful insulation installation.

Not just metal, any non permeable skin.
And the harder you try to prevent the condensation the worse you are going to make the problem. With a very good vapor barrier on the inside of the insulation, there wont be as much condensation on the skin on any given night, but the water that does condensate will be trapped. Forever.

Its very easy to reach the point of diminishing returns on insulation, I wouldn't go past two layers bubble wrap on a van. After that decent air exchange is going to steal more heat than can radiate out the walls. Which you need decent air exchange, it hurts to pump out hot air and let cold air in, but you have to get all the moisture out.

Hank
I disagree with just about all the conclusions you all are reaching here.
Bob
(03-16-2015, 10:44 PM)907KHAM687 Wrote: [ -> ]Not just metal, any non permeable skin.  
And the harder you try to prevent the condensation the worse you are going to make the problem.  With a very good vapor barrier on the inside of the insulation, there wont be as much condensation on the skin on any given night, but the water that does condensate will be trapped.  Forever.  

Its very easy to reach the point of diminishing returns on insulation, I wouldn't go past two layers bubble wrap on a van.  After that decent air exchange is going to steal more heat than can radiate out the walls.  Which you need decent air exchange, it hurts to pump out hot air and let cold air in, but you have to get all the moisture out.  

Hank

The trick is just to have enough heat to compensate for the ventilation and to keep you warm at the same time.
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