I live in humid east Texas. I am building an all metal 30 x 40 building on concrete (vapor barrier under concrete) that will be occupied periodically. Walls are 10 feet and pitch is 1:12. I want to foam the underside of the metal roof and build no ceiling. Can I use open cell foam of 5 to 6 inches? Do I need a vapor barrier? Will I have excessive  humidity build up when not occupied? Mold can be a problem down here.  Are there any long or  short term potential health problems? Walls will be studded and batts installed.  I'm reasonably sure the foam will stick when applied properly. Space will be have A/C and heat. Any other tips or concerns would be appreciated. Thanks.

Ran

You said it will have AC/heat but how often will you run that if it's only occupied periodically?

A metal roof IS a vapor barrier. Putting a second vapor barrier on the interior of the roof assembly would keep open cell foam from ever drying should there be a leak.

To meet code you need an ignition barrier between the foam and the interior space though. (Half-inch gypsum, or most 3/8" OSB or plywood would qualify.) Since you don't want to build a ceiling, you probably don't want to build a cathetral-ceiling either? There are spray-on ignition barriers that would work, but for the money you'd be better off going with 2" of closed cell foam at the roof deck with 3" wet-spray cellulose or superfine wet-spray fiberglass (JM Spider) blown-in-bag on the interior for the ignition barrier, which would boost the R up to ~ R23-25, compared to R21-22 for 6" of o.c.SPF. The wet-sprayed goods have water-activated adhesives- once they're up they stay in place, and won't need gypsum or wood for support on the interior. (IIRC it takes 3" of either to be legal.)

On average 1 or 2 times a month

Code is not a problem. Safety is. Along with long term health concerns. Due to only periodic use, cost is also concern. I have a quote of  spraying 5 inches of open cell on the ceiling  for $1.55(reasonable?) but gotta make sure no health probs and won't fall off. Also will the breathe enough. I know I can de-humidify with the A/C but with a lot of idle time will humidity tend to build up in the house. Also can foam be painted black with a latex paint? Thanks for you reply.