Material for rainscreen strapping
I’ve searched for this numerous times and haven’t found an it answered anywhere:
I’m planning 3″ of Rockwool comfortboard outboard of the sheathing and WRB, followed by 3/4″ of vertical strapping then wood siding. (following Rockwool’s guidance for screw type and spacing through the strapping into studs)
Is there an ideal material for the strapping? I lean toward exterior grade 3/4″ fir plywood, ripped to 3″ widths, for a balance of strength, consistency and cost effectiveness, but have heard of folks also using 1×3 dimensional SPF.
What about pressure treated? Is there enough of a long term benefit to make the additional cost worth it?
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We used 2x4 furring over 4" of Rockwool. Using thicker furring helps to prevent compressing the Rockwool --- something that's very easy to do.
We used the spider drive Headlok screws to attach the furring through the Rockwool and into the 2x6 framing. A Fastenmaster engineer told me to make sure at least 2" of screw gets embedded into the framing (excludes sheathing). The screws work great.
Cutting down strips of plywood seemed too time consuming, while I had read complaints from builders online about 1x3's being more susceptible to splitting.
If we hadn't used the 2x4's, we would've used 1x4's.
With a ventilated rainscreen gap between your siding and the Rockwool, pressure treated wood shouldn't be necessary.
I have a lot of photos showing our process on our blog: https://kimchiandkraut.net/2019/07/11/siding-1-of-2-continuous-insulation-with-a-rainscreen/
Thanks Eric. I'll check into the spiderdrive headlock screws.
I've already spec'd and installed all window surrounds (we've installed in-betweenie's, nailed to sheathing) So changing depth of strapping away from 3/4" won't be an option - though I do like the idea of 2x4 rigidity.
Thanks for the info and for sharing your blog.
Another consideration is that many PT products can be corrosive to fasteners. Not sure about headloks specifically but it may be a risk to put on the scale (even if the company claims PT ready, since they're relying on a coating. Perhaps someone else can speak to how they hold up real-world...)
The consensus (from my readings here at GBA, I don't have real world evidence) is that PT isn't needed since the drying potential should exceed the wetting potential by a good margin.
Plywood vs 1x strapping seems to be region and builder preference. Some advantages of strapping is that it's a less processed product, already ripped to size. Plywood requires ripping, has more exposed end grain, but is less likely to split.
*edit: strapping would be most likely to split near the ends, so if there are problems there, a quick pre-drill may help (and still require far less time than ripping a bunch of ply)
Thanks Tyler. Drilling pilot holes at least at the ends seems like a smart thing to do. Appreciate the insight.
Rockwool says: "Pressure treated strapping is only required in high rain areas, though it is recommended in all jurisdictions. "
Thanks Jon. This is the statement I read which got me onto this whole question.
Plywood strips make sense in regions like here in Coastal BC where they are available in all lumberyards, and are being installed directly onto a solid substrate. Otherwise they are too time consuming and lack the rigidity for use over Rockwool.
That makes sense. Thanks Malcolm.
I found using material direct from sawmill gave me 12 ft. lengths and a full 3" wide. Here we can get hemlock or larch, both rot-resistant woods.