Tip #1: Over the plywood roof sheathing, add a layer of peel and stick vapor barrier to stop vapor transmission into the rigid insulation system which will be the next layer above the plywood deck.
Tip #2: Put 2 layers of 2.25 inches totaling 4.5 inches of EPS or XPS closed cell rigid insulation on the top of the deck with horizontal or vertical joints never matching up and R-19 fiberglass batt insulation on the under part of the deck. This will give the area an R-value of 38. Or, you can do 2 inches of XPS on the top and 2 inches of closed cell spray foam underneath to act as an air barrier, then add the R-19 batts to the under plywood deck side.
Tip #3: Be sure to find deep roof trusses. These should be deep enough for 12-16 inches of insulation and also have enough room for a 6-inch air gap between the roof sheathing and the insulation top.
Tip #4: Adequate ventilation is key when it comes to insulating your flat roof, so it’s imperative that you have vents around the perimeter of the closed soffit area. These can be in the form of soffit vents, fascia vents.
Flat roofs are definitely unique and require some different processes. If you’re overwhelmed with insulating a flat roof by yourself, you should contact a contractor that has extensive experience working with flat roofs. It’s better to know what you’re getting into before you start diving into the insulation process yourself. Getting advice on what the best, easiest, and most cost-productive methods are helpful and will allow you to make the right decision so you don’t get in over your head.