overhangs for thick foam roof – GreenBuildingAdvisor

Don, tHis is what I did for my house:

I ran horizontal purlins spaced every 2 feet. In between the purlins was 1.5″ of foam. Then I placed sheets of 8.5″ thick foam (staggered) on top. I used long screws through the foam to hit the horizontal purlins. The purlins are an easier target than hitting the trusses. I suggest you use 2×6 purlins for an easier target.

Keep in mind that thick sheets of foam may want to slide downward over time. Build a lip along the eaves so they cannot slide down.

Over the foam, I installed Mento 3000 as an air/ water barrier. Then I did 2×4’s on edge over the mento. I used really long screws to go through the 2×4’s and 8.5″ of foam in order to hit the purlins. 5/8 sheathing over the 2.4’s. The 2x4s provide a generous air gap for the ridge ventilation.

There are more than one ways to accomplish the outcome you’re seeking. Focus on air sealing, keeping the dew point outboard the sheathing, and staggering foam. Joe Ltiseberik (the very smart building scientist) didn’t stagger the foam on his own home and paid a price. An inquisitive carpenter can help you achieve your desired outcome. Good luck