Rob-Goose said:
so basically i have to put in a moisture barrier then on to of the insulation if there's going to be a gap.
There is a lot of debate as to moisture barriers in vans. Lots of people assume the same rules apply as in homebuilding... but they don't.... at least not exactly.
There should only ever be one moisture barrier because, once moisture gets between the two barriers, it is very hard for it to get out. That's when you get mold and mildew. In a home, that barrier is close to the inside in areas where it gets cold. And it is close to the outside, in hot, humid environments. Basically, the barrier needs to be on the warm side of the insulation, so the barrier itself doesn't get cold and cause moisture to condense on it. Some people are going to challenge this, saying that Tyvek House Wrap is put on the outside of houses even when there is a moisture barrier on the inside. But the first line on the
Tyvek web page says it, "helps prevent the infiltration of air and water,
but lets water vapor escape to prevent rot and mold inside walls."
However, in a van or car, you already have a moisture barrier: The metal wall of the vehicle. In warm environments, this works great, but in cold environments, this is a problem. One that I don't believe is solved by adding a second moisture barrier. If you
do choose to install a second moisture barrier, you have to make dead sure that you have an absolute seal, That space between those two barriers must be utterly hermetically sealed. If you do that, then it can significantly increase the R-Value of your whole installation, because no air can get through. Here's the rub. Almost no one can do that. If you have to put a screw through your vapor barrier, well now moisture can get in there. And, in a vehicle, those things are almost certain to eventually get damaged.
Now, lots of people will jump in here and tell you that the metal wall of the vehicle is not a complete vapor barrier because there are drain holes at the bottoms of the doors and such. But, if you look carefully, those drain holes are between two separate layers of the metal: The outside of the body, and the inside structural frame. The inside of the vehicle is still usually pretty well sealed from the outside of the vehicle.
So, if you install an additional vapor barrier, you are just adding work to your project and increasing the chance that you will get mold and mildew in that space between the two barriers.
So, what do you do?
First, leave out the vapor barrier. You already have one.
Next, you want your insulation glued as closely to the wall of the vehicle as possible, so no moisture can get between the insulation and the wall of the vehicle. Yes, this is a bit of a vapor barrier, but it is localized to each separate piece of insulation, so it is easier to keep a handle on. Then seal between every piece of insulation, all the way down to the metal wall. The idea is to keep as much air from inside the vehicle from contacting the inside surface of the metal wall of the vehicle. {But don't use Great Stuff foam sealant. That stuff is open cell foam (despite what their web site tries to trick you into believing) and will hold moisture. I have read old posts about people ripping out Great Stuff foam and finding rust all over the inside of their vehicle, but only where that foam was.} Just use flexible sealant like silicone. My favorite is Lexel, because it stays more flexible than silicone and sticks better to more stuff, but it is pretty expensive to go squirting it all over the inside of a vehicle. But be careful using cheap household caulk. That stuff gets stiff and will eventually pull away from the metal.
Finally, build your actual wall so it will
breathe. Now that you have done everything you can (other than an easily damaged moisture barrier) to keep air from getting to the metal wall, you need to accept that some moisture will get between your wall and your insulation. The point is to make sure it can find its way back out again. My preference is simply a fabric wall covering with quilt batting behind it. But that can be hard to keep clean. Only put solid paneling on the parts of the wall that you will see. Behind all your cabinets, just put some strips of wood or fabric or netting to keep the stuff in your shelves from beating up the insulation. Or maybe drill a bunch of holes in the paneling back there. Instead of solid paneling, use strips of paneling or thin wood, or even plastic strips. Just make sure that any air that gets behind the paneling can easily get back out. Remember, the paneling is really only there to protect the insulation (and for better looking YouTube videos). The insulation is really doing all the work.
Now, around your cooking area, you will want to keep that large amount of moisture from getting behind the wall, if possible. In this case, the interior surface of the wall (the surface that is facing you) should be your only additional moisture barrier. That way you can clean it as well as easily see if it is damaged. But, you will still get some moisture behind that wall. So you still want the space behind that area (between the paneling and the insulation) to be able to breathe.
Don't let people with tough-guy sounding user names fool you. And remember, X years of experience in construction does not mean someone truly understands the physics of what is going on and how building in a van is different from building in a house. I am sure at least someone is going to get in here and aggressively challenge my intelligence or experience. Yeah, don't let that fool you either. Just think about the science and make your own decision.
Good luck.
