Illustration by Tim Healey for “Why and How to Build an Insulated Overroof” in Journal of Light Construction.
This detail is a good example of how to have a thin roofline but still sufficient insulation for an R6.6 roof. The advice may vary for a specific project, but in general it’s much more robust to ventilate under the metal roofing using an above sheathing ventilation setup. This can be done with 40mm of thickness if a slim profile is required.
Note that the insulation between the truss chords is directly in contact with the air control layer. This is required to prevent thermal bypass and other performance issues.
I recommend reading the article by Travis Brungardt in the Journal of Light Construction—it points to the significant benefit of early contractor engagement and the value great builders bring to a project when they have a solid understanding of both their trade and building science. This is even more important on projects where the architect lacks an understanding of designing for building performance and durability!
Comments 1
This is a great detail. We have used SIP panels, which are foam filled, instead of the 2×6 buildup on top filled with mineral wool. On top of 14” truss joists filled with dense pack cellulose. This is for cold climate Canada – or Arthur’s Pass.
Of course, Western Canada has an oil industry, making foam products far cheaper and easier to obtain. We also have a rock wool plant and a cellulose plant.
The Structural Insulated Panels are used the same way: 2×6, with 7/16” sheathing top and bottom, cantilevered to provide overhang. With foam bonded to plywood, the resulting sandwich has considerable strength, allowing less lumber use, with framing really only providing the extra structure for the overhang. Less lumber, low U value, the foam can also be graphite blown for better performance and compressive strength.
For the future: BASF has now produced a graphite blown foam that is based on vegetable oils rather than fossil fuels. Makes me wonder if that’s really better, with greater implied land use, but an interesting option. If it can be made with waste vegetable oil…