Do I need a vapor barrier?

[cobraguy19]

I am planning a 30x50 metal shop and I plan on using foam board insulation. I live in Oklahoma where is gets humid in the summer. I was wondering if I need a vapor barrier between the metal roof and the foam board insulation to prevent moisture from dripping down? Thanks.

 

[Scotto]

If you spray foam/caulk between the foam boards then you don't need to IMO.

 

[Firebrick43]

I agree with Scotto

EPS, XPS, and polyiso all are wind proof and vapor retardant enough that if installed well at the perimeter of the panels are well sealed there will be little issue.

 

[dcg9381]

If you spray foam/caulk between the foam boards then you don't need to IMO.

I assume this is an R-panel type roof.. there would still be gaps... Any moisture would "run down" the foam potentially.

I'm in TX, the only time I've run into this is when it's really humid outside and the temperature of the shop is a lot lower than outside overnight.. In that case (uninsulated shop / no AC) I could see moisture inside on the R-panels. If you cool the shop (HVAC), that should be your humidity control.

Using a moisture barrier like say Tyvex on R-panel, the tyvex isn't touching the ribs in the R panel. I assume to properly use it, you'd need an OSB sheet, moisture barrier, then R-panel.