I am not a fan of spraying to the back of the roofing for a couple of reasons. It will make it very difficult to ever repair or replace the roof. Also, it can make it very difficult to ever troubleshoot roof problems. To answer your question, I think I'd pose that to your foam contractor. It probably would work but I'd sure confirm it with them.
Hello, I am currently building a pole barn style home. With this I have the metal roof on purlins, currently there is no vapor barrier which is due to the plan to used closed cell spray foam. However I did not know the risk of applying directly to the metal. I saw on another post with similar question that using fan board in between the purlins would be ideal before applying foam but that was with a standing seam metal roof which mine is not. If that would still be the ideal way to go would it be possible to use a reflective board secured to the Purlins and spray foam to the back side, non radiant side.
