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How to properly insulate a tongue-and-groove ceiling?

Richard Callas | Posted in General Questions on

I live in climate zone 5B in northern California and I’m finishing a shop building that I plan to heat in the winter. My plan for the ceiling is to cover it with tongue and groove pine. The rafters are 2×8′, 2′ OC that are vented at the ridge and with soffits at the eaves. I’ve installed R21 Kraft faced insulation between the rafters (Kraft side facing the interior of the building).

To minimize heat loss through the tongue and groove, I’ve considered using either gypsum as an under layment or a semi-permeable membrane like Intello. To make a more informed decision, I hope you can answer the following questions:

1. Since I’m using Kraft faced insulation that I’ve stapled to the underside of the rafters, is it worth going to the extra work of installing gypsum or a membrane?

2. Are semi-permeable membranes suitable for ceiling applications and can they be placed against Kraft faced insulation?

3. Can you help me weigh the benefits of gypsum versus a semi-permeable membrane for the application I’ve proposed?

Thanks,

Richard

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Replies

  1. GBA Editor
    Martin Holladay | | #1

    Richard,
    1. You need an air barrier above your fiberglass batts; I suggest ventilation baffles, installed in an airtight manner.

    2. Your insulation R-value doesn't meet minimum code requirements.

    3. You need an air barrier on the underside of the insulation; either drywall or Intello membrane will work, as long as it is installed in an airtight manner. You might also consider installing a layer of rigid foam on the underside of the rafters, especially because the fiberglass batts don't provide the necessary R-value.

    4. More information here: How to Build an Insulated Cathedral Ceiling.

  2. Richard Callas | | #2

    Hi Martin,

    Thanks very much for your response and I have a couple of questions based on your earlier suggestions I hope you can answer.

    You suggested that, "You might also consider installing a layer of rigid foam on the underside of the rafters, especially because the fiberglass batts don't provide the necessary R-value". If I add rigid insulation to the underside of the rafters and tape the seams, would drywall or Intello membrane still be needed?

    If I have the following ceiling layers (listed in order from the inside to the roof sheathing): tongue and groove, Intello, rigid foam, fiberglass batt, could that cause problems if water vapor moves through the tongue and groove and the Intello only to be trapped by the rigid foam?

    Thanks again,

    Richard

  3. GBA Editor
    Martin Holladay | | #3

    Richard,
    Your choice of a material to use as your interior air barrier -- drywall, Intello, or rigid foam -- is less important than careful installation. Any of these products can work.

    Whatever material you choose, pay attention to airtightness during installation.

    There is some evidence that certain types of rigid foam can shrink, raising questions about the durability of taped seams. Rigid foam manufacturers often claim that modern foams won't shrink as much as older formulations of foam. It's up to you to decide whether these manufacturers' claims are reassuring enough for you to use taped rigid foam as an air barrier.

    Moisture won't become "trapped" between Intello and rigid foam. The interior side of your rigid foam is supposed to be warm and humid during the winter; the exterior side of your rigid foam is supposed to be relatively cold and dry during the winter. These conditions are different, and they are separated by the rigid foam. One side is supposed to be more humid than the other. Nothing is "trapped."

  4. Richard Callas | | #4

    Martin,

    I have one last question about an internal air barrier. I just learned from a contractor employed at my local building supply that starting January 1st, the California Building Code will require the installation of a vapor barrier of at least 4 mil on the room side of ceilings. He also said that Polyethylene was the material to use.

    I've read online of problems with installing plastic sheeting under tongue and groove, but perhaps that isn't a real concern because of insulation behind the sheeting. If Polyethylene sheeting will not cause a problem for that application, it doesn't seem necessary to spend the extra money for a semi-permeable membrane like Intello. Have I missed something?

    Thanks,

    Richard

  5. GBA Editor
    Martin Holladay | | #5

    Richard,
    Clearly, if you plan to install a layer of interior rigid foam -- a layer with very low permeance that prevents drying to the interior -- there is no need to invest in a smart vapor retarder with variable vapor permeance.

    I am very surprised at your report that the state of California will mandate the use of 4-mil interior polyethylene. Frankly, I think the rumor is untrue.

  6. Richard Callas | | #6

    I called my local County Building Department and spoke with the lead supervisor to see if he could confirm the rumor that a new requirement for vapor barriers in ceilings would be required beginning this year.

    He said that after January 1, 2014, installation of a vapor barrier (4-mil) against the underside of rafters and immediately under finished ceilings is required by the California Energy Code for heated structures using fiberglass batts or blown insulation. Apparently if the insulation used is considered impermeable (e.g., rigid foam) no vapor barrier is required.

    Thanks again for your advice.

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