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Community and Q&A

Finishing insulation on unvented sloped roof with mineral wool over polystyrene

Erich Riesenberg | Posted in General Questions on

I have an unvented, sloped roof. I cut and cobbled 5 inches of expanded polystyrene into roof rafters, and 3 inches into attic wall studs. Then placed 2-6 inches of expanded polystyrene across the rafters and studs, to create a final air barrier and address thermal bridging.

I plan to cover the expanded polystyrene with 1 or 2 layers of Comfortboard IS mineral wool. Each layer is 2 inches thick. Expanded polystyrene seems ideal except for flammability, so I want to address that with mineral wool.

My question is about the safety of the rafters, studs and sheathing, from condensation or other issues.

Is it safer to 1) place 2 layers of Comfortboard over the 1 layer (or more) of expanded polystyrene or 2) place 1 layer of Comfortboard over 2 layers (or more) of expanded polystyrene? Or are both equally safe? My guess is the main factor is interior humidity, which should be normal, with snowy winters and humid summers.

Thank you.

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  1. GBA Editor
    Martin Holladay | | #1

    First of all, we need to clarify what you mean by "over" and "under." When you write that you intend to install mineral wool insulation "over" the polystyrene on your attic ceiling, I think you mean "under" (that is, on the interior side of the polystyrene, not on the exterior side of the roof sheathing).

    If you implement your plan, the main risk of moisture problems arises from your decision to use the cut-and-cobble method to insulate an unvented cathedral ceiling rather than from the use of mineral wool. There have been reports of moisture damage in unvented cathedral ceilings insulated with cut-and-cobble insulation. For more information on this issue, see Cut-and-Cobble Insulation.

  2. Erich Riesenberg | | #2

    I have read that and I suppose every related article numerous times. I am not worried about using expanded polystyrene, it seems the risk is from people who are sloppy. Each layer is well sealed with caulk and Great Stuff Pro, and some has been visible for two years now with no damage and no gaps due to expansion and contraction.

    With a layer or 2 or 3 covering the rafters, it seems unlikely that vapor will reach the rafters, even if a precise answer is apparently impossible.

    Thank you.

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