Insulating A Flat Roof

[Jody Sperling]

I am completing a live-in flip on an old house with a flat roof. We had to replace a large section of framing due to leakage and rot. Now, I don't know how to insulate the roof to avoid condensation and moisture issues. Can I use closed cell foam directly on the decking and joists? Do I have to drop the ceiling. What is the best way to go?

I've watched dozens of DIY videos, but this one is more complex than I expected and confusing.

[James Rohr]

Jody, by day I’m an Architect and Building Enclosure Consultant (in Omaha). 
As you know, moisture/ air/ vapor/ thermal control is key for the long-term performance of your house and occupant comfort. Given you are trying to add insulation after the roof has been redone adds a level of difficulty and if not installed properly can actually lead to other issues. 

You post a lot of great info and I’ve learned a ton from you over the last year since joining BP.

I’d be more than happy to give you some guidance (or at least share some cautionary tales) on your specific situation if you’d like to DM me.