Installing Metal Roof over Spray Foam insulated roofs

Thank you. It sounds like this will be a non-vented roof assembly. I assume the spray foam is closed cell which is a good product for forming a vapor barrier and having good R Value.

Most metal shingle products are designed for direct to deck installation along with underlayment as prescribed by code and the roof manufacturer. There are a few that can be installed over battens and a few that require battens.

The general practice as you have described and I have summarized would be to keep with the roof manufacturer instructions. In most cases, that would be direct to deck with underlayment.

There are some products such as breathable underlayments, reflective underlayments, and entangled mesh products that could be used for extra energy benefits or extra security in regards to letting moisture drive out of the building. However, based on this project, I see no essential reason to use them.