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Community and Q&A

Vented roof with valleys

brentwilson | Posted in General Questions on

Imagine a ventilated cathedral ceiling roof with some valleys in climate zone 6B. It would have a vent channel in each rafter bay of a bit over 1”. In addition, it would have 2x4s as cross strapping on top of the rafters providing some cross ventilation below the sheathing. In this scenario, are the rafter bays in the valley areas (with no soffit vents due to the valleys) going to get adequate ventilation?

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  1. Expert Member


    Unfortunately the answer is almost certainly not.

    1. brentwilson | | #2

      So you don't think the 2x4 cross-strapping would provide enough airflow under the sheathing in the valley areas?

      1. Expert Member
        Dana Dorsett | | #3

        It depends- there are too many unknowns to say for sure whether it will work. Climate matter, the design of the ridge & soffit venting does too.

  2. brentwilson | | #4

    My hope was that the 2x4 cross strapping on top of the rafters would provide enough cross-ventilation into the valley rafter bays. If not, how easy would it be to spray foam those bays when the sheathing is up above the rafters by the depth of a 2x4? Would it be okay to run 2x4s as both cross-strapping and parallel with the tops of the rafters in order to box out these valley bays for spray foam? Or would it work to keep the vents under the sheathing and cross-strapping, then 2" rigid foam in the rafter bays, then spray foam in the rafter bays?

    1. Expert Member
      MALCOLM TAYLOR | | #5


      Those are pretty small valleys. If the covered porch and garage have attics allowing air-flow from below, you are just left with a few small potentially problematic areas that many well be fine with the cross ventilation.

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