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Should the rafter bay be sealed if there is a skylight installed in it?

Hi,
I’d like your recommendation on sealing rafter bays of a currently vented cathedral ceiling.

I’m doing a remodel of a small room (20 X 20 feet); the roof/ ceiling were built as a vented roof assembly complete with a ridge and soffit vents.

We’ve have complete access to the underside of the roof deck. We plan to install air channels for venting , 4 inches of felt faced polyisocyanurate sheets foamed in place and 3-½ inches of mineral wool before covering the assembly with sheet rock. The problem is there are two skylights (they have to stay); how do you insulate rafter bays that contain the skylights? Obviously they will block air flow through the bay. Specifically, should we consider that rafter bay as non-vented since it is technically it is, and insulate the bay accordingly? Should we eliminate the air channel above and below the skylight and place either polyiso or EPS directly on the underside of the decking for the entire length to the rafter bay? If the bay were to be treated as a non-vented space would it suffice to seal that portion of the ridge vent with a piece of stiff vinyl and butyl caulk?

We are in Pennsylvania, zone 5A. They are 2 skylights each one bay wide. The rafters are 2x10s.

Thank you in advance for your time.

Asked by Anthony Jackson
Posted Tue, 07/15/2014 - 12:48
Edited Tue, 07/15/2014 - 13:34

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Anthony,
I provided an overview of methods for insulating this type of roof assembly in an article called How to Build an Insulated Cathedral Ceiling.

In that article, I wrote, "A vented cathedral ceiling only makes sense if the geometry of your roof is simple. You need a straight shot from the soffits to the ridge. That’s relatively easy on a gable roof without any dormers or skylights, but if the geometry of your roof is complicated — with features like hips, valleys, and dormers — it’s impossible to assure air flow through all of your rafter bays."

Your roof is not a good candidate for the vented roof approach. I don't recommend trying to mix a vented approach with an unvented approach; it complicates details at the soffits and ridge, and introduces a variety of complications.

I suggest that you create an unvented roof assembly by installing rigid foam above the roof sheathing, or spray polyurethane foam under the roof sheathing. More details are provided in the article.

Answered by Martin Holladay, GBA Advisor
Posted Tue, 07/15/2014 - 13:33
Edited Tue, 07/15/2014 - 13:34.

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