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Durability issues with wooden rainscreen furring

Bernard Lam | Posted in Energy Efficiency and Durability on

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.

https://www.armatherm.com/products/z-girt-structural-thermal-break-cladding-attachment/

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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Replies

  1. Expert Member
    Malcolm Taylor | | #1

    Bernard,

    Nice house! I hope you post pictures when it is finished.

    There are a couple of assumptions in your case against using wood as strapping I'm not sure I agree with.

    The strapping in a rain-screen cavity is not expected to get wet. The cladding should be detailed so that no bulk water gets past it. For a rain-screen to have to drain moisture something has gone wrong. That's a back-up function. The primary ones are facilitating drying, and avoiding water getting behind the siding by capillary action..

    The reason we worry about wet sheathing is that it is located in the wall assembly where it is the First Condensing Surface. That's why we try and keep it warm. We don't worry about the studs, or the rain-screen strapping because they aren't in a position to accumulate problematic amounts of moisture as it moves through the wall.

    I don't know enough about the composite alternatives to comment on them. Perhaps the answer is a simple as using PT 1"x4" or PT plywood strips to assuage your concerns?

    1. Bernard Lam | | #3

      ok you have convinced me. I guess my feeling that wood of insufficient thickness deteriorates over time in unconditioned spaces is unfounded. I guess you do need at minimum bacteria which needs moisture. And if we are confident that the rainscreen sees no water then the wood, however thin, should stay put forever. Hard concept to accept given the average person's experiences with wood. I guess nothing lasts forever. Ironically repeated heating cycles might actually cause composites/fiberglass to become brittle and fail over time. So wood might actually be better. But the metal clips and rail system is still the most durable.

      Having said that, I do really like the idea of the ability to use mineral wool batts which are more cost effective per R. This can be achieved using the clips/rails. I'll have to do more cost analysis to see if its worth it. If there's a time to spend more on metal/composites to substitute for wood, this is the time with 400% rise in lumber prices. Its $5 a piece of 1x4x8' for me, and $80 per sheet of 3/4" ply.

      And finally, keeping everything metal in front of the rockwool sounds really good in a wild fire prone area. Afterall the county made me do 2 layers of typeX drywall for a 1-hr rating at soffits. class-A fire rated siding (metal) and roof (tempered glass), even windows cannot be vinyl (melts too fast) and decks have to be metal, concrete, or composite. Though if we are really honest to ourselves, I doubt the house is worth saving if a forest fire really got close enough to ignite the wood furring behind the metal siding. Just the smoke contamination would condemn the building. Maybe I'm over thinking it.

      Its going to be EPS+1x4s vs CavityRock batts + clips and rails. I'll try to price this out on monday and make a decision.

  2. Expert Member
    Akos | | #2

    You can also rip some composite deck boards or trim and use that as battens.

    1. Bernard Lam | | #4

      I have thought of that. But I wonder if composite decking can be used as a structural component. If I can't find data for approved tests done for pullout resistance etc. my engineer can't spec for fastening schedule. In a high wind zone I'm not allowed to just wing it.

      1. Expert Member
        Akos | | #6

        One option is to strap out your walls with horizontal 2x3s on edge (PT or SPF) 17.5" OC. You can install 2.5" AFB or Ultrabatt which is meant for metal studs between them. Your panels can now attach to the 2x3 directly. The 2x3 can be spaced off your WRB using larger washers or running a couple of saw kerfs to allow for drainage.

        The horizontal strapping gets you close to 95% of the benefit of continuous insulation without any of the hassle or cost of dealing with clips and semi rigid mineral wool. Way lower material cost, much quicker install. I have done this for roofs before and it works great.

        P.S. The way most commercial installs around me are done is with the furring channel held off the walls with metal L clips through the exterior rigid. This avoid most of the thermal bridging of Z grits, allows for easy shimming to get the wall dead flat and uses standard off the shelf pieces.

        1. Jon R | | #7

          > horizontal strapping gets you close to 95% of the benefit of continuous insulation

          A good idea in terms of lower priced exterior insulation. But do the thermal bridging and resulting sheathing cold spots cause moisture problems, canceling all but the R value advantage of continuous exterior insulation? US code is clear about the need for *continuous* insulation to protect sheathing from moisture entering the wall via a Class II or III vapor retarder.

          Similarly, fasteners are thermal bridges too - do they ever cause very localized sheathing rot (or excessive fastener corrosion)?

          1. Expert Member
            Akos | | #8

            Condensation in building assemblies is not the same as the condensation you see on a window or metal surface, there is no liquid water building up and dripping. The moisture moves through adsorption into the wood structure slowly increasing its moisture content. Even an assembly that we would consider to have "condensation control" will have some moisture getting into the assembly part of the year.

