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Community and Q&A

Historic re-roof and cathedral ceiling

JPanzer | Posted in GBA Pro Help on

We live in an historic district and are about to replace our 100-year-old slate roof with synthetic slate. (EcoStar) The remaining original construction is 2”x5” rafters 24”o.c., with 1”x2” skip sheathing. Several years ago (before we owned the house) fiberglass insulation was installed between the rafters, with a plastic vapor barrier below. Below that, a portion of the attic was finished with GWB screwed to the rafters. 

I’m planning a two-phase project, with Phase One being to replace what’s above the sheathing and Phase Two addressing what’s below. The attached drawings show Phase One and what remains of the existing construction below.

Another issue of significant importance is that the historic review commission grudgingly approved shifting the roof plane up two inches – and is extremely unlikely to approve any.

I sketched up the attached plan for Phase One. Pure genius – or so I thought for ten minutes.  While realizing that this design would not come close to meeting my desired insulation value, which would be addressed as possible during Phase Two, I figured I had solved the vapor problem and the roof ventilation issue in a pretty slick manner, until I realized that this design resulted in naked fiberglass exposed to the airflow of the ventilation channels above. 

My first question is:  Would an encapsulated insulation (JM ComfortTherm or CertainTeed EasyTouch) address the “naked” fiberglass exposed to the ventilation channel issue? 

My second question is:  To achieve the desired R-value, would foil faced polyiso inside the rafters be the way to go. Or, could I use polyiso with OSB on the interior – with finished drywall.

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  1. Expert Member
    Dana Dorsett | | #1

    High R/inch foam board thermally bridged by rafters doesn't perform as well as the same R-value using a lower R/inch goods, due to the shorter thermal bridge through the rafter. But in the vented assembly it's fine. A layer of continuous polyiso on the interior side covering the rafter edges would be a much better performer, since it thermally breaks the rafters.

    High-density "cathedral ceiling" fiberglass, or rock wool batts don't suffer the same performance hits as low density goods when left "naked" to the vent channel. But half-inch foam board between the vent cavity and batts is a sufficient air barrier at any batt density. Despite having extremely low vapor permeance even foil-faced polyiso works fine in this application, since the higher permeance of the half-inch path through the wood rafter provides a sufficient drying path.

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