The cork arrived three-to-a-pack, and we've been storing it in our barn all winter.
Click to enlarge.
Photo Credit: Alex Wilson
Avoiding foam insulation
The primary reason I’m excited about using cork insulation on our house is that I don’t like some of the chemicals used in conventional foam insulation. Extruded polystyrene is made with a blowing agent, HFC-134a, which is a very potent greenhouse gas that is contributing to climate change, and nearly all foam insulation materials contain hazardous brominated or chlorinated flame retardants. I’ve most recently written about these concerns here.
Cork, by contrast, contains nothing but cork—nothing! As it is produced today by Amorim Isolamentos, S.A., the granules are poured into large vats and heated with steam in an autoclave at about 650°F for 20 minutes. The heat expands the granules by about 30% and releases a natural binder, suberin, that exists in the cork. There are no added ingredients.
Isn’t cork a limited resource, or isn’t there a cork blight?
I get these questions whenever I mention cork. As far as I can tell, these were rumors that were started by companies making synthetic bottle stoppers for the wine industry that were trying to take away market share from natural cork. No, to the best of my knowledge there isn’t a blight.
Bundles of cork awaiting installation.
Click to enlarge.
Photo Credit: Alex Wilson
Cork is a somewhat limited resource, so cork insulation will never come to dominate the rigid insulation market. But the resource is not disappearing and clearly it is a renewable resource.
The sad part of the story is that as synthetic corks and screw-lid wine bottles have replaced traditional natural-cork bottle-stoppers, the demand for cork has dropped. I’m told that in some parts of the western Mediterranean region, cork oak forests are being cut down and the land converted to other uses.
Shipping cork to Vermont from Portugal
I’ll admit that shipping cork across the ocean is a significant downside. While ocean shipping is very energy-efficient (far more efficient than shipping over land), the fuel used—a low grade of diesel—is very dirty. I struggled with that as I thought about the use of this material for our house. I’ve reviewed an analysis Amorim Isolamentos has done on the carbon footprint of their material, and it’s not too bad.
Detail of the cork.
Click to enlarge.
Photo Credit: Alex Wilson
Ultimately, I decided that by publicizing our use of this material I would help generate demand that might help preserve the cork forests. I don’t expect that the U.S. will ever become a huge market, but for people wanting natural and rapidly renewable building materials or who have chemical sensitivities, cork is an option that can be considered. (Relative to chemical sensitivities, care should be taken to make sure that there isn’t sensitivity to the odor of expanded cork, which has a somewhat smoky smell—no doubt due to the heating.)
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Our use of cork insulation
We are installing cork as a layer exterior insulation on our farmhouse. The air barrier for the significantly rebuilt early-1800s house is a fully taped and airtight Zip sheathing layer. On the interior of that will be 7-1/2” of cavity-fill insulation. On the outside of the sheathing is the 6” layer of cork.
The building enclosure is designed so that the cavity-fill insulation layer can dry to the interior (if it ever gets wet), while the cork can dry to the exterior. On the outside of the cork will be a layer of housewrap (a high-performance German product, Pro Clima Solitex, distributed by 475 High Performance Building Supply), vertical strapping to create a rainscreen, and wood clapboard siding.
Installation proceeding.Click to enlarge.
Photo Credit: Alex Wilson
We ordered the cork with shiplap edges so that joints would not extend all the way through the material. We had debated ordering 3" thick cork and overlapping the joints—and that strategy would have worked fine—but we decided that we could save on labor with the thicker panels.
Next week I’ll provide some specifics on how the cork insulation is being installed. Eli Gould and his crew have done a wonderful job at figuring out how to work with the stuff—some of the details are quite tricky.
Alex is founder of BuildingGreen, Inc. and executive editor of Environmental Building News. In 2012 he founded the Resilient