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Adhering/fastening mineral wool to walls

user-2812795 | Posted in Green Building Techniques on

Quick question. building a new house.. now at “wrap house with Zip sheathing — taped seams” stage. Zone 4c/5 border near Long Island Sound shore.

Soon, we’ll likely cover with 4″ of mineral wool. Ostensibly fastened/tied down with 2×4 furring screwed through the m.wool. then, fiber cement attached to the 2×4.

A. Thoughts on squash block requirements between the studs (to prevent the fiber cement from bowing and or breaking because of the unsupported void which is required for rain/drain plane. and or to prevent gradual wind blown pressure on the siding…

I was thinking of cutting small triangles of 1.5″ thick polyiso foam of 2 or 3″ wide.. and gluing them to the back of the fiber cement pointy side up (let any liquid water drain down).

Also, this technically has the siding attached to pt studs which are attached to the structure which is probably not acceptable under the code.. shouldn’t the siding be attached directly to the studs? yes, hardie and certainteed both want you to attach directly to the sheathing with no mention of a rain/drain plane void behind the siding. duh!

B. Any additional thoughts on using polyiso instead of mineral wool? I’d get about the same cost, easier to install, tighter compression, and a higher R value by an order of almost 53% improvement (r 26 – r17 = r9 over r 17 = 53%). but no outward drying if it was ever necessary – even given that the interior side of the sheathing will almost never ever be cold enough for condensation.

thx in advance!

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  1. Expert Member
    Dana Dorsett | | #1

    Siding doesn't have to be attached directly to the studs- you can hang it on the furring as long as the furring is attached directly to the studs with sufficient fastener strength, numbers, & depth to support the siding.

    If the sheathing is half inch, the insulation is 4" thick and you're using 2x furring (6" total depth of furring + insulation + sheathing), if you have 16" o.c. studs and a furring board per stud you can get there with 7.5" long pancake head timber screws through-screwed to the studs 24" o.c.. It's a bit of an awkward length, but it's still do-able with practice. It's a lot easier of you used 1x furring and ring-shank for hanging the siding for better retention despite the somewhat lower penetration.

    If you went with 3" roofing iso you'd be at about the same exterior R, and WAY more than is necessary for dew point control at the sheathing (even if you had 2x12 studs filled with cellulose), no interior vapor retarder needed. That way you'd be looking at 6" timber screws.

    To be clear on the timber screws, one vendor stocked by some box-stores in a few lengths would be the FastenMaster HeadLok. (There are others. Pancake heads are important to keep from splitting the furring in this type of assembly, especially with 1x furring.)

  2. GBA Editor
    Martin Holladay | | #2

    GBA has published two articles written by people who have installed exterior mineral wool over wall sheathing. In their articles, they describe exactly how the insulation was attached.

    Here is a link to the article by Shannon Cowan and Patrick Walshe:
    Installing Roxul Mineral Wool on Exterior Walls

    Here is a link to the article by Mark Yanowitz:
    Wrapping an Older House with Rock Wool Insulation

    Here are links to two articles that discuss the engineering behind fasteners used to attach furring strips:

    Installing Mineral Wool Insulation Over Exterior Wall Sheathing

    Fastening Furring Strips to a Foam-Sheathed Wall

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