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Community and Q&A

Skylights and cathedral ceiling – venting the downslope “blocked” bays (or don’t)

rcke2 | Posted in Energy Efficiency and Durability on

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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  1. Expert Member
    Akos | | #1

    I'm in colder climate up here and have never done anything special for skylights. The important part is detailing the opening well so that it doesn't become a big air leak. Provided you seal it up well, it should be fine.

    The board sheathing also has the gaps between the boards from one bay to the next, which does allow for a fair bit of drying.

    If it does ever become an issue, you can always add shingle vents.

  2. rcke2 | | #2

    Aha. Yes. Thank you for the reply and mention of the board gaps. I totally wasnt thinking of that although it’s really small. Looks like I’ll only get about 0.75 square inches per rafter. Better than zero but not by much.

  3. Expert Member
    BILL WICHERS | | #3

    +1 for properly sealing the skylight wells
    +2 for properly sealing the skylight wells
    +3 for properly sealing the skylight wells
    +4.... you get the idea ;-)

    I have this problem right now. I have some condensation issues where air can leak a little up to the flashing in the winter, where it condenses, runs down inside the drywall, and causes the taped drywall joints to lift. I’m going to be opening all the skylight wells up (8 of them), sealing the small gap between the drywall and framing with canned foam, and putting things back together. It’s a nightmare of drywall finishing with all the little corner details.

    Don’t be like me. I thought I could seal the perimeter but it wasn’t enough. Make sure there is NO place for air to circulate in the are around and inside the skylight wells.

    The rest of my cathedral ceiling is unvented and insulated with closed cell spray foam and I’ve had no issues with that part. I’m not a fan of vented cathedral ceilings, but in my case it’s such a low slope I didn’t have a choice.


  4. Expert Member


    I don't know that much about y0ur much warmer climate, but here the rafter bay below the skylight would be problematic. When you are framing the rough opening I'd fill the air-gap with foam, converting that portion of roof into an unvented assembly. The area above the skylight connected to the ridge vent is short enough it would be fine.

    I wouldn't bother with holes. The area necessary to get any useful ventilation is too large, and in the area of the rafter that is experiencing the highest loading.

    1. rcke2 | | #6

      Yes this seems the smart way to go on this. Thanks. I have fiberglass batts, and rock wool batts handy .... could easily spray foam it from can because it’s such a short run and I can see both ends of the space at the moment before skylight framing blocks me....any vote on which? Or all the same?


      1. Expert Member
        MALCOLM TAYLOR | | #7


        I'd stick to foam. You don't want any air-permeable insulation up against your sheathing. If nothing else it's a code violation.

        As Akos said, in most circumstances it works alright, but it's a marginally safe assembly to have a bay designed to be vented with no route for air-movement. Things like that work until they don't.

        I'm dealing with a similar situation on my own house. While re-roofing I found several bays where the insulators didn't provide baffles, and both the sheathing and rafters show some localized rot. Of course here in the PNW it's a lot more of a problem than it probably is where you are.

  5. Expert Member
    Akos | | #5

    I'm in the land of 2.5 story buildings, so there are a LOT of skylights. I've never seen one with any venting detail around it. At least in my climate (zone 5), venting the bays does seem to be an issue.

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