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Rigid foam over stone wool?

RaWe | Posted in General Questions on


I am re-doing my attic and have removed the old fiberglass blown in insulation. I am replacing it with Rockwool and would also like to add rigid foam on top of the joists. Will this create any problems?


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  1. rhl_ | | #1

    Is this on the floor or in the joist bays? You need to worry about moisture control. In either location if the fluffy stuff goes inside the bay, and then the rigid stuff goes over the studs, that will be a problem.

    In an idealized world, you can reverse the order, putting the rigid stuff inside the bays first, then backfilling the rest with fluffy stuff. This is called cut and cobble [+ batt]. The trouble with this approach is that it can be hard to get a good air seal. Even it it works out at install time, the house can shift, and the air seal can break. (this is one reason why exterior insulation is preferred).

    If this is on the attic floor, consider dense pack cellulose.

  2. RaWe | | #2

    Thank you. I was afraid the get this answer. Will stick with just stone wool.

  3. GBA Editor
    Martin Holladay | | #3

    I agree with Ryan. Just add more mineral wool if you want more insulation.

  4. Expert Member
    Peter Yost | | #4

    Does your ceiling/attic insulation plan have an air barrier? It needs one.

    I don't understand why installing rigid insulation to the exterior of the cavity insulation would pose a problem; we install continuous rigid insulation on the exterior of wall assemblies all the time, to warm the air-permeable insulation to the interior and also if we seal those exterior insulation panels, we achieve an air barrier. So long as the "exterior" (topside) rigid insulation has sufficient R-value based on the R-value of the cavity insulation for the climate, should work fine.

    I am not disagreeing with the economics of just adding more loose fill insulation. With an air barrier, that solution makes the most sense but I want to make sure I understand the problem the proposed rigid insulation solution poses.


    1. GBA Editor
      Martin Holladay | | #5

      If RaWe is talking about horizontal rigid foam above mineral wool batts on an attic floor, the two main problems I'm thinking of are: (1) it will be impossible for maintenance workers or electricians to walk on the rigid foam without damaging it, and (2) taping the seams of the rigid foam will be a real pain.

      1. RaWe | | #6

        Yes, that's why I stick with a double layer of Rockwool.
        Thanks to all for your inputs.

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