I've worked for a few clients that have spray foamed their shops and their houses. From what I've seen, 2 inches of closed cell gives good results. Even with the shop doors wide open in the heat of the day, it was significantly cooler inside the shop then outside. No AC in the shop, just the insulation.
In another metal shop, my client went with open cell. The cost was a lot less and it was over a foot thick. The results felt good too. He had a couple of window AC units in there and even though the roll up door wasn't insulated, it was nice and cool in there. He didn't believe me when I told him that open cell has a reputation of rusting out the metal, but a couple years later I ran into him at Lowes and he was trying to find something to deal with the rust on the metal that had just started to show up.
I've worked on and been in a few houses with open cell sprayed to the roof and the results have been catastrophic. One was only two years old and the shingles where curling with a lot of rock missing. It really looked like a 40 year old roof that was at the end of its life. It was for sale for $850,000 and nobody bought it. The last I heard, it's still for sale almost five years later with a much lower price, but not enough to deal with the foam issue.
The house I'm working on right now has open cell foam on its roof. It was done about 5 or 6 years ago by the previous owner. The shingles still look good on it, but the gable ends are rotting away. They used MDF siding above the brick and it's all deteriorating really bad. MDF is alright f it's kept dry and painted. Since the underside was never sealed, the moisture in the open cell foam put moisture right on the MDF siding. I'm not messing with that repair, it's massive and extremely difficult to get to with the peak being 30 feet up. The bids to replace it are ranging from $55,000 to $93,000
When I added on a 3 car garage to my house, I had a valley that didn't give me very much room for insulation. Here in Texas, R60 is the best you can get for keeping your attic area cool. That's about 2 feet of blown in insulation. I had 7 inches in that area so I went with the spray foam kit listed above. Kraken spray cans. I found that the first half of the can went on pretty good. It's unbelievably messy with the spray going way off to the sides of where you are spraying. What did go on the metal was pretty good, but then once the can was about half empty, it sputtered and instead of spraying several inches wide, it liked to spray more like Great Stuff, but not as fluffy. It also didn't stick as well and a lot more of it fell to the ground. Overall, I was able to get it all done with several more cans of Great Stuff to fill the areas that needed it, but I'm not going to do that again.
I think the best way to do a barn if money isn't an object is to spray 2 inches of closed cell, then get your R value from fiberglass. Air penetration is the biggest battle and what defeats AC more then anything else. Once you get the building sealed in by blocking the air, it's pretty easy to retain your temps with fiberglass.
The walls are easy, but the ceiling can be a challenge. Since heat rises and cold falls, the ceiling is where 90% of your heating and cooling issues happen. Walls are mostly to stop the air from getting in, which is why R13 or R19 works. Windows and doors are less then R5, but if sealed from the wind, it's not an issue. The attic needs to have the highest R value possible to get the best results!!!!
When I build my shop, I'm going to have a metal roof and wood trusses. I'm going to have a metal ceiling and spray 2 feet of Attic Cat loose insulation up there for an R60 rating. The metal roof will have a full ridge vent. To me, this is the best way to deal with the heat in Texas for the money. Walls will be fiberglass bats, and my exterior sheeting will be taped to keep out he wind. I do not plan on using foam except for filling gaps where Great Stuff can do a better job.
