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Community and Q&A

Insulating a cathedral ceiling

Richard Uhler | Posted in Energy Efficiency and Durability on

What is the most economical way to insulate an 8/12 cathedral with 2×6 rafters, 1×4 “sheathing” with 4″ gaps, no barrier…you can see the underside of the metal roofing? The roof is in good shape so I don’t want to remove it. I can fir the rafters and vent if necessary but would rather not vent unless necessary. I plan on installing a wood celing without drywall. The house is in Charlottesville Va which is mixed humid. Sorry if you already answered this but I couldnt get back to the other place I asked it.

Thanks, Richard

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  1. GBA Editor
    Martin Holladay | | #1

    You have two choices: either build a vented assembly, or an unvented assembly.

    If you want to build a vented assembly, you need a roof without valleys, hips, dormers, or skylights. If you have that type of roof, venting is possible. Here are the steps:

    1. Install soffit vents and a ridge vent.

    2. Install 1"x1" so-called "sticks" in the corners of each rafter bays, up against the sheathing, followed by site-built ventilation baffles. (For more information on these sticks and the type of ventilation baffles I'm recommending, see Site-Built Ventilation Baffles for Roofs).

    3. Insulate your roof according to the recommendations for vented roof assemblies in this article: How to Build an Insulated Cathedral Ceiling.

    If you want to build an unvented assembly, only one type of insulation will work: closed-cell spray polyurethane foam. If you go this route, you need to first install some type of physical barrier in each rafter bay to prevent the spray foam from sticking to the underside of the metal roofing. I would use cardboard.

  2. Richard Uhler | | #2

    How much CCF would be needed if I do a hybrid with either cellulose or Roxul under? Looking for about an R40 range. Can I do the cardboard under the 1x4's and will the gaps between them be an issue?

  3. GBA Editor
    Martin Holladay | | #3

    The gaps between the 1x4s are not an issue if you install closed-cell spray foam, as long as you insert tight cardboard or OSB between the rafters to prevent the spray foam from contacting the metal roofing.

    Virginia is in Climate Zone 4. According to the 2012 International Residential Code, the minimum R-value for roof assemblies in Zone 4 is R-49, not R-40.

    Here is a link to an article that explains the needed ratio between the foam insulation layer and the fluffy insulation layer for the type of assembly you are describing: Combining Exterior Rigid Foam With Fluffy Insulation. (Although the article discusses rigid foam + fluffy insulation, not closed-cell foam + fluffy insulation, the relevant ratios are the same in either case.)

    In your climate zone, if you install a total of R-49, the closed-cell spray foam layer must have a minimum R-value of R-15. That's about 2.5 inches of spray foam.

    If you install less than the code minimum insulation, and stop at R-40, you need a minimum of R-12.5 in the spray foam layer -- call it 2 inches of spray foam.

  4. Richard Uhler | | #4

    Could I cut and cobble 2" of polyiso against 1x4s and then 1" of CCF filleted on edges at rafters to increase seal with balance of fluffy stuff under? Thanks for all you do for the building industry...I've been reading your blogs for years!!!

  5. Richard Uhler | | #5

    Do I need vapor barrier between fluffy stuff and wood ceiling?

  6. GBA Editor
    Martin Holladay | | #6

    Using the cut-and-cobble method for an unvented cathedral ceiling isn't recommended. It's risky. To learn more, and to read about cut-and-cobble failures, see Cut-and-Cobble Insulation.

    On the interior side of the fluffy insulation, you don't need a vapor barrier. However, you do need an air barrier. The easiest, cheapest, more durable air barrier to use when installing a board ceiling is drywall. First install the taped drywall. Then install the boards.

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