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Insulating an existing cathedral open beam roof

Carla Culbertson | Posted in Green Building Techniques on

Hello all –
I have an open beam cathedral structure with 2×6 t and g ceiling over 4×8 rafters at 48″ oc over Glulams. There is an existing “over-roof” of some sort of spray foam insulation with plywood sheathing over that, but no regularly spaced sleepers. We want to install solar, and the solar company wants assurance there is adequate structure. We are remodeling, and need to repair portions of the roof anyway, so I am thinking we will tear off the existing over-roof, place new sleepers, and spray foam in between directly to 2×6 t and g ceiling below. My question is then how to close it up, and whether to vent or not. It is a low slope (2.5:12), located in Marin county has a torch down roof, which i think it best as it is a hill top location wit high winds at times. I cannot place much insulation (if any) over the sheathing bc the solar company wants direct connection to the plywood. Since it is a remodel, i am hoping we can get away without the required R5 over the sheathing as a result of this. Or we have to install stantions to plywood, then install rigid around them all? But bc it is south west facing, it can get VERY hot and I am concerned with no ventilation that we could get moisture on the underside of the sheathing with hot days and cool nights- especially because the spray foam will have to  be placed from above, and thus under and before placing the final roof sheathing – thus there will not be a true attachment of the spray foam to the underside of the sheathing creating an air barrier. This is feeling like a complicated scenario and any information on how best to do this would be great! thank you, carla

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Replies

  1. Steve Knapp CZ 3A Georgia | | #1

    Hi Carla,

    This article should be helpful (https://www.greenbuildingadvisor.com/article/how-to-build-an-insulated-cathedral-ceiling). Can you post some pictures of the roof assembly?

  2. Expert Member
    Akos | | #2

    For a flat roof, I would look at a ballasted solar solution. It would eliminate all the unnecessary holes through the roof and you can easily install them over rigid insulation.

    This makes the inevitable repair much easier as you can move array section to access the membrane underneath. It would also let you keep the existing structure as is and just install the new insulation on top with a new torchdown membrane.

    You'll have to get an engineer to look at the roof structure, but this would cost you less than ripping everything off and starting from scratch.

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