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Best way to vent (or unvent) roof?

I am looking for suggestions on how to handle insulating this roof. The main problem is that where there are gables, there aren't any eaves for venting. The house is in Zone 6, I am looking to get R100 by insulating the rafter plane with dense pack cellulose. Am I better trying to build out a channel from an open eave to create venting or going unvented by fitting 5" of foam in the upper roof and finishing with cellulose. Neither idea seams great and spray foam would be very costly.



Asked by Brian Van Handel
Posted Nov 4, 2012 7:11 PM ET
Edited Nov 5, 2012 7:07 AM ET


3 Answers

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Since it looks like your roof has at least one valley, it's not a good candidate for a vented roof.

It looks like a new house, which raises the question: isn't this a little late in the construction sequence to be designing the roof insulation?

Ideally, you would have included a thick layer of rigid foam above the roof sheathing before you installed the roofing. Is it too late for that?

Answered by Martin Holladay, GBA Advisor
Posted Nov 5, 2012 6:48 AM ET


Martin, the house has a 12/12 pitch and is located on a steep hill. I had looked at guidelines from Fastenmaster and if memory serves me right, 5-6 inches of foam meant screws 6" on center. Not very buildable. Originally, I had planned on a tji roof and unvented roof throughout with 5-6" spray foam on the underside. The design was changed to trusses, which allow roughly 50% of the roof to be vented. Venting is by far a cheaper option as the trusses have enough depth for R100 of cellulose. I was looking for opinions on was to do with the rest of the roof that isn't easily vented.

Answered by Brian Van Handel
Posted Nov 6, 2012 12:50 PM ET


Unvented roofs need either:
1. Rigid foam on top of the roof sheathing, or
2. Closed-cell spray foam on the underside of the roof sheating.

Another option, of course, is a SIP roof.

Answered by Martin Holladay, GBA Advisor
Posted Nov 6, 2012 2:15 PM ET

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