Sounds like a great project. First of all, I want to stress ... you need to make sure that whatever you do, it is with the sanction of the manufacturer of the roofing you choose.
I would strongly encourage a vapor barrier behind the ceiling boards. Then I would suggest several inches of urethane foam followed by another layer of decking, underlayment, and the roof.
Now, although I did not include it above I really would also suggest a vented airspace (with intake and exhaust vents) on top of the insulation and beneath the second layer of decking. You might get by without condensation issues without this ventilation but it is a good safeguard and will also help with energy efficiency. It does not need to be a huge airspace -- 1" should be okay. It also is required by the International Residential Code.
Todd Miller
2011-07-05 09:52:31.000000
I am in the process of designing a year round structure in Newport, Maine. We would like to have a metal roof on the exterior and from the inside a cathedral ceiling with exposed rafters. Through the rafters you will see 3/4" T&G boards. From the T&G to the bottom side of the metal roofing what is your recommendations for any type of roof decking and insulation? What would a typical section look like with thicknesses?
Thank you!
Joseph Roscia
2011-07-05 09:13:59.000000
Great suggestions and recommendations Todd.
Definitely need a complete, unbroken, and air tight vapor retarder on the other side of the T&G. Ice and water shield can do this as can spray foam. I prefer a vapor retarder that is adhered to the T&G (needs to be airtight).
Spray foam or rigid foam for insulation is probably best. Rigid will be cheaper but is harder to get down and make for intimate contact with the entire structure. Spray foam will be more expensive but can eliminate a couple of steps and make the labor portion of the project easier to somewhat off set the cost.
Here is a link to an article on just such and installation as Todd reference with the breathable space above the insulation and between the roof deck and roof.
http://danperkinsroof.com/1108_JCL_Perkins_A.pdf
This is just one method of doing it. If I were doing it new construction and were going to use spray foam in other sections of the home, that is the route I would go.
