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Community and Q&A

Furring orientation over exterior mineral wool panels

user-1045979 | Posted in Green Building Techniques on

Our office (architectural) is considering using exterior mineral wool with furring strips to create a rain screen. The assembly would be like this, from interior to exterior:

5/8″ GWB painted w/ semi-permeable paint
2×6 studs w/ r-21 fiberglass batt insulation
1/2″ plywood sheathing
Henry Blueskin WRB (fully adhered housewrap w/ 29 perms) which would double as air-barrier
2″ Roxul Comfortboard IS
1x furring strips
vertical wood board siding

Normally the strapping/furring would be vertical but I was wondering if there’s a problem turning it horizontally to support the siding. My thinking is that trapping water wouldn’t be an issue because the water could drain behind the furring through the mineral wool — almost a secondary drainage cavity.

Does that work? Thanks,


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  1. Expert Member

    There is no problem at all from a drainage standpoint. Our code, which stipulates a continuous drainage plane at the sheathing, accepts what you are proposing. You do lose some drying capacity as you no longer have a continuous airspace from the venting at the base.

    What might or might not be problematic is the weight of the siding. Installing furring over Comfortboard is a more delicate task than over foam. It tends to compress and can leave an uneven nailing surface. I don't know if it would be more pronounced with horizontal furring, or whether it might be more prone to slumping with the weight of the siding.

    Using vertical furring with heavy siding you can mitigate this problem by attaching the top of the material to continuous blocking at the soffit of the same depth as the insulation. Perhaps in your situation it would be cheap insurance to call for blocking at the soffit the depth of the insulation and furring to which you would nail the top of the vertical siding boards.

  2. charlie_sullivan | | #2

    If you haven't seen it, you might read the discussion about reverse board and batten siding that is going on in parallel to this. That might be an option to consider, or perhaps it's just a different name for Malcolm's suggestion of vertical furring strips with your vertical siding.

  3. Expert Member

    I guess the cavity created by reverse board and batten siding is in a sense a rain screen, but both it and regular B&B are so poor at stopping rain from getting behind them it's hard to recommend either as a good choice as cladding in any climate where bulk water infiltration might be a problem.

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