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Insulating scissors trusses — best practice or economics?

user-2435739 | Posted in Green Building Techniques on

I’m going to build a one story house with walk out basement in northern Idaho, east of Coeur d’Alene. The main roof will be scissor trusses with a 16″ heel. With an 8/12 pitch roof and 4/12 pitch ceiling. Due to the spans, there will probably be a 7′ to 8′ space at the peak between the roof and ceiling. The roof will be standing seam metal and the finish ceiling will first get a taped drywall ceiling and thane 1x T&G boards over that.
I am considering CC spray foam under the roof deck and an unvented ceiling. Another option mentioned to me was to forgo the high cost of the foam and use batts with blown in on top of that with a vented roof assembly.
For the latter, would I need a vapor barrier somewhere in the assembly and or would I need rigid insulation between the roof sheathing and the finish metal roof?
I’ve been in construction over 35 years and lately the more I read up on newer thoughts and ideas, the more I find out what I don’t know.
My last name is not Rockefeller so I am torn between what may be the best practice for the situation and economics. Any advice would be appreciated.

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  1. GBA Editor
    Martin Holladay | | #1

    If you choose to install a vented roof assembly with a vent channel under the roof sheathing, then you should not install any rigid foam insulation above the roof sheathing. The two approaches are incompatible.

    Building codes do not require a vapor barrier in roof or ceiling assemblies, but some building codes require a vapor retarder (a less stringent barrier). If your local code requires a vapor retarder, this requirement can be satisfied with vapor retarder paint.

    From a building science standpoint, the vapor retarder isn't very important. What is much more important is to make your ceiling assembly as airtight as possible. An air barrier is much more important than a vapor retarder.

    For more information on these questions, see:

    Do I Need a Vapor Retarder?

    Vapor Retarders and Vapor Barriers

    Forget Vapor Diffusion — Stop the Air Leaks!

  2. user-2435739 | | #2

    If, by using spray under the roof sheathing, making that the outside of the building envelope, does the size of the attic space under it matter (between the finish ceiling and the spray foam roof)?
    In what situation would you install rigid foam under a metal roof?
    When would you install sleepers on top of the foam to vent under the metal roof?

  3. GBA Editor
    Martin Holladay | | #3

    Q."If, by using spray under the roof sheathing, you make the sheathing the outside of the building envelope, does the size of the attic space under it matter (between the finish ceiling and the spray foam roof)?"

    A. No. But you need to install enough insulation under the roof sheathing to meet minimum code requirements (say, R-38 in many climate zones). If you don't want to install R-38 of spray foam, you need to install R-38 of something (for example, flash-and-batt insulation).

    Q. "In what situation would you install rigid foam under a metal roof?"

    A. Rigid foam can be installed on top of the roof sheathing if you want, no matter what kind of roofing you want to install. This approach works with metal roofing, or asphalt shingles, or many other types of roofing. The main advantage of this approach is that it stops thermal bridging through the rafters. For more information, see How to Build an Insulated Cathedral Ceiling.

    Q. "When would you install sleepers on top of the foam to vent under the metal roof?"

    A. Both vented and unvented roof assemblies can work fine. Vented assemblies are more traditional, and are especially useful in cold climates, where ice dams can be a problem. However, it's hard to make a vented approach work if the roof has a complicated geometry, with valleys, hips, skylights, and dormers.

    If you prefer a vented approach, all of your insulation needs to be below your ventilation channel. So if the roof includes rigid foam on top of the roof sheathing, the vent channel needs to go above the rigid foam, not underneath the roof sheathing.

  4. mackstann | | #4

    If there is no HVAC in the attic, then I don't see any reason to spend money on foam. Why not just air seal the attic floor and blow in cellulose?

  5. user-2435739 | | #5

    Since it will be a scissor truss roof, there is really no viable way to go with the "flash & batt", say an inch of foam to seal the underside of the sheathing and then put batts tight underneath that. As I understand it, if you combine insulations, they must be tight together with virtually no space in between.
    So it seems to be either foam the minimum R38 under the roof sheathing for an unvented scissor truss attic space.
    Or ... use batts and blown in on top with a vented scissor truss attic space and hope, over time, it doesn't settle to much being on a sloped ceiling. Also being sure the ceiling plane is completely sealed.
    With the right amount of insulation and a metal roof, I would assume that ice dams will not be an issue with scissor trusses.

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