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Community and Q&A

Mineral wool roof

Ben Woline | Posted in Energy Efficiency and Durability on

We are in the beginning design phase of our new house. The current concept is a 1,600 sqft ranch with a 4/12 pitch shed roof to allow clerestory windows along the south side. We are in the middle of Iowa, on the edge between climate zone 5 and 6.

The single roof plane would be a cathedral roof with both interior and exterior insulation. No valleys, no skylights, no dormers, no can lights, just a single roof plane with fascia mounted vents at the top and bottom.

I am curious if we can make an option work using mineral wool batts on the inside and mineral wool boards for the exterior insulation. We are considering the Roxul Comfortboard 110 under slab, exterior foundation wall, and exterior wall so by having it on the exterior of the roof as well we may be ordering enough that a truckload would be economical.

After reading several articles, including how to insulate a cathedral ceiling, the below concept appears to be a slight modification to a vented approach since the ventilation channel is above the exterior insulation. With that in mind, I think I have it correct that the following should work.

From the inside out:

– 5/8″ GWB
– 3/4″ Furring Strip Electrical Chase
– Intello Plus Smart Air Barrier and Vapor Retarder
– Rafter cavity filled with 11″ Mineral Wool Batts tight to sheathing. R46.
– Sheathing
– Delta Vent SA Vapor Permeable WRB which should also act as air barrier.
– (2) Layers of 3″ Mineral Wool Rigid Boards with seams offset. R24.
– 2×4 on flat Furring Strips running diagonally to allow air flow and to support metal roof. Otherwise 2 layers of 3/4″ furring stiprs first vertical for ventilation and 2nd horizontal to support roof. Likely attached with Heco Topix Therm Fasteners.
– Standing Seam Metal Roof

Please let me know if you think this detail would pose any concerns.

Ben

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Replies

  1. User avatar GBA Editor
    Martin Holladay | | #1

    Ben,
    While exterior mineral wool insulation is routinely used for flat (low-slope) roofs, especially in Europe, its use on steep roofs is unusual. You will be experimenting. John Straube, the Canadian building scientist, recently told me about his plans to build an outbuilding with exterior mineral wool on a sloped roof. He considered the project feasible.

    I would never use diagonal furring strips under a standing-seam metal roof. You are going to drive the roofer crazy, because it's hard to know where the furring strips are, making fastening frustrating. If you go ahead with your plan, I advise you to install two layers of furring strips.

    This roof won't be cheap, so you need to have a compelling reason to use these details.

  2. User avatar
    Stephen Sheehy | | #2

    It would be easier and cheaper to use trusses and 18" of cellulose above the ceiling and skip the exterior insulation altogether. From inside:
    Drywall
    1x4 strapping
    Intello,
    trusses
    cellulose (blown in from gable end access panel or through ceiling access panel
    vents
    sheathing
    roof underlayment (check with roof manufacturer or installer for what to use)
    roof, skipping the furring strips

    Maybe just use a high south wall if you want clerestory-type windows and vent the roof the usual way.

    Also, having the roof pitched down toward the north side makes any solar installation a problem and the clerestory windows on the south may create some overheating or require an awkward overhang.

  3. Christopher Riordan | | #3

    Martin, Ben and Stephen,

    I’d like to readdress Ben’s original question as I am trying to accomplish a similar roof sandwich. Massachusetts, zone 5, by the coast. I am in the planning stages for a 2 story 1,250 sf residence designed to passive house standards.

    Roof will have a middle section that is a cathedral ceiling, and the two ends, (for the HRV, etc.) will be unfinished attic space.

    Roof sandwich from inside out:
    GWB (at cathedral only)
    Intello or similar
    10:12 sloped roof - 2x10 Rafters @ 24” o.c. with cavity filled Roxul batt insulation (+/- 9” thickness)
    Plywood roof sheathing
    Air barrier membrane (Delta Vent S or similar)
    6” thickness Roxul comfort board, overlap seams
    1x4 wood strapping runs in line with the rafters at 24” o.c. and pins the Roxul to the rafters with Heco-Topix fasteners
    Plywood sheathing
    Roof underlayment
    Asphalt shingles

    The 1x4 strapping creates continuous vent channels from the eave to the ridge vent.

    The wall assembly is similar with 2x6 studs and Roxul insulation strapped to the sheathing to create a rain screen. In short, I (and I think is Ben is after this as well) am trying to employ the “same” wall assembly to the roof, employ simple construction practices, avoid polystyrene, use a single source type of insulation - in this case mineral wool, for the entire building.

    Precedents I’m looking at are Little Compton, RI passive house and echome.net Demo House.

    I would love to hear your opinions. Thank you.

  4. User avatar
    Stephen Sheehy | | #4

    Christopher-I'll give you the same advice I gave Ben. Use raised heel trusses, with the upper chord at 10/12 pitch and the lower cord at whatever cathedral ceiling pitch you want. We did something similar in our house, only the attic space (for us just a utility chase) was on the front (north) of the house as opposed to the two gable ends. Everything below the bottom truss chord is conditioned space. Above was filled with cellulose and above that a conventionally vented roof (ridge vents and soffit vents).

    I think this would be cheaper and less complicated to frame and insulate. All that Roxul (and long screws) is expensive and lots of builders are not familiar with attaching squishy mineral wool on top of the roof sheathing.

    So, from inside, I'd do the whole roof like this:
    GWB
    1x4 strapping
    Intello or similar
    trusses filled with 16-18" of cellulose
    vent channel at eaves running at the top chord to a point well above the insulation.
    Sheathing
    underlayment
    roof shingles.

    Then just frame the attic floors at 8' or whatever ceiling height you choose.

  5. User avatar GBA Editor
    Martin Holladay | | #5

    Christopher,
    1. I've never put mineral wool on the exterior side of a sloped roof. If you want "simple construction practices," as you tell us you do, I suggest that you use EPS on the exterior side of the roof sheathing. If you want an "outside-of-the-envelope, bleeding-edge experiment," go with exterior mineral wool on your roof.

    2. That said, I recently spoke to John Straube about his building project using mineral wool on the exterior side of a sloped roof assembly, and he said everything went fine, and that the roof didn't feel squishy once the second layer of plywood was installed. (Of course, John wasn't doing the actual installation -- he was directing a contractor.)

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