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Rockwool on roof sheathing

pbout | Posted in Green Building Techniques on

For a roof with exterior insulation on top of the sheathing, is it possible to use rockwool instead of rigid foam?

I was reading here about how mineral wool, as an an air permeable insulation, was not a good choice for inside of stem walls, as air can still contact the cold stem wall surface and condense. Would the same apply to roofs, avoid air permeable insulation on top of the deck, because condensation will still happen?

This is for a compact roof assembly in CZ3-coastal, zip 95060. Roof detail is attached.

Roxul sells products for roofs:, but based on what I read about condensing in the crawl space, I’m skeptical.

Links I am referencing:

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  1. GBA Editor
    Martin Holladay | | #1

    The brief answer is that you can substitute semi-rigid mineral wool panels for rigid foam in any roof assembly that requires rigid foam. The more extended answer is that while this approach is fairly common on flat commercial roofs, especially in Europe, it is almost never done on sloped roofs -- especially in North America. Because of this fact, you are unlikely to be able to locate a roofer who is willing to do it.

    That said, building scientists who have experimented with this technique (including John Straube) say that it works.

    The air permeance of the mineral wool isn't much of an issue. However, if you are building an unvented roof assembly, you still need to follow the usual ratio rules -- you want enough continuous exterior insulation (in this case, semi-rigid mineral wool panels) to keep the lowest level of roof sheathing above the dew point during the winter.

  2. Justin_DeSilva | | #2

    Thanks Martin,
    I share the same question as Phil. I’m planning an exterior rigid insulation solution for walls and pitched roofing, and ideally would like to use the same rigid insulation for both. Presently I’m considering Rockwool Comfortboard 80.
    You mention a critical aspect regarding the dew point of an invented roof. Does this mean I would require 2 layers of Comfortboard 80 for an effective roofing system?

  3. GBA Editor
    Martin Holladay | | #3

    The "ratio rules" for the ratio of the R-value of the above-the-sheathing insulation to the R-value of the entire roof assembly are explained in these two articles:

    "How to Install Rigid Foam On Top of Roof Sheathing"

    "Combining Exterior Rigid Foam With Fluffy Insulation"

    Ideally, you'll specify mineral wool insulation that has an R-value that meets the ratio rules for rigid foam, as explained in these articles. Note that these ratios are climate-dependant -- the colder the climate, the more insulation you need above the roof sheathing.

  4. LaciB | | #4


    Can I put a thin layer of rockwool above the roof deck just to stop the thermal bridging if I use 7" of ccsf under the deck in Zone 5?

  5. A2HomeOwner | | #5

    I will be retrofitting a cathedral ceiling built in the early 1990s, and I am considering polyiso exterior to the rafters. Would a semi-rigid mineral wool “board” be a good alternative?

    Current roof (typical early ‘90s): 2x10 rafters. R30 fiberglass in the cavities with a weak attempt venting from the soffit to the ridge vent (small vent chutes, not really connected to soffit vents). No vapor barrier on the interior. Asphalt shingles. Plenty of ice dam problems.

    The current plan: Metal roof (Decra metal “shingles”). We want to get the cathedral to about R60. If we leave the R30 fiberglass, we’d need about R24 exterior to meet the 41% exterior guideline. Our roofer and I are thinking polyiso - 6 inches given the ~4.5 R value at cold temperatures. We are in zone 5 (Michigan). So that would be R30 fiberglass, existing roof deck, polyiso, new roof deck, membrane, metal roof. We plan to seal the soffits and have no ridge vent for an unvented roof assembly.

    My question: Would something like Roxul comfortboard be a good alternative? We’d still have 6 inches (R24) exterior. Performance shouldn’t degrade over time for a rockwool product. I have not worked with comfortboard types of products before. How well would they work for an inclined (fairly steep pitch) roof, and could we get enough purchase for the metal roof?

    1. Expert Member
      Michael Maines | | #6

      Seth, I just did a project with a polyiso over-roof, and another one that I designed is being built now. I don't think you would want to install a metal roof over 6" or more of mineral wool. What we did on the second project, and I would recommend, is to frame a new roof above the existing, and infill with mineral wool batts. Dense-packed cellulose would have been my first choice for embodied carbon reasons, but we couldn't figure out the logistics, and mineral wool insulates a little better anyway. Mineral wool is not very susceptible to wind-washing so we just used rafters 1 1/2" deeper than the insulation, to allow a vented assembly.

      1. AlexPoi | | #8

        Did you use I joists to avoid thermal bridging? And if so did you use an wrb on top of the ijoist to protect the osb web? I'm planning on doing the same thing but not sure if I need to worry about protecting the joists from water. My wrb will be at the sheating level under the joists.

        1. Expert Member
          Michael Maines | | #9

          Alex, no, we ran 2x furring across the existing roof and then 2x6 rafters vertically as rafters, partly because the existing structure is a timber frame with purlins at the roof. But even with stick framing, the horizontal furring provides a thermal break. IMO thermal breaks are sometimes overrated; it's the R-value (or U-factor) of the entire assembly that really matters, and I-joists are expensive, though worth it in some applications.

          We created an airtight and water-resistant layer above the existing roof deck, then installed full roof sheathing above the new rafters, and used a synthetic underlayment below the new roof shingles.

          A vented metal roof with no sheathing directly below the roofing will create condensation due to night sky radiation, so if you go with a vented roof plan for water accumulation.

  6. user-6184358 | | #7

    They did it recently on This Old House - "The Jamestown Net-Zero House" series

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