Shingles over rainscreen over Roxul
pbout
| Posted in Green Building Techniques on
Any tips on installing shingles (cedar or fiber-cement) over a rain screen, which is installed over 1.5″ of exterior mineral wool? I was thinking vertical furring strips fastened to the studs, then horizontal furring strips on top of those to provide a shingle nail base. Sounds like a lot of extra work but the only other way I’ve seen (in one of Martin’s articles) is to install horizontal strips with dado cuts on the back, which sounds like even more work.
If I use 1×4 for both furring strips, then I think I’ll need to buck out windows by another 0.75″ as well; not hard just will take planning.
Any advice appreciated, thanks.
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Replies
Phil,
One approach is to install a second layer of plywood or OSB sheathing.
But if you are installing mineral wool rather than rigid foam, you don't need the solid sheathing or two layers of furring strips if you don't want them. Nor do you need dado cuts on your horizontal furring strips.
Just install one layer of horizontal furring strips without any dados. Remember, the mineral wool insulation allows drainage.
Martin, thank you. If we install a second layer of sheeting, would that be on top of the vertical furring strips? Do I then need to add another waterproof layer on the sheeting, behind the shingles?
Also, I posed the question to Hardie, Nichiha, and Allura, three fiber cement manufacturers.
Hardie sent this response, along with a technical bulletin:
"See method 2 of attachment for installing our product over a non-nailable sub straight. We would require a solid sheathing if the individual shingles are being used and vertical furring for the shingle panels."
Allura sent this response, along with 2 technical bulletins:
"Attached to this message are 2 technical bulletins that address your statement of materials layering in your initial message."
Nichiha sent htis respone: " " (ok, they didn't respond)
The Hardie shake panel installation sounds reasonable, and I could keep the vertical furring strips as well.
Nichiha has the best looking fiber-cement shingle IMHO, and the have a shingle panel as well, similar to the Hardie panel. But according to this website, this isn't available in California. http://www.nichiha.com/residential/detail/nichistaggered-straight
Nichiha has installation instructions for their panel, but no mention of an over-the-rainscreen installation http://www.nichiha.com/images/uploads/NichiProducts_Install_Guide_4-2018.pdf
Phil,
In most cases, builders are forced to follow manufacturers' installation instructions -- it's a code requirement.
I suppose you can sandwich a layer of exterior mineral wool between two layers of sheathing if you want, even though it's a little odd. If you do that, you don't need furring strips under the second layer of sheathing. You would, however, need a water-resistive barrier (WRB) on the exterior side of the second layer of sheathing.
Builders in Maine who install a second layer of sheathing over continuous exterior insulation (because in Maine, builders like to install cedar shingles as siding) usually use rigid foam, not mineral wool, for the continuous insulation. Some of these builders just install nailbase instead of foam plus a second layer of sheathing.