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Community and Q&A

Flashing a shed dormer with mineral wool exterior insulation

blueskye | Posted in Green Building Techniques on

We are using exterior mineral wool insulation, but can’t find a detail for where the side wall of a shed dormer meets the roof.

1. Where does the step flashing go – taped to the Zip sheathing or outside and under the mineral wool?

2. If the flashing goes on the Zip sheathing, should the bottom of the mineral wool be raised slightly from the roof so water going down the flashing won’t wick in from the bottom? I know mineral wool is supposed to be hygrophobic…


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  1. Expert Member

    If you are using Zip sheathing and no additional building paper or house wrap, the sheathing is your WRB, and the flashing should be integrated with it. Like the siding, the mineral wool should be held up off the roofing. I would be tempted to use an L flashing to cover the bottom of the insulation with a 6 degree slope to drain any water that gets in.

  2. GBA Editor
    Martin Holladay | | #2

    Malcolm gave you good advice. Any wall flashing, including step flashing, needs to be integrated with your wall's water-resistive barrier (WRB). If your wall has housewrap or asphalt felt as a WRB, then the flashing needs to be integrated with that layer.

    If you have decided to use Zip sheathing as your WRB, then the flashing needs to be integrated with the Zip sheathing. In that case, you will be depending on the Zip System tape to direct water that flows down the WRB over the top lip of the step flashing. Some builders are suspicious of any system that depends on tape rather than an overlap for long-term performance; make your own decision on that point.

    It's a good idea to keep the mineral wool an inch or two above the roof sheathing. Supplementary flashing to protect the bottom of the mineral wool from incidental water and insect entry isn't a bad idea.

    Keep buildability in mid: the step flashing will need to be installed, and the roofing will need to be completed, before you begin installing the mineral wool on your walls.

  3. Expert Member

    Martin brings up a good point about build-ability. There is a very good chance you are going to need to replace the roof during the lifespan of the house. Exterior insulation means that you will not have access to the step-flashing without removing both the flashing, furring and siding. It might be a good idea to hold the siding and insulation up a bit higher and slip the step flashing under a strip of flat stock so that it can be replaced or used again when you re-roof.

  4. blueskye | | #4

    Thank you, Malcolm and Martin!
    The buildability point is an excellent one. Considering the roof replacement probability, I'm wondering if a house wrap WRB outside the mineral wool might not be the way to go for the dormer walls. That would put the flashing back outside the exterior insulation with adding a huge amount of work or material.

  5. Expert Member

    That's what I'd do. Integrating your flashing with house wrap, which relies on lapping not tape, is a lot easier to get right, and you can now run your exterior insulation right down to the roof deck so there is no gap.
    Aslong as your roofer nails the step flashing to the roof, not wall, the replacement is fairly easy.

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