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Community and Q&A

Air gap between batts and ceiling deck

Doelman | Posted in Building Code Questions on

I’m building a house with an unvented cathedral ceiling in zone 3.  On top we have, in order from out to in, a standing seam metal roof, peel and stick membrane, plywood, 1.5” polyiso, plywood, and then 16” i joists.  I’m planning on using R30 rockwool batts in the joist bays.  Should the batts be in contact with the roof decking or should I leave an air gap?  Building code is IRC 2015.


  1. kbentley57 | | #1

    If the ceiling is unvented you'll want the insulation to contact all surfaces with no void space.

    1. Doelman | | #2

      The i joists are 16” deep, I can’t fill the entire thing

      1. jollygreenshortguy | | #3

        In IRC2021 section R806.5 is the relevant section. It doesn't specifically require that the batt insulation be in contact with the underside of the roof sheathing. But if you look at the various configurations it covers, it seems to lean towards having full contact between all the layers, rigid, batts and sheathing.
        Check out R806. and R806.
        I've heard other sources say that is best practice as well. I believe the idea is to avoid a gap so that there is no chance of convective air currents happening in that gap.
        Anyhow, the safest thing would be to fit the batts tight up to the underside of the sheathing. Simpson StrongTie makes a product called "IS", insulation supports. They're simply wires that can be pushed up against the batts to hold them in place.

  2. Expert Member


    He is what Martin wrote in this recent blog:

    "Keeping the insulation in contact with the underside of the roof sheathing

    Building scientists recommend, and building codes require, that the insulation installed between the rafters must be in direct contact with the underside of the roof sheathing. An air gap between the sheathing and the fluffy insulation (or the spray foam insulation) is not permitted.

    Complying with this requirement can be tricky: for example, if you are using fiberglass batts, the batts have a tendency to sag away from the roof sheathing, creating an unacceptable gap between the sheathing and the fiberglass. That’s why many builders find it easiest to use open-cell spray foam under the roof sheathing rather than fiberglass, mineral wool, or cellulose.

    If you’re tempted to just buy thicker-than-necessary fiberglass batts, hoping that the thick batts do a better job of staying in contact with the roof sheathing, remember that thicker fluffy insulation can mess up your ratio calculation, resulting in the need for thicker rigid foam in order to keep the sheathing safe from winter condensation."

    1. Doelman | | #7

      What exactly is the draw back if your batts aren’t tight against the roof decking? Code requires R5 rigid insulation in my zone to prevent condensation, I have almost double that, so it seems to me condensation shouldn’t be an issue, air gap under the roof decking or not.

      Preventing ice dams is the only other reason I can think of, but that isn’t an issue in zone 3.

      1. Expert Member
        Michael Maines | | #10

        The most likely problem is at the roof perimeter, where a gap between the sheathing and insulation would allow a thermal break at the exterior wall, negating the effect of the rooftop insulation.

        1. Doelman | | #11

          Couldn't you insure there isn't a gap when you insulate the rim joist bay? I could even double up the insulation in those bays just to make sure it's all tight.

          1. Expert Member
            Michael Maines | | #12

            That would work. But it's often overlooked. Here's a photo of the floor of a bathroom I'm renovating. It's on the second floor, built around 1980. The fiberglass is about 2" below the subfloor and just out of sight is the rim joist. It wasn't the cause of the rot you see but the cold air surely didn't help!

  3. Expert Member
    Akos | | #5

    I've run into this with I-joist and there is no easy solution. The insulation wires don't work as they are meant for dimensional lumber. There are insulation pins you can get, these are adhered to the roof deck for holding the insulation tight.

    The simplest is probably to get faced batts and staple the facer to the I-joist OSB web. Make sure to get batts for metal studs as they are wider and will fit snug.

    1. Doelman | | #6

      My i joist are 20” OC so I can trim down normal 24” batts. Why don’t the insulation wires work? You can attach them to the i joist webs?

      1. jollygreenshortguy | | #8

        I'd suggest you give Simpson StrongTie a call and ask them about their "IS" insulation supports. I have a hunch they'd work for you.

        1. Doelman | | #9

          Yeah I think those would work!

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