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Low slope attic/cathedral ceiling ventilation in climate zone 3c

Andy Kahn | Posted in General Questions on

I have been remodeling my 90 year old bungalow in Oakland, Ca. It has never had ventilation in the attic and shows no signs of problems. I have converted about a quarter of the roof to a cathedral ceiling with 4″ of polyiso foil faced insulation carefully foamed into the rafter bays and with a 1 1/2″ space above the insulation. in the rest of the attic, I added a foot of blown in fiberglass after foaming/caulking any gaps I could find and reach. We are getting ready to redo the composition shingle roof but none of the ridge vents I have found are recommended for my 2 1/2 / 12 roof slope. I’m wondering if I have a greater chance of problems from blown in water at a ridge vent or from having an unvented attic and cathedral ceiling.

Any ideas on whether the ventilation is necessary or on better ways to provide it on the low slope roof?

Thanks for any ideas


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  1. GBA Editor
    Martin Holladay | | #1

    I don't recommend venting a low-slope roof. If you want to do it, however, recommendations can be found in the following article: Insulating Low-Slope Residential Roofs.

  2. Andy Kahn | | #2

    Thanks Martin,

    After rereading your article, I realized my roof doesn't quite meet your low slope definition, but it is still difficult to ventilate effectively. I'm glad it doesn't seem critical in my climate.


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