Venting/insulating outlook bays TGI roof
jordan_harder
| Posted in General Questions on
The house I am building has 14” tgi rafters on a 12/2 single pitch roof. I am planning to seal backed mineral board or eps to the bottom of the web caps then fill below with fiber insulation leaving 1 1/8” vent channel under the sheathing. Dropping a joist on either end to accept 2×8 outlookers which rest on stepped up rake walls. Had somewhat overlooked the insulation detail of these 2’ by 10” or 20” bays between the end joist and the rake wall.
any ideas?
do I need to just fill them with rigid to eliminate due point?
Upward venting would be blocked by outlooks unless drilled..
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Replies
Hi Jordan.
I have seen an assembly with dimensional lumber, not TGIs, where the outlooks are dropped and inch or so from the top of the rafters so that the venting in the last bay is not blocked. The last rafter and the fly rafter would need to be close enough for the sheathing to span without support. Wish I could remember where I saw that. If it was in FHB, there's probably a drawing to study.
It's also possible to attach a ladder-style rake overhang to the gable wall that doesn't tie back into the roof with outlooks. If the overhang is not too deep, some structural screws and of course, the roof sheathing tying the overhang back to the TGI rafters can do the trick. This is a common detail when an overhang needs to be extended to add exterior insulation to a gable wall in a deep energy retrofit.
These are just thoughts of course. You should probably consult an engineer or insulate those bays in a way that don't need venting.
Jordan,
I'd suggest the detail Brian described. Drop the look-outs another 1 1/2" run 2"x4's at 24" on top of them parallel to the TGIs. The lookouts probably only need to be 2"x4"s or at most 2"x6"s.
Another issue is whether 2/12 sloped roofs can be effectively vented.
https://www.greenbuildingadvisor.com/article/insulating-low-slope-residential-roofs