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Insulate vented cathedral attic ceiling above gable joint

mfleck | Posted in General Questions on

Hi all,

Im deep in my attic insulation project. Here is my plan: I’m using 1-inch foam to create a 1 inch air vent against underside of roof. I’m sealing with spray foam. Then will come 3.5 inches of unfaced rock wool up to the bottom edge of rafters, and then 2-inch polyiso layer under rafters, followed by 5/8 drywall.

Ive got 80% of the ridged foam cut and cobbled in and have one more stretch to do, but it runs over the opening to a gable running perpendicular to the main roof. This section has access to the main ridge vent, but not the soffit venting.  What’s the recommended approach here?  Do I follow the same approach, but just seal off the bottom of the venting gap since it can’t go down to a soffit?  So, is it ok to just have ridge venting for this section without soffit venting?

thanks, everyone.


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  1. Expert Member
    Akos | | #1

    It depends on what you are doing in the gable section. If that is left with insulation on the floor and a gable vent, then you can use the air from the gable vent to feed the new channels going up. If this is the case, make sure you air seal the gable section for the attic space. Important to block up all air flow paths around the joists and rafters.

  2. mfleck | | #2


    The gable (might not be the right term here) is above the bathroom that juts off the main rectangle of the building. The gable has the same roof structure as the rest of the attic (straight shots from soffit to ridge vents). I am planning to insulate the gable section in the same way as the rest of the attic.

    So, I don’t see how I could have intake air coming into this vented part of the roof that is cut into by the opening to the gable. It has access to the ridge vent, but nothing but conditioned air at the bottom opening to the gabled section (see pictures).

    Is it possible to vent this section? How do I do this?

  3. GBA Editor
    Brian Pontolilo | | #3

    Hi Matt,

    Just to be clear, you are planning to insulate the roof line in the space over the bathroom as well? If so why? Unless you are going to be using that space for something, you could air seal and insulate the attic floor in that area, which is a more cost-effective strategy and the follow Akos' advice and let that vented area interface with the roof venting over the main volume of the house.

  4. mfleck | | #4

    Thanks Brian and Akos,

    That makes sense.

    So, I need to find a way to seal that opening to the gable space above the bathroom so that the air in that unconditioned/vented space above the bathroom can move freely into the vents that I create on my last roof line (but not into the main attic space)? Then, I'd insulate the bathroom ceiling. Is that right?

    Should I use faced insulation in the attic floor above the bathroom?

    1. Expert Member
      Akos | | #5

      Pretty much. I would air seal the ceiling above the bathroom first then block off any air paths at the ceiling joist level to the conditioned attic. After that top up the insulation and finally install the air barrier between the two spaces. You might want to build a small hatch in case you need to get in there.

      Our code calls for vapour barrier at the ceiling for an unconditioned attic space. Consensus seems to be the presence/absence of a vapour barrier doesn't matter in this location, so go with whatever is easiest. Just make sure you don't inadvertently build a cold side vapour barrier.

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