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How can I fix a poorly ventilated cathedral ceiling insulated with kraft-faced insulation?

I am in the planning stages of renovating a home that has cathedral ceilings with 2 x 8 rafters and R-19 kraft-faced insulation. Ventilation is poor at best. There is frost on the underside of the roof sheathing.

I would like to turn it into a hot roof by sealing up any vents and either replacing the the insulation with closed-cell foam or adding 2 two-inch layers of polyiso on top of the roof deck (leaving the existing insulation in place). If I choose the latter, will I be trapping moisture with the kraft facing?

Asked by Frank Ball
Posted Jan 22, 2013 9:20 PM ET
Edited Jan 23, 2013 2:56 PM ET

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Frank,
Your proposal to install 4 inches of polyisocyanurate on top of your roof sheathing will work in climate zone 6 or anywhere warmer.

In colder climates (climate zones 7 and 8), the rigid foam needs to be thicker, in order to keep the temperature of the roof sheathing above the dew point during the winter.

The kraft facing on the fiberglass batts is optional. It won't do much to help your roof assembly, nor will its presence cause any problems.

For more information, see How to Build an Insulated Cathedral Ceiling

Answered by Martin Holladay
Posted Jan 23, 2013 6:38 AM ET

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