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Cathedral ceiling insulation with 4 1/2 inch thick rigid foam and ridge vent in southern Calif

Robert E | Posted in Energy Efficiency and Durability on

This is in So Calif, average daily temp range 50 deg F to 85 deg F. Coastal moisture (less than 1 mile from ocean)

I have a cathedral ceiling that is 8 ft x 20 ft , symmetrical about the peak and the peak is about 4 ft high

The rafters are 2×8

Putting 4.5 thick rigid foam in between the rafters and wondering about all the debate I read about airflow in cathedral ceilings

I could fit the foam tight on sides, maybe spaced away from roof interior by 1 inch for air gap and then NOT block off the ridge vent by stopping short by an inch or two


Fit it tight on sides, fit it tight against roof interior, and tight against peak corner, essentially preventing the ridge vent from venting

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  1. GBA Editor
    Brian Pontolilo | | #1

    Hi Robert.

    Cut-and-cobble is not a reliable method for unvented cathedral ceilings. If you want to use rigid foam, use it (in coordination with soffit and ridge vent) to create a ventilation channel. Make sure it is air sealed. And then you can fill the rest of the cavities with fibrous insulation of any type. Unvented options would be to use rigid foam above the sheathing and fibrous insulation in the rafter bays or closed-cell spray foam on the underside of the sheathing plus fibrous insulation filling out the rafter bays. For a more detailed look at these options, read this: How to Build an Insulated Cathedral Ceiling.

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