Input requested for furring strips
Happy New Year, hopefully everyone’s 2021 is better than 2020.
I am currently in the design phase for a new home in climate zone 6B Western Wisconsin and will be using 1.5″ Rockwool 80 (R-6) outside of a 2×6 wall (R-21). The building inspector recommended thickening the 1x furring strips to adequately meet the 1.25″ fastener penetration requirements of both Certainteed’s Monogram and LP’s SmartSide. Initially I was thinking of ripping 5/4 deck boards to use as furring strips, but that seems like overkill for a ventilated rain screen. My second idea is using 2x furring strips (higher cost compared to a ripped deck board) and then using 1.25″ Rockwool 80 to increase the insulation thickness and keep a quarter inch ventilation gap behind the siding.
Any input is appreciated.
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Replies
Not sure I follow your setup. Are you proposing installing an extra layer of rigid between the 2x strapping? If that is the case, it should work fine but you'll have to do a lot of trimming to get it in there.
A simpler assembly would be to mount 2x2 horizontal strapping 17.5OC, install the 1.5" rigid between then strap this out with vertical strapping of your choice. This would avoid having to deal with the squishy mineral wool, can all be nailed (no long screws) and give you about 95% of the assembly R value compared to CI insulation. I would check the fastener table of your siding as most allow for nailing into 3/4" material.
You can also bump the horizontal strapping up to 2x3 on edge and insulate with 2.5" AFB which would give you more R value be significantly cheaper than your 1.5" comfortboard 80.
Note that 5/4 decking is typically closer to 1-1/8" thick, so if your inspector is trying to rigidly enforce the "1-1/4" fastener depth" requirement, deck boards might not pass muster. If this is the case, 2x lumber is probably your best bet unless you're willing to run a bunch of stuff through a planer to shave it down.
It's pretty common to use strips ripped from sheets of 3/4" plywood for furring in rain screen gaps. Some of the siding manufacturers even have fastener schedules for this, so that would probably be your best option if you can find the info you need from LP and/or CertainTeed. A call to either of them should get you an answer pretty quickly since this isn't an unusual request.
Another option might be 1" plywood, which can be special ordered. You'd still not be at 1-1/4" thick though, but it might be enough for a siding manufacturer to give you a "this is OK" letter for the inspector even if they didn't think 3/4" plywood was thick enough.
Bill