Insulating Cathedral Ceilings

Closed-cell foam is denser than open-cell material and dries stiff, making it more difficult than open-cell foam to clean off the face of the joists when it has to be sprayed in a shallow bay.

It can be done, however, and a horse curry comb seems to be the tool of choice for this task.

When sprayed in a cavity, closed-cell foam expands from 30 to 50 times its liquid volume, making it easy to apply in very narrow cavities—for example, in a complex vaulted ceiling (3) that couldn't be insulated with anything but spray foam.

When recessed lighting is unavoidable, best practice calls for installing a piece of foil-faced rigid foam above the fixture ...

... or isolating the fixture from the insulation by installing it in a metal box.

If rigid insulation is added above the deck, it needs to be in contact with the roof sheathing. If roof ventilation is added (as required for some roofing), it must be added above the insulation.

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