Durability issues with wooden rainscreen furring
The most recent post on GBA is all about moisture concerns with exterior insulation. But no one seem to be concerned with the long term structural integrity of the furring strips. I see the wooden 1x4s failing before the sheathing.
I agree with all “modern” building techniques that improves energy efficiency and durability. But one detail I cannot get over, is the use of 1×4 strapping over rigid foam. As a rainscreen, its expected to get wet. Exposed to temperature extremes, and most importantly, its most often the structural connection for the siding.
My situation is a bit more difficult than usual:
– 165mph design wind speed (131mph 3sec exposure C)
– Corten “western reveal” vertical panels, consealed fasteners (with 1.5″ air gap to the building due to its profile)
-The usual 2×6 16OC studs with tapped 7/16″ zip sheathing.
-Zone 5, class A/ignition resistant exterior
-Tall (30’+), difficult to access exterior, above 40+ degree slope. Top of building down to road 70’+ (see attached picture)
With the required R5 continuous insulation, and vertical panel siding, I’ll have to do horizontal strapping. Luckily the metal panel profile is already a giant rainscreen. But I can’t help but to feel that in 50+ years the strapping will fail and the rusted strips of decapitating metal fly off into the sunset. Granted, I don’t care to live for another 50 years, but I assume others do.
So I started looking into commercial techniques, and learned that they have been doing exterior insulation forever and use metal Z-girts as standard. And now with these lumber prices, Z girts might not be more expensive than 1×4 strips of plywood. Problem solved. Except then its not really continuous insulation anymore due to the metal z girt thermal bridging. So I look into thermal isolation clips that hold girts and rails like Knight wall systems, Smartci, etc.
Another good solution is furring master’s FM3 hat channels, other than having to buy 10000′ minimum. But I’d still have a problem. The corten manufacturer recommends any metal in contact with the corten to be stainless steel, not just galvanized. So apparently I cant use metal girts/rails/hats, clips or not.
Just as I’m about to break down and go back to 1x4s and let future generations deal with my siding liability, I learned that there’s a composite z girt thats thermally isolating and structural.
This has the advantage of allowing me to use “semi-rigid” rock wool batts – which is lower in cost compared to “rigid” insulation like comfortboard or even EPS. So my next challenge is to find this composite girt, hope that its not crazy expensive, and continue to complain about how that’s not industry standard, how 1x4s are acceptable, why I didn’t choose to use metal faced SIPs like a warehouse would, etc. etc. And then just to find out the composite Z girts are out of stock, or they won’t sell to builders directly, and need my lumber yard to order 10000′ minimum.
Anyway, are there any better suggestions? Other similar products that’s easily obtained? Is it a good idea to do rockwool BATTS on top of taped zip? Whatever I install, I never ever want to have to go up and access the siding again, that’s why I chose weathered steel. I expect it to last 100 years without intervention.
But maybe my concern about flying siding is moot when the roof avalanche off the solar roof will kill cyclists below on an annual basis anyway.
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