GBA Logo horizontal Facebook LinkedIn Email Pinterest Twitter X Instagram YouTube Icon Navigation Search Icon Main Search Icon Video Play Icon Plus Icon Minus Icon Picture icon Hamburger Icon Close Icon Sorted

Community and Q&A

Vented cathedral ceiling except 2 rafter bays

JohanH | Posted in General Questions on

We have an early 1900 bungalow that we are looking at converting the attic this winter.

Gable roof, 8:12 pitch. I was planning on keeping the assembly vented (soffit vents with 1.5″ site-built eps baffles and ridge vent already present). However towards one end of the space is a valley rafter. So I would have 2 rafter bays (maybe runs of 6ft and 3ft) which can’t be vented at the bottom. All other bays are straight shots from the ridge to the eave or would extend past the planned knee wall.

Do I switch my entire plan to unvented and go with ccSPF? Buy a small diy spray foam kit for those 2 bays? It’s getting colder here (zone 5) so that’s another factor if we don’t want to delay until warmer months. Any other ideas?

GBA Prime

Join the leading community of building science experts

Become a GBA Prime member and get instant access to the latest developments in green building, research, and reports from the field.


  1. Expert Member


    I would stick with a vented assembly, and use cut & cobble in those two bays.

  2. JohanH | | #2

    Thanks Malcolm. What about the reports of unvented cut and cobble cathedral failing?

    It's not a huge area. But replacing jack and valley rafters does not sound fun if it rots.

    Any value in adding a 1.5" ventilation space even though the bottom won't be open? Normally I'd guess no but given the short runs, I could see some air being able to move.

    1. Expert Member
      MALCOLM TAYLOR | | #5


      "Any value in adding a 1.5" ventilation space even though the bottom won't be open?"

      Yes. Between the cut & cobble, and a good interior air barrier you should be fine, but keeping the bay open removes the possible problems of moisture trapped between the foam and sheathing. How deep are the rafters? You could bore a few 1" holes an inch from the top to connect them to the bays on both sides.

  3. Expert Member
    Akos | | #3

    As long the rafter bay is vented on one end it will work fine. This is common enough and generally doesn't create issues.

    The wood framing allows a fair bit of moisture movement as well, so these bays can also dry towards the ones that are fully vented.

    Generally the key to any of these assemblies is to focus on air sealing. Knee walls is not the best start for that, figuring out those details out is more important than these two rafters.

    1. Expert Member
      MALCOLM TAYLOR | | #4


      "As long the rafter bay is vented on one end it will work fine"

      Unfortunately it doesn't here. It's fairly common to find one or two bad bays among a well performing roof, the only difference being no continuous vent path.

    2. Deleted | | #6


  4. JohanH | | #7

    This article/ video makes me think I might be ok providing 1.5" ventilation space even though the bottom isn't open.

    We have original 1x plank decking then OSB on top. There's up to 1/2" gap between boards which should mimic the design in the video and provide some level of horizontal airflow. Not ideal but hoping it will suffice with the short runs. Thoughts? Please correct if wrong.

    Malcolm, I'm hesitant to drill near the top of a rafter. Effectively it's a joist at an angle and covered by the same guidelines for holes and notches.

    1. Expert Member
      MALCOLM TAYLOR | | #8


      I think you are right - that horizontal pathway adds a lot. Well air-sealed, it should be fine.

Log in or create an account to post an answer.


Recent Questions and Replies

  • |
  • |
  • |
  • |