Insulating Roof With Exterior Rigid Foam
justfount
| Posted in Energy Efficiency and Durability on
I am planning a new home build and have been following GBA, Building science and Dr. Joe. The house is a 44 x 30 ,1 1/2 story using attic trusses and 2 20 foot gable dormers. 9′ tall basement, deciding between 5 foot ICF walls with 4 foot pony wall on top.
However, I am confused with the information overload. I am in climate zone 7 Upper Peninsula of MI. I would like to use the continuous exterior rigid foam method but find that for my climate zone it requires SO much exterior foam to meet the the ratio requirements. How can I build in this climate to have good R factor, and allow the sheathing etc to dry.
Thank you
GBA Detail Library
A collection of one thousand construction details organized by climate and house part
Search and download construction details
Replies
According to this article by Martin Holladay, How to Install Rigid Foam to Roof Sheathing, you’ll need R-49 of rigid foam (about 12.5 or 14 inches of EPS, 10 inches of XPS, or about 9 inches of polyiso). Take a look at the drawing in the gallery for the assembly details. He also discusses an alternative method whereby you add some rigid foam to the exterior and some to the inside of the roof sheathing.
Your alternative is to vent the roof. With a complex roof, shingle vents may help.