Replies

1. GBA Editor
   Martin Holladay | Sep 14, 2011 08:31am | #1

   Walter,
   First of all, I'll post a link to the detail under discussion, so that other GBA readers know what you are talking about. Here is the link:
   https://www.greenbuildingadvisor.com/system/files/sites/default/files/GBA_INDIVIDUAL_PDF_FILES/GBA_PDF_Series-9/9-00800.pdf

   The reason that the detail drawing shows a gap between the old fiberglass batt and the roof sheathing is that this drawing shows a retrofit job. The drawing assumes that the gap is a pre-existing ventilation channel. When additional rigid insulation is installed on top of the roof sheathing, the bottom and top of the existing ventilation channels need to be carefully air-sealed with rigid foam panels and spray foam, as shown in the drawing.

   In your case, if you have existing 2x6 rafters without any insulation, it's possible to fill the entire 5.5-inch-thick rafter space with cellulose -- as long as you also include plenty of rigid foam on top of the roof sheathing, as you describe.

   Your final question is unclear. If I understand your question correctly, you wonder whether it makes any sense to include ventilation channels above the rigid foam. If you can afford to do so, that would be a Cadillac job. From the bottom up: gypsum drywall, cellulose-filled rafter bays, existing roof sheathing, two layers of polyisocyanurate with staggered seams, new plywood sheathing, 2x4 sleepers installed from eave to ridge to create vent channels, another layer of plywood, and then asphalt felt and shingles.

2. 
   WEG | Sep 14, 2011 09:00am | #2

   Thanks Martin. Is there a way to eliminate the plywood over the foam in the Cadillac scenario? Gypsum, cellulose, existing plywood, two layers iso, sleepers, plywood, felt, shingles. Thanks

3. GBA Editor
   Martin Holladay | Sep 14, 2011 09:06am | #3

   Walter,
   Yes. I should have pointed that out. You're right.