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Insulation in cathedral ceiling on addition

user-2423385 | Posted in Energy Efficiency and Durability on

I have an addition on a balloon framed 1880 house that I am working on. The addition is comprised of three parts: Main Addition (kitchen) which is 11×14 and is in the center with the roof at approx 16′ on peak. There are two smaller additions on each side of this kitchen and the roof slope is reduced significantly on each side. There was no insulation and no venting in this structure. I built out the main kitchen area with 2×10’s and plan on insulating with R’30 fiberglass and leaving a 1.5 inch area and drilling holes into the two additions to add venting. I will be completely rebuilding both roofs on the smaller additions as there was water damage at some point that destroyed some of the structure so I’ll be rebuilding the roof as well and adding the R30 to the new additions. My question is: when I rebuild the roof will it add to the R value if I put down foam sheathing on top of the roof with the venting in the “attic” space? If it doesn’t I’ll have to re-think my approach and maybe go with an unvented scenario and add the foam sheathing when I do the roof. If I go with the non-vented approach will there be any risk of sealing up the ceiling without having the foam sheathing installed right away. The kitchen won’t be in use and the building won’t be heated or cooled until the roof is complete? Thanks for your help

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  1. GBA Editor
    Martin Holladay | | #1

    You didn't mention your climate zone. It's worth mentioning that your proposed insulation R-value (R-30) won't meet minimum code requirements anywhere in the U.S. except in Climate Zone 1. (That's the southernmost tip of Florida.) Everywhere else, you'll need at least R-38 or R-49.

    I don't recommend using fiberglass batts to insulate a cathedral ceiling -- especially if you intend to install the batts without a ventilation baffle that separates the top of the batts from the ventilation cavity. More information here: How to Build an Insulated Cathedral Ceiling.

    Q. "When I rebuild the roof, will it add to the R-value if I put down foam sheathing on top of the roof with the venting in the attic space?"

    A. No. If you have a ventilation gap -- either a gap above the insulation in a vented cathedral ceiling, or a ventilated attic -- then it makes no sense to install rigid foam insulation above the roof sheathing. The ventilation gap makes the rigid foam useless.

    Q. "If I go with the non-vented approach, will there be any risk of sealing up the ceiling without having the foam sheathing installed right away?"

    A. Maybe. The answer depends on your climate zone. Again, I urge you to read my article, How to Build an Insulated Cathedral Ceiling.

  2. user-2423385 | | #2

    I'm in northeast Wisconsin. I've read your article and it's very informative. I want to stay away from the use of spray foam as it's drawbacks of extreme cost and a few people I've talked to have horror stories about it separating of mold issues (I'm sure construction and venting issues go along with these problems however). Thanks

  3. user-2423385 | | #3

    So going back to your article...I guess my question would be can I install one inch of rigid foam, taped with fiberglass R30 below with no venting and be fairly safe until I can install the foam on the top of the roof sheathing later this summer? And would I have to put R25 on the roof...that's 5 inches of foam..sounds like a lot of rigid foam on the roof? My combined total would be R60 and is there such thing as too much insulation. I have to tackle similiar issues with my main hip roof. There is much confusing information on this topic :) . Thanks again.

  4. GBA Editor
    Martin Holladay | | #4

    It sounds like you are in climate zone 6. If you want to create an unvented cathedral ceiling with a combination of rigid foam and fiberglass batts, you need at least R-25 of rigid foam above the fiberglass. That's what you need to prevent condensation or moisture accumulation problems in your roof assembly.

    The alternative approach would be to install spray polyurethane foam.

  5. user-2423385 | | #5

    Thanks for that clarification. I guess the expense is going to be high no matter which direction I go..I'll get a quote from somebody on spray foam before I proceed. The people who do it are pitifully lacking in website and advertising information in our area. I CAN add soffit channels and a ridge vent, but is a roof that has a low pitch changing to a steep pitch going to be ok for ventilation? Would it be ok to use Fiberglass in a vented system or a combination of rigid/fiberglass. Thanks again..just trying to get all my bases covered.

  6. GBA Editor
    Martin Holladay | | #6

    I don't think it's a good idea to ventilate a low-slope roof. Unvented roof assemblies make more sense for low-slope roofs.

  7. user-2423385 | | #7

    I guess my only real choices then are to go unvented in which case the roof will be sealed for a few months until I rebuild the roof? Or when I rebuild the roof ends to increase the pitch of the roof by running new rafters on the outsides to run up to meet the steep pitch rafters? Thanks!!

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