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Musings of an Energy Nerd

Filling Rafter Bays With Fluffy Insulation

New cathedral ceiling research confirms that cold-climate builders need to follow existing code requirements

A variety of insulation techniques were tested in the cathedral ceiling of the Massachusetts test hut. All of the rafter bays were unvented. [Photo credit: Building Science Corporation]

For years, building scientists have investigated whether fiberglass insulation or cellulose insulation can be used to insulate an unvented cathedral ceiling. In general, the answer has always been no, because this approach can lead to damp roof sheathing, especially in cold climates.

I’ve written at least four articles on this topic in recent years:

Here’s a summary of the information covered in the above four articles:

The two researchers who have investigated this issue in the greatest depth are Kohta Ueno and Joseph Lstiburek of Building Science Corporation in Westford, Massachusetts. In July 2016, I reported on one of their research projects: a field study near Chicago showing that installing a diffusion port at the top of unvented rafter bays insulated with fiberglass or cellulose failed to prevent damp sheathing (especially near the ridge). The data suggested that in Chicago, the best way that builders could stay out of trouble was to follow the conventional vented approach.

Following up on that Chicago study, Ueno and Lstiburek obtained funding to conduct a three-year study of similar roof assemblies in Massachusetts. In the Massachusetts study (unlike the Chicago study), some of the rafter bays included an interior “smart” retarder — that is, a vapor retarder with variable permeance. The researchers wanted to know whether the inclusion of a smart retarder (in addition to a diffusion port at the ridge) would be enough to keep these rafter bays safe. The Massachusetts research was funded by the U.S. Department of Energy’s Building America Program and a consortium of insulation manufacturers.

Ueno and Lstiburek recently published their findings. Interestingly, the data gathered in Massachusetts reinforces and confirms the GBA advice summarized in the bullet points above. Of course, the main reason that the building community can be confident of this advice is that…

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  1. Russell Miller | | #1

    EXCELLENT as always Martin.

  2. Malcolm Taylor | | #2

    As usual, Ueno has it right. An assembly that needs two attempts at getting the installation right, and relies on maintaining conditions perfectly, isn't going to do well over time. I frequently come across vented-roofs that suffer from moisture problems because the air-sealing, which may have been fine initially, has been compromised as the building ages. In structures built and occupied by humans that is to be expected, and building assemblies should be designed with enough resilience that they can still function when they suffer reasonable deterioration from the conditions they were originally built in.

    1. Tyler Keniston | | #5

      "I frequently come across vented-roofs that suffer from moisture problems because the air-sealing, which may have been fine initially, has been compromised as the building ages."

      Aright, that confused me Malcolm. Isn't venting supposed to reintroduce some of the resiliency un-vented lacks? Or are you simply suggesting that one should choose foam over sheathing if they want resilience?

      1. Malcolm Taylor | | #6


        I'm cautioning against building assemblies that are on the edge. If a roof or wall needs perfect air-sealing to avoid moisture damage, chances are at sometime it will fail. Assemblies should be robust enough that they can deal with more air-leakage than they initially experience. We should build-in a margin of error.

        In a way this goes against that has been the dominant advice here on GBA - that a well air-sealed roof makes ventilation less important. I think we should build as though the ventilation will become more important over time. Provide more ventilation than the code mandates as minimums, and use materials that can cope with more moisture if it becomes present.

  3. User avater
    Peter Engle | | #3

    Absolutely on-point. I just looked at another house today (CZ4) with unvented cathedral ceilings filled with FG insulation. They replaced the roof 5 years ago because of "leaks," but they didn't install chutes, air seals or vapor retarders. Surprise, surprise, the sheathing is rotten again. Is this concept really that hard?

  4. User avater
    Deleted | | #4


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