Roof insulation
I am rehabbing a house located in zone 5, Pennsylvania. The room I am working is a kitchen with a cathedral ceiling. The room is completely gutted. The rafters are 2 x 10 on 24 o.c. The roof pitch is about 15 degrees.
I am planning to install Accuvent corrugated sheets for the roof venting and air blocking. Additionally, I am thinking of using 4 inch polyisocyanurate and 3 ½ inches of mineral wool in each rafter bay. I plan would include foaming the polyiso in place for air seal and filling the remainder of the bay cavities with rock wool. The entire ceiling would be covered with ½-inch sheetrock that would be painted with latex.
My questions are:
• Do you foresee any problems with the aforesaid configuration?
• Locally I can find only fiberglass-backed polyisocyanurate in 4 inch thickness – do you foresee any issue with the fiberglass-faced polyiso versus the foil-faced polyiso?
• Does the aforementioned application require a vapor barrier or vapor retarder?
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Replies
If you are putting up rigid panel insulation, and foaming it in place, why not have it serve as the vent channel? The you can do away with the AccuVent expense and labor. I am doing much the same in my cathedral ceiling, although I am using XPS. I am cutting some 2" spacers, then press fitting the XPS between the rafters and foaming it in place, and air sealing it. You need to select appropriate thickness of rigid insultion, making sure it is thick enough so that the boundary stays above the dew point. At 4", you should be OK in CZ5.
Anthony,
You are proposing a variation of the "cut-and-cobble" method of insulating. (For more on the cut-and-cobble approach, see Cut-and-Cobble Insulation.)
Your decision to include a ventilation channel makes sense; it reduces the risk associated with the cut-and-cobble approach.
Like Kent, I think that you can skip the use of the AccuVent baffles by installing "sticks" in the corners of your rafter bays as spacers, and that you can install thinner rigid foam if you want. For more information on methods used to insulate cathedral ceilings, see How to Build an Insulated Cathedral Ceiling.
Q. "Locally I can find only fiberglass-backed polyisocyanurate in 4-inch thickness – do you foresee any issue with the fiberglass-faced polyiso versus the foil-faced polyiso?"
A. No.
Q. "Does the aforementioned application require a vapor barrier or vapor retarder?"
A. In some climate zones, building codes require an interior vapor retarder. This requirement can be satisfied with vapor-retarder paint. You should avoid the use of interior polyethylene. In any case, pay strict attention to airtigtness when installing your ceiling. For more information on this issue, see Do I Need a Vapor Retarder?