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sjay2121 | Posted in Energy Efficiency and Durability on

We’re building a small commercial/industrial building in a zero lot line location, zone 4C, and are looking for a good cost effective system for insulating a Scissor truss roof in an un-vented application.
The two options are either 2″ of spray foam applied to the interior of the roof sheathing followed by either blown in fiberglass or cellulose to fill the entire cavity.
Second option is R10 or R15 XPS on the roof deck and also fill entire Scissor truss cavity below with blown in as above.
Is one of these a better choice? Other options?


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  1. GBA Editor
    Martin Holladay | | #1

    It takes an awful lot of blown-in insulation to fill the attic created by scissors trusses.

    Remember, too, that in Zone 4C, at least 20% of the total R-value of the roof insulation (in a flash-and-fill job) needs to come from the spray foam. The thicker the blown-in cellulose or fiberglass, the riskier this assembly becomes from a moisture perspective.

    For more information, see "Insulating Scissors Trusses."

  2. sjay2121 | | #2

    When planning to use the poly-iso insulation and we're noticing it comes with a fiberglass backing or a paper/felt backing. Is there a preference for this type of application? We're concerned about whether the paper/felt is durable or if it's subject to moisture during installation, although this is what the local roofing supplier has available


  3. charlie_sullivan | | #3

    I'm glad you are moving to polyiso. XPS inuslation is made with gases that are >1000x as bad for the climate as CO2. Polyiso is a much better choice.

    I don't have experience with paper/felt backing.

  4. walta100 | | #4

    Consider finding used foam it has become widely available at very good prices. Be sure to apply it in 2 layers with staggered joints.


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