            The local cold/warm spot from the small thermal bridge of the wood strapping or from metal fasteners will cause some extra adsorption locally, quantifying that amount is well outside anything I could do, but my guess it falls into the squat category.

            The only time I've seen corrosion and rust is around window water leaks and on roof fasteners where there was lot of interior air leaks and heavy condensation on the roof deck. If you are near salt spray, might be a different story.

            I've take out a small section of siding installed with rain screen over rigid at my own home that is about 7 years old now. Everything underneath was pristine, just like the day I installed it.

          2. Jon R | | #9

            > near salt spray

            In February, I was replacing roofing panels where the screws were completely rusted through to the point where the panels were falling off. Yes, this involved salt dust and radiant cooling causing condensation (not sorption),

        2. Bernard Lam | | #14

          I see what you're getting at - essentially there'd be 2 layers of non-continuous insulation, one in front of the WRB and one behind, and the only "full depth" thermal bridge that goes through both layers are where the girts and studs intersect. That's actually exactly the same as using the composite z girt horizontally, but wood is a lot easier to source. the building department may object, as they require the outer insulation to be continuous. I do wonder if the thermal conductivity of 2x3s is much more than fiberglass girts in practice.

          But yes, your way would allow me to use batts instead of boards. which does have a cost difference. The commercial way with clips and rails would also achieve the same thing, but possibly more durable. A year ago I'd believe the 2x3's would be much cheaper. Today, I think I'm going to get some quotes on the clips and rail system.......if they let a residential owner builder buy it....

  3. Jon R | | #5

    > no one seem to be concerned with the long term structural integrity of the furring strips

    Rainscreen: "Pressure treated strapping is only required in high rain
    areas, though it is recommended in all jurisdictions. " Also says 1x3 ??

    https://cdn01.rockwool.com/siteassets/o2-rockwool/documentation/technical-guides/residential/comfortboard80-installationguide.pdf

  4. AlexPoi | | #10

    As long as the wood can dry faster than it get wet, it won't rot. Your cladding will stop most of the bulk water coming in and with a 3/4 inch ventilating space behind it you'll get plenty of drying. Just make sure you have an entry and exit points at the top/bottom of your all.

    Exterior furring has been code for a very long time here in eastern Canada. Nobody is using pressure treated wood and yet your never hear about it failling apart.

    1. Expert Member
      Malcolm Taylor | | #11

      Alex,

      "Exterior furring has been code for a very long time here in eastern Canada."

      I'm pretty sure Coastal BC has the only code requirement for exterior furring in Canada.

      1. AlexPoi | | #12

        I could be wrong but I think it's code as well in Quebec :
        9.27.2.2. Minimum Protection from Precipitation Ingress
        Except as provided in Sentence (6), exterior walls exposed to precipitation shall
        be protected against precipitation ingress by an exterior cladding assembly consisting
        of a first plane of protection and a second plane of protection incorporating a capillary
        break, where
        a) the number of degree-days is less than 3400 and the moisture index is
        greater than 0.90, or
        b) the number of degree-days is 3400 or more, and the moisture index is
        greater than 1.00.

        I have never seen a house built without a rain screen here.

        1. Expert Member
          Malcolm Taylor | | #13

          Alex,

          That's very interesting. The language is similar to that in our code. I knew that exterior insulation was becoming commonplace in Ontario and Quebec, but I didn't know rain-screens had been mandated. I wonder if the Maritimes will follow suit?

  5. Expert Member
    Peter Engle | | #15

    A bit late to this party, but I do have one comment on the above: The OP should do some more research before using PT furring with Corten steel. The chemicals used in PT are very corrosive to steel in general. Even though Corten is designed to have a rugged corrosion layer that protects the rest of the steel, the galvanic couple could be too much for it. I have no direct data on this, but raise it as a potential concern. I'd be very comfortable with the composite systems, if they are available and reasonably economical.

    1. Bernard Lam | | #16

      It's true. I can't use ACQ. Even those commercial clips and rails has galvanized rails which should touch corten. So pretty much wood or the fiberglass z girts is the only way to go.

      1. Jon R | | #17

        I'd say "ACQ can't touch the corten". So a thin plastic separator (eg, tape) might work.

        You have good overhangs and I'm guessing not excessive rain - I suspect that untreated wood furring would be fine.

        You could pre-paint untreated wood to make it considerably more durable.

    2. Expert Member
      Malcolm Taylor | | #19

      Is ACQ still the most common pressure-treated lumber in the States? Around here it has largely been replaced by MPS, which doesn't have the same corrosion issues.

  6. Steve Knapp CZ 3A Georgia | | #18

    Haven't read the thread. Did anyone mention thermally modified wood (for example, Americana)?

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