Cork as both cladding and continuous insulation?
NICK KEENAN
| Posted in Green Products and Materials on
I’m working with an architect who has suggested a 2″ layer of cork as both the exterior cladding and continuous insulation layers. I’ve never worked with the material. Does anyone have experience with it?
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Replies
Hey DC.
I have no experience working with it, but I like the way it looks and it's an environmentally responsible choice, assuming it holds up. Do you have a link to a specific product that the architect would like you to use? I'm curious...
I remember seeing some cork on design sites, didn't really look all that good though:
https://www.dezeen.com/2019/02/13/cork-cladding-house-hotel-architecture/
Maybe EIFS over cork might be better.
I used cork on the gable ends of my remodel of my house. Mainly because of ease of installation, and durability on exposure to weather. I am a bit of a coward at heights, so wanted a durable and light material that would be easy for a ladder coward to install as exterior insulation (I am very slowly remodeling, so the gable ends are exposed to weather until I can get siding on) . I used two 2" layers of roxul comfortboard, with 3" cork over that, all fastened to the wall with long screws (heco topix worked better than headlok) through 1x4 battens, as I am going to side eventually. I pinned the roxul to the wall with non structural screws, then used longer non-structural screws to pin the cork over the roxul, and then secured it with the 1x4 battens. Biggest issue was hitting the studs with long screws, as with all thick external insulation sheets.
I loved working with the material, very easy to use, cut, and attach, no itch (even roxul makes me itch), and has a slight pleasant smoky odor. The non exterior grade cork is a little brittle, but you would use the exterior grade which is less brittle.
The cork has weathered well, it's slowly changing color. Main issue other than expense, is that Flickers love it. I have three holes were flickers tunneled in to make nests.
I think there are some projects in PNW using exterior grade cork as insulation and cladding. You could google and see what comes up.
I actually tried to convince my wife to go with the cork as an insulation and a cladding, but she had non of that. You can't paint it, and she didn't want grey brown siding.
DC,
Using the cork both as insulation and cladding would preclude a rain-screen gap. I don't know enough about it's permeance to know how you would detail it properly. The wall might have to be detailed to dry to the interior, and include a mater-proof membrane behind the cladding?
That's a good point. The cork I used was ship lapped, and I saw no evidence of leaking around the window opening or any where else, but I air and waterproofed my sheathing with Prosoco R guard liquid applied weatherproofing before insulating (again for ease installation on a ladder, being a coward is expensive...). Those products are great as well, but expensive. You could put a drain screen gap behind the insulation though, using battens.
David,
Good to hear from someone with experience. I had wondered about bird damage. Woodpeckers are due back here soon. My concern with mounting it on battens is that the air-gap negates its effectiveness as continuous exterior insulation.
Thanks for pointing that out Malcolm. I thought a drain screen was just a small gap (1/4" )with top sealed and bottom open. Would that gap be large enough for a convective loop? I was thinking of using that detail with Larsen trusses and weather barrier on the rest of the house, so I don't have to use sheathing over the trusses.
Edit to add. Plywood sheathing detailed as air barrier, larsen truss and insulation, then weather barrier with 1/4" plywood battens external to batten, then siding nailed through the battens into the truss.
I actually like the Flickers nests. They sit with their heads poking out and watch me in the yard. I guess I would think differently if enough of them swiss cheesed my insulation, or if I wasn't going to side over. When we side the house, I'm going to take leftover cork scraps and build a flicker nest panels to see if they will nest in those instead of my siding.
David,
I don't know at what width a cavity with an open bottom short-circuits the effect of exterior insulation. Maybe someone can chime in.
Our local power company replaced the pole outside our house recently. I'm waiting to see how our resident woodpecker reacts when he comes back from his migration. he used the metal top of the old one to amplify his pecking and claim territory. Hopefully he approves of the new one.
Ha, our local resident Flicker does the same thing on our metal chimney cap. Makes me jump every time he does it.
As far as convection goes, looking at window pane gaps being around .5-.75 inches, i'm going to assume that .25 inches will not cause appreciable convection loops.
David,
This from Building Science Corp:
"But won’t the tiny gap cause a loss of thermal performance of the foam sheathing? Yes. How much? About 5 percent of the thermal performance of the foam sheathing (not the entire wall assembly) with the 1/8-inch gap, less with a smaller gap."
So assuming you can extrapolate to a 1/4" cavity, the loss doesn't appear at all significant.
I am the last one here to be concerned about sustainability. Wonder about that with cork.
I love the looks of it. Makes a great floor particularly over old concrete.
Hi Mike.
I don't know enough to say for sure how sustainable cork is. I do know that it is a natural material that can be reclaimed, reused, and is biodegradable. I'm not sure about the energy intensity of harvesting and manufacturing it into products. And there's shipping, and durability to consider. I'm curious though, do you have a specific concern when it comes to sustainability, something I'm not seeing?
Brian,
This first came up when synthetic corks began to replace the natural ones in wind bottles. Cork is entirely sustainable. When cork use declines, the trees are cut down and the land used for other purposes.
That makes sense. Thanks Malcolm.
No. Just a hip shot that perhaps I should not have said.
I watched a fascinating video of a UK house that was cork on the inside, outside and everything in between. https://www.youtube.com/watch?v=t76Wjw1ZVkQ
A possible guide regarding the gap size affecting insulation value.
A recent(ish) Fine Homebuilding article or letter responded to a similar question about rain screen gaps and the risk of fire spread. I believe they advised that gaps up to 1" did not appreciably increase the risk of fire spreading behind the cladding. Martin has frequently advised on GBA that air gaps under or close to 1" will have virtually no value in venting roofs.
I would have to suspect that small air gaps would not be able to provide sufficient air flow due to flow resistance, similar to the relative water delivery rates in piping of different sizes. This resistance to flow even allows an air film to be assigned an R value, even when the air flow is entirely unconstrained as in heat exchangers. The value is often given as .16-.17 for open surfaces and about .83 to 1 for trapped air gaps like windows. The very small gap available for air exchange likely makes the drain gap behind insulation come in on the .8 or higher side. I suspect the engineers on this site would be better able to define the values properly for "sandwiches" made of different materials.
As a practical experience matter, the 1/4" air gap I have at the sheathing surface behind my 6+ inches of outsulation does not seem to have impacted my overall efficiency/R-value. I think trying to breath through a straw for a few minutes would suffice as a demonstration of how much differential in pressure would be necessary to exchange a significant portion of the air volume contained in the 1/4" gap.
hi, Looking for advice on cork cladding - my builder is not sure how to secure guttering etc to/through the cork. any advice please?