Roof insulation

[Chevyshortbus85]

Hello there,

I am working my first built and it is a shorty Chevy 1985.
I've already started to do my new insulation but I may have made a mistake.
I want to know what is the best to prevent condensation.
I got a sweet deal on spray foam and foamed the walls and ceiling myself with the plan to put this double reflective vapour barrier on top of it. Unfortunately someone told me that it was probably going to creat condensation between the vapour barrier and the roof. That lead me to do some more research and read that a proper vapour barrier would work but that such a thing is nearly impossible with van conversion. That the best option is to have more ventilation.

My bus already has 2 holes trough the roof for ventilation.. one for the airflow for the condensation from the propane fridge and the other one just has a general opening for air. I am planning to get a mini wood stove for dry heating.


So the big question: What do I risk if i have these layers ( from outside to inside ) for my roof.
Outside metal shell, 1-1.5 inch of spray foam, 1 layer of double reflective vapour barrier, 1/4 inch fir tree planks


Thanks ahead for helping me out.

[Drew Bru]

Originally Posted by Chevyshortbus85

So the big question: What do I risk if i have these layers ( from outside to inside ) for my roof.
Outside metal shell, 1-1.5 inch of spray foam, 1 layer of double reflective vapour barrier, 1/4 inch fir tree planks

Sounds like it should work just fine. I think most spray foam installations are like what you're planning. I wouldn't think you need to worry.

[milkmania]

Originally Posted by Chevyshortbus85

*snip*
Outside metal shell, 1-1.5 inch of spray foam, 1 layer of double reflective vapour barrier, 1/4 inch fir tree planks


Thanks ahead for helping me out.

what kind of R Values are being expected from "1 to 1.5" spray foam?
not sure which route I'm taking

I'm learning
R-3.5 to 3.6 per inch open cell spray foam
R-6.5 per inch closed cell spray foam

I know poy-iso foam board is R-5.0 per inch.

[Tango]

You do NOT want to use open cell foam. Open cell = "Sponge".

However...it is excellent if you want to hold lots of moisture in your roof & walls.

[milkmania]

Originally Posted by Tango

You do NOT want to use open cell foam. Open cell = "Sponge".

However...it is excellent if you want to hold lots of moisture in your roof & walls.

Got it, thanks

[Yukon Cornelius]

I'm skipping the vapor barrier, letting it breath is the concept. Much the same as cabins up here, as the bus will be exposed to hot and cold cycling dependant on usage, and weather I want to any moisture to be able to breath as oppose to trapping it.

Condensation will form on the warm side, so if your spray foam has been done well all you have to worry about is a thin isolator between the spars and your ceiling material. Then from there airflow/cycling/exchanging should help regulate the moisture difference.

disclaimer.....random redneck ravings