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Community and Q&A

Hybrid insulation approach for cathedral ceiling

Mark Oatney | Posted in Green Products and Materials on

I have an open-beam cathedral ceiling and live on the northern california coast (lots of rain, high winter humidity).  I’m on the cusp between 3c and 4c climate zones.  Mold and interior condensation are constant concerns.

I’m planning on two 2″ layers of staggered and taped polyiso, 1″ of toprock plus above that, 1/2″ nailbase above that, and asphalt shingles.  I’d put tyvek both under the foam and above the nailbase (and tape the sheathing under the foam).

– do i secure each staggered layer of polyiso separately, or all at once?
– should I glue the toprock onto the polyiso for better stability?  I keep reading rockwool is for low slope roofs only….. 
– any thoughts on how much safer the rockwool makes the polyiso with wildfires?
– does the above seem a helpful recipe for my condensation concerns? (we have fishtanks, four people, no dehumidifier, wood heat, single pane windows)


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  1. GBA Editor
    Brian Pontolilo | | #1

    Hi Mark,

    If this were my project, I'd probably consult an engineer for a proper fastener/spacing schedule. I believe that Rockwool offers a fastening schedule and specifies fasteners and penetration for fasteners, but the instructions also say that when strapping will be installed, fastening can be minimal and the strapping fastening can substitute for their instructions. I would imagine this is the same for a layer of sheathing installed over the insulation. However, you have three layers of insulation, including two different types, so again, a professional should probably specify fastening for this assembly.

    If your condensation concern is cold sheathing as a condensing surface, then yes, this work should help with that. However, you may also need to mechanically control humidity as you begin to tighten up your home. Perhaps with a properly sized and installed A/C, perhaps with a whole house dehumidifier. The roof work is one step towards tightening up and insulating the home. The next step is proper HVAC.

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