Vapour barrier anyone?

[Camp4Life]

I never see comments on using vapour barriers when building walls and roofs. Does anyone use it? I'm considering doing some kind of wrap on the outside just under the outer roof. The wall structure will have foam cutouts for insulation and I'm also considering a vapour barrier on top of that before installing the inner plywood finish, or maybe that sticky foil stuff. Is this overkill or do people do this? It gets quite cold and damp early on in the summer as well as getting into fall so I'm worried about condensation.

Along the same lines, I wonder how this will affect having an aluminum skin with a wrap right underneath it. The aluminum skin will only be on the front wall and halfway up the roof. Will this cause problems with condensation getting stuck between the aluminum and the wrap?

Here's my wall design:


The outer walls are blue. The inner structural walls are green with cutouts for foam insulation. There will be a 1/8" plywood finish on the green for the interior wall. I'm thinking of putting the plastic vapour barrier on top of the green structural part before putting the interior plywood wall.

Looking for any advice anyone has to give

[lrrowe]

I used foam with a vapor material on one side and then taped joints or areas where the barrier tore off.

[Camp4Life]

I used foam with a vapor material on one side and then taped joints or areas where the barrier tore off.

I assume the vapour barrier was on the inside of the insulation yes?

[lrrowe]

The insulation was rigid foam, 3/4 in thick. I doubled them up with the barrier to the inside.

[tony.latham]

There are reasons that sheetrocked walls (homes) have vaporer barriers. Tiny campers? Meeehh...

My walls (and roof) are sandwiched plywood/foam/plywood. I seal the inside with three coats of polyurethane and the outside with epoxy and cover with .040 aluminum.







My belief is that you can't allow moisture––liquid or vapor––in the wall. If I were to build without a thin layer (1/4" or 1/8") of sealed plywood under the aluminum, I'd be concerned that if moisture finds it way under the aluminum it would end up in the capillaries between the foam and the wood and cause severe problems.

I don't want any moisture touching bare wood.

You will get some condensation on the inside of your camper walls if you don't ventilate at night. Insulated walls drastically cut down on the condensation problem. (Even when we've failed to keep air moving through our 'drop at night, with three layers of poly, I've never seen any damage to the wood and don't believe the moisture penetrated into the walls. To me, that's the solution to half the water problem.

That's my opinion.

Tony

[Camp4Life]

Yes I plan to seal all the wood as well. But what about condensation building up under the aluminum sheet? Isn't the sheet floating and just attached at the edges? Or is the entire sheet glued to the wood?

[tony.latham]

Yes I plan to seal all the wood as well. But what about condensation building up under the aluminum sheet? Isn't the sheet floating and just attached at the edges? Or is the entire sheet glued to the wood?

The sheet floats. For whatever reason, I've never been able to detect any condensation on the inside surfaces. My gut tells me that the energy from the sandwiched walls––that is moving outwards or is just there because of it's mass––keeps the air temperature in this limited space above the dew point.

It's common for us to get dew on our camp chairs but not on our teardrop.

My old Hunter had the aluminum glued to the walls but it had lifted here and there. So either way, you're going to have a void that you need to seal in my opinion.

I'll be curious to hear other's thoughts on this issue.

Tony

[woodywrkng]

I applied a coat of poly to the top of the 1/8" plywood underneath the aluminum skin, but really, there's no reason you should have enough moisture buildup inside the cabin to create a problem. Your cabin should be ventilated when you're inside of course, and if you lock it up tight for the winter, there's nothing inside creating moisture. A non-issue in my opinion. My teardrop has two intake vents near the head of the bed that cannot be closed, as well as exhaust vents high above the foot of the bed, so short of a monsoon, moisture will never build up inside.

[Andrew Herrick]

I certainly won't claim this way is the best ... but so far, it has worked for me.

I've used house wrap (a moisture barrier, not a true vapor barrier) between the aluminum exterior and the plywood sheathing. This housewrap covers every square inch of the walls and roof. It is properly overlapped, taped and stapled at all joints. At the base of the teardrop, the housewrap is left about a 1/2-inch longer than the wall sheathing, and some sort of sealant/tape - I've used different systems and haven't decided on the best - is used to flash the corner seal between the house wrap, wall and the subfloor.

Ideally, this wraps the entire camper in a water-resistive cocoon that prevents intrusion should rain ever penetrate the exterior joints. That isn't likely, but hey, looking 20 years down the line. The aluminum exterior "floats," so there is generally enough space between the house wrap and the exterior for the water to drain down and out. If water ever got behind the house wrap ... well, theoretically, the housewrap will allow vapor to permeate through, which should prevent mildew.

Again, not claiming this solution is better than epoxy. I don't have experience with that method, and I'm still refining this one. But it does seem to be much faster and cheaper than glassing?

[elcam84]

On a camper with varying climates that it operates in a moisture barrier isn't a good thing. The barrier typically goes on the warm side of a structure. And vapor barriers can go on either the inside or outside depending on climate and in regions in the middle you don't want a barrier at all.

It's best not to do a vapor barrier as you don't want moisture to be trapped in the walls if and when it finds a way in through some small leak.

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[Andrew Herrick]

On a camper with varying climates that it operates in a moisture barrier isn't a good thing. The barrier typically goes on the warm side of a structure. And vapor barriers can go on either the inside or outside depending on climate and in regions in the middle you don't want a barrier at all.

It's best not to do a vapor barrier as you don't want moisture to be trapped in the walls if and when it finds a way in through some small leak.

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Now, I'm not disagreeing with you, here ... but there's a difference between something like Tyvek, which allows moisture vapor to escape but sheds bulk water, vs a 6-mil plastic sheet, which is a true water barrier. So while as I completely agree that one ought not swaddle a camper in plastic sheeting, there might still be justification for a water-resistive barrier like Tyvek. Any thoughts?

Also, the whole debate about warm-side vs cold-side ... as I understand it, most of that has to do with the location of the dew point. In a home, that's a big deal, because the dew point will vary within a six-inch R-30 wall depending on the inside and outside temperature. But on a camper with 3/4-inch plywood walls, you don't get that kind of variation. Not sure how that affects the location of the moisture barrier

[rowerwet]

Unlike a rather airtight house, a teardrop must be ventilated well for safety and comfort when sleeping.
I leave my tear windows open for ventilation year round.

[elcam84]

Tyvek is a good air and moisture barrier and like you mentioned does allow moisture to slowly permiate. Poly plastic stops most everything however 6 mil is not a total moisture/water barrier and it will still let moisture through. In construction if you need a true water barrier there are materials that are like pool liner but anyway...

The general rule with vapor barriers in construction is if you aren't sure if or where to put one don't do it at all. A barrier in the wrong place can cause major damage. I have seen my fair share of restaurants with lots of moldy drywall and rotting framing due to a vapor barrier on the inside. In our climate that's a recipe for disaster as it traps moisture. But the corporate architect puts it on the plans and they want it there so....

Personally with a camper I want it to breathe as there is no telling when or where you will have a leak and they are closed up and not climate controlled so breathing is good for moisture control.

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[Camp4Life]

Thanks for all the input guys. I think I'll stick to only putting a barrier under the aluminum sheeting on the front of the trailer just because a thin sheet of alum will get condensation under it pretty easily with changing temperatures. I think I'll leave a small gap open on the underside to let it breathe and let any condensation drip out the bottom.