Skylights and cathedral ceiling – venting the downslope “blocked” bays (or don’t)
Hey – I have a single story house in Southern California R value zone 3 (1 mile from coast in Manhattan Beach/Los Angeles area).
Looking to add skylights in a cathedral ceiling. Rafters are 2×8 struct #1 that are 16 on center.
The size of the skylights (30 in x 46in) and the size of the ceiling results in only about 36 inches of rafter bay on the below side (so that much would be blocked) and about 24 inches on the top side from them (which vents to ridge vent like the other rafter bays will that are unobstructed by skylight framing)
This cathedral ceiling is already insulated with Polyisocyanurate (4.5 inch R Max Thermasheath) spaced 2 inches away from roof deck and the roof is one layer of new 30 year asphalt shingle, heavyweight tarpaper, and the roof deck is original NO plywood, only 1×8 lumber. Need advice on what to do with the bays which will become blocked and unable to vent to the ridge vent.
Opinions seem to be 50/50 here on attempting any sort of side/cross venting for the adjacent vented bays. I agree that holes of sufficient size will weaken the rafter (and yes, the only holes i could drill would be in those 2 inches where i am clear of the polyiso foam)…..and smaller holes which don’t add a knock down factor to the rafter capacity would be ineffective for air flow
Should I properly fill the 24 inch length of rafter bay below the skylight with fiberglass and call it a day?
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