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I've been building my van out and have been having a lot of trouble with getting foam board to conform to the slight curvature of the walls of my van. I've got a Promaster 2500 136WB. I tried using 1" polyiso board, cut to fit the spaces between the ribs of my van with spray adhesive but I just can't get it to stick along the curvature of the van body. I've watched countless videos and read dozens of sets of instructions but I just can't seem to get it to work. 

My only thoughts going forward are to either layer thinner layers on each other, a much more expensive prospect, or just glue up strips of the insulation along the curve of the walls. Alternately, I've considered more flexible alternatives but the soft stuff doesn't seem to have a high enough R-value by comparison or other serious downsides like weight or water absorbancy or cost. 

Does anyone have any thoughts?
Why do you want to follow the curve?

It's called kerfing
I believe a heat gun will allow you to bend it. Check the Foamies fourm at tnttt.
Sorry left out the description, link is to google, filtering foamie discussions on the topic.

Adjust keywords as you like.

And no, heat guns are not a great option with thick boards, kerfing is a PITA but the way to go.

But IMO likely unnecessary for this use case, unless curves are desired for aesthetic reasons.
If I can't get the insulation to adhere to the van along the curvature, there will be open spaces behind it where condensation can collect. That's why I want it to conform to the curvature of the walls.
In a van, there will always be condensation. The only answer is proper ventilation. Be sure the weep holes are open at the bottom of the walls.
20/20 hindsight which you already figured out.
For curved surfaces, (many posts on the subject), 1/2" polyiso layered is the way to go.
That's kind of the way I've been leaning, Johnny B.

Weight, I have heard and read nothing of weep holes. Would you please elaborate?
a thin layer of spray foam directly over the steel

make sure not to plug the drip holes or whatever channels the maker put in for draining water out from the inside of the panels

also seal up the gaps between sheets, corners

cover any exposed protruding metal, prevent bridging

in short, create a well sealed envelope

a good vapor barrier just under the innermost lining

Finally yes, strong controlled ventilation like a Fantastic Fan or MaxxAir is also important, don't let humidity build in the first place
John61CT, these sound like good suggestions to me. Thank you.

Is spray foam a sufficient adhesive to hold the foam boards in place? I was planning for spray adhesive, probably 3M 77 or 90 and then caulking with silicone and spray foam.

Because of certain limitations, there are areas of the van (primarily the wheelchair bay in back) that I won't be able to insulate for space reasons. I'm going to close that off as much as I can from the rest of the van. A vapor barrier is probably not going to happen for this reason, but I do plan to ventilate well, both with a MaxxAir fan and an AC to bring down the humidity.
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