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Community and Q&A

Upgrading Insulation in Cathedral Ceiling

PaulH7 | Posted in Energy Efficiency and Durability on

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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  1. Expert Member
    Peter Engle | | #1

    How much insulation is in the 12" cavity now? And, do you have any idea how consistent it is? Where are you located? If the vapor barrier is well done, going to an unvented assembly can work. Some installers have had luck installing dense pack cellulose over top of the batts. This compresses the batts and fills all of the uneven spaces with cellulose. 12" of mixed FG and dense-pack would be a pretty good R-value.

  2. PaulH7 | | #2

    I am in Bozeman, MT.
    There is roughly 12" of fiberglass in that cavity now, and the overall building quality of this kit home is fairly good but since there is no way to see that cavity I won't know until I open it up from the top. I think the vapor barrier is well installed but still not confident that it would prevent moisture from entering the cavity after it was dense packed with blown in insulation. Maybe a layer of spray foam would be the only way to insure an adequate vapor barrier to allow for a non-vented cavity.

  3. the74impala | | #3

    Have you seen the vapor barrier? How well sealed is it really? If you have a well sealed 12" of insulation, more likely an r-38, is that not enough for you to expect to avoid ice dams? I think you have some serious leaks. Or missing Batts of insulation.

  4. PaulH7 | | #4

    Sorry for the delay, I keep expecting the email notification to work:)

    It is quite possible that the bats are not fit well or are missing in areas. I have seen the vapor barrier in places like bathroom fan penetrations, it is clear plastic and about 10 mil. There is also heat loss due to thermal bridging of the roof joists.

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