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.