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Insulating a gambrel roof

Tom Sloss | Posted in General Questions on

I have a newly constructed gambrel roof with dormers. The bottom pitch is 12/12, and the top pitch is 4/12, with a ridge vent installed. I live in zone 5, and the roof is 5/8″ plywood, titanium felt, ice shield on sides, bottom, and dormer roofs. What’s the best way to insulate the bottom pitch, and the attic floor (below the top pitch)? If I don’t vent the bottom and dormers, do I need to vent the end walls above the insulation to use with the ridge vent, or should I seal off the ridge vent, or go with no vents except for the ridge vent? I can use cellulose on the attic floor, and cellulose or spray foam (if needed) on the bottom pitch. I cannot vent from the dormers to the main roof, but I can vent to the dormer roof. But, the rafter joist spaces from the dormers to the peak of the main roof cannot be vented. I can extend the 2×6 joists to get more cellulose. If I use cellulose on both the lower roof and the attic floor, do I need any vapor control? Or, will air sealing the inner living space suffice?

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Replies

  1. GBA Editor
    Martin Holladay | | #1

    Tom,
    First of all, I'm going to provide some advice that is too late for you, but may be helpful to GBA readers. Here is the advice: You need a plan for how you will insulate your home before you begin framing. Once the house is framed and the roofing underlayment has been installed, it's very late in the process -- too late for many insulation options.

    For more information on this issue, see "Plan Ahead for Insulation."

    Yes, you can use cellulose to insulate your attic floor. For more information on this issue, see "How to Insulate an Attic Floor."

    Concerning the 12-in-12 roof (the lower section of your gambrel roof): You will be creating an insulated sloped roof assembly. To learn about all of your options for insulating this roof, see "How to Build an Insulated Cathedral Ceiling."

    If you have rafter bays that can be vented, then those rafter bays need to be insulated with an unvented approach. At this stage of construction, that leaves you with just one option for those rafter bays: You'll need to install an adequately thick layer of closed-cell spray foam against the underside of the roof sheathing.

    For the other rafter bays, you have two choices:

    1. You can ask your spray foam installer to insulate all the rafter bays the same way.

    2. You can use a vented approach for the rafter bays that can be vented, and spray foam for the rafter bays that can't be vented.

    1. Tom Sloss | | #2

      If I go with closed cell foam (2" min if I understand correctly for vapor barrier) sprayed on the 12/12 lower roof underside and overlap onto the upper roof, that would prevent any venting to the upper section as well. However, I do have a ridge vent installed. Should I close that off so there is no circulation at all? I am able to build vents from the soffit up the 12/12 roof and then to the 4/12 roof (with a "bend" at the pitch changes) to above the attic floor insulation on all the bays excluding the dormers. I can also vent the dormers to the peaks of the dormer roofs and/or to above the same attic insulation if that is better. These vents would be foam board cut to fit between the rafters and held off the roof deck by spacers of wood or foam board. I was thinking of 1 1/2" space. Then spray foam on top of that.

      Is there any problem with the cellulose on the attic floor (above the sealed envelope), with no heat and the open ridge vent? No concerns about the cellulose absorbing moisture?

      1. Expert Member
        Zephyr7 | | #3

        Your idea is similar what I’m having to do on my house. I have something of a cape cod, but spread out so the kneewalls are full height to a low (~7 foot) ceiling on the second floor. I have to vent the upper roof through a section of the first floor roof. I’m using cut and cobble polyiso to fill the rafter bays in those areas leaving a 2” vent channel above the polyiso. This was more economical than spray foam.

        There is no reason you couldn’t use a piece of thin 1/2” polyiso to define a vent channel and then spray foam under that. It’ll cost more but should work fine.

        Bill

        1. Tom Sloss | | #4

          I guess another question is whether spray foam needs to be applied under the vent channel? Would cellulose or open cell foam work in this area as well?

          1. Expert Member
            Zephyr7 | | #6

            You could use cellulose, but you’d need a way to support it. I’m not so sure about open cell spray foam. I’m inclined to think it would be ok due to the venting, but there have been issues with open cell spray foam in attics so I’d avoid it to be safe.

            Bill

      2. GBA Editor
        Martin Holladay | | #5

        Tom,
        You wrote, "2 inch minimum if I understand correctly for vapor barrier."

        The minimum thickness of the spray foam has nothing to do with creating a vapor barrier. It has to do with installing enough spray foam to keep the inner surface of the cured spray foam warm enough in winter to avoid condensation.

        This article explains what you need to know: "Flash-and-Batt Insulation."

        In Climate Zone 5, the building code requires that the spray foam layer have a minimum R-value (if you're talking about a roof assembly) of R-20. That means you'll need about 3 inches of closed-cell spray foam, not 2 inches, as a minimum thickness.

  2. Tom Sloss | | #7

    I was counting the 1" foam board used in the vent. If needed, I can go with 3". Another question I would ask is if it would be advisable to put 1-2" of foam board over the inside of the joists to help with the thermal bridging? Is it a concern to have a vent directly under the roof sheeting, 3" closed cell foam under the vent, then foam board under that? If the foam and foam board are not in contact with each other, is there any concern?

  3. GBA Editor
    Peter Yost | | #8

    My home is a 100-year+ old four-gabled gambrel roof - major pain the butt to renovate/drag kicking and screaming into high performance.

    But just wanted to let you know that in my retrofit, I did an unvented roof assembly on the steep lower pitch (18:12) and then kept the gable venting for the attic formed by the upper shallow pitch (4:12).

    You can find the project here: https://www.greenbuildingadvisor.com/green-homes/deep-energy-makeover-one-step-at-a-time

    Including a footing-to-ridge cross section.

    The only things I would do differently, with hindsight:

    1. Yank all of the original 1950s vintage fiberglass batts in the attic and then vacuum the entire attic before doing a complete air sealing package rather than the admittedly piecemeal air sealing I did from the attic and from below room by room.
    2. Increase the depth of rigid interior insulation on the steep slopes of the gambrel (interior rigid) to bring the R-value of the steep slope up closer to the R-value of the attic.

    Peter

    Peter

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