Upgrading Insulation in Cathedral Ceiling
I have researched and read through the various articles here and still am not confident of what to do with my cathedral ceiling upgrade and would appreciate some input or direction.
I have a 1991 home with vaulted, ie cathedral ceilings constructed as follows from the bottom up:
– tongue and groove wood ceiling overlaid with clear plastic sheeting (10 mil or so), no can lights and minimal electrical and other penetrations;
– on top of the moisture barrier is a 12″ thick vented cavity with fiberglass bats;
– then plywood sheathing, tar paper, asphalt shingles; roughly 2000 sq feet of 4:12 roofing, full vented in soffit and ridge vent.
Hail damage is giving me the opportunity to upgrade the insulation in the re-roofing process and I know there are critical issues with moisture barrier and many options on the insulation. I think I’ve considered all the main options with the following:
– SIPS sheathed on one side, would need as-built drawings and this is probably the most expensive and slowest option;
– rock wool with 2×4 furring to support new additional layer of roof sheathing,
– rigid foam board, with or without 2×4 furring, seems EPS (expanded polystyrene) is best for cold climate (Bozeman, Montana, zone 5/6),
– adding or replacing insulation in existing cavity, could cut horizontal access slots through existing sheathing and blow (tight pack) fiberglass or other blow in insulation on top of existing fiberglass bats; or remove all existing sheathing and fiberglass, spray 1-2″ of foam insulation, re-add fiberglass bats, and new sheathing to create a roof that doesn’t need venting due to spray foam barrier.
Tight packing the existing cavity could effectively terminate venting action which seems to not be needed since there is a vapor barrier above the ceiling, but could be risky due to possibility of moisture entering cavity from above or below over time.
Simply adding insulation above existing roof sheathing seems to offer marginal gains since the existing cavity would still be ventilated. This would be the first phase of a deep energy retrofit, with walls and windows to be upgraded next. This is a Lindal Cedar Home and is built to 1990 code but certainly offers plenty of room for energy efficiency improvement. We absolutely need roof melting cables to prevent winter ice damming and roof leaks. I know there is no single or simple answer to this complicated challenge.
Any input would be greatly appreciated:)
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