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Adding 1 inch of cork – Correct build-up for Hardiboard?

user-5254480 | Posted in General Questions on

I am in the midst of a remodel located in Seattle. Due to some permitting issues we needed to re-sheath the entire house. This led me to pursue the idea of adding exterior insulation. I already have my windows and framing is done (so I can’t go the dudley box route) and I am planning on an outie installation.

My current plan is this:
– 1/2″ sheathing OSB (all joints sealed with tape and foam in some areas)
– 1″ cork (screwed to framing)
– Wrap and tape with Siga Mavjest
– 3/8″ furring strips for rainscreen
– Hardipanels

My questions are 1) What is the best way to attach the windows? The windows have built in flanges. I read the “Installing Windows In a Foam-Sheathed Wall” post, but I am confused as to if I can use the built in flange and screw through the cork into and through the sheating? 2) Should I do anything to seal the nails used for the furring strips? 3) Any other ignorant gaps in my plan ? 🙂


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  1. GBA Editor
    Martin Holladay | | #1

    Q. "Can I use the built in flange and screw through the cork into and through the sheathing?"

    A. Yes. In this respect, the cork is no different from 1-inch-thick rigid foam. For more information, see Nailing Window Flanges Through Foam. You may also want to read Installing Cork Insulation.

    Q. "Should I do anything to seal the nails used for the furring strips?"

    A. No. For more information, see All About Rainscreens.

  2. charlie_sullivan | | #2

    Cork is an interesting idea for this application. I'm guessing that its advantages include:
    * Higher vapor permeability than EPS, so you don't have to worry as much about traping moisture and respecting guidelines for minimum thickness.
    * Higher mechanical rigidity than mineral wool, so siding on top is easier.
    * Avoids the toxic flame retardants in EPS, and is biodegradble, so you can cut it with a saw and let the dust fertilize the ground.

    I imagine that it's also expensive compared to mineral wool or EPS.

    Did I get that right?

  3. user-5254480 | | #3

    Thanks Martin. I re-read the Windows Flange post again and am sharing it with my installer.

    Charlie - yes. You got most of the points. Cork vs. XPS is about 30% more expensive per sq/ft. With cork you get about .5 more perm, better fire resistance, better sound dampening. And yes green wise - I'll sleep better knowing that 2000 sq. ft of cork will get recycles one day if needed :-)

  4. Expert Member

    Eric, I'm surprised that the cork is only 30% more. That's great!
    If you are running furring strips I'd be inclined to fasten the cork with a roofing nailer. Good big heads and fast.

  5. user-5254480 | | #5

    To be fair the XPS base price is a Home Depot price. I could get is cheaper via insulation distributor. The challenge with the cork is that there are only a few folks distributing it in the USA. Fortunately I am really close to one of them so shipping is cheap :-)

  6. user-5254480 | | #6

    Martin - I reread the links. I am still a bit confused. My situation is that we are using an outie install. I read the innie or outie post, but for outies it only talks about building out a box.

    I am looking more at the picture frame approach noted here: From:

    In that build up he is using 2" of foam. He tucked 1" under and 1" wood window frame. This gives enough R value between the sheathing and the outer wall. Is that an acceptable approach?

    If so I could move to 2" of cork and do the same. I was also thinking that my picture frame could be 1.5" to allow for a possibly deeper rain screen. Either way with the 1" tucked behind the frame you get the R value where it needs to be.

    I am fine moving to 2" from 1" as I read your Calculating thickness article and it dawned on me that I have 2x4 on lower and main and 2x6 on upper floor and I am in Marine Zone 4 so I am on the upper limit for the 2x6 with 1" cork.



  7. GBA Editor
    Martin Holladay | | #7

    Q. "In that build up he is using 2 inches of foam. He tucked 1 inch under a 1-inch wood window frame. This gives enough R-value between the sheathing and the outer wall. Is that an acceptable approach?"

    A. Yes. I described that approach in the article you linked to (Installing Windows In a Foam-Sheathed Wall). In that article, I wrote that one acceptable approach is to "Install a picture frame of 1x4 lumber, installed on the flat, on the exterior surface of the rigid foam. The 1x4 picture frame is screwed through the rigid foam to the sheathing and framing."

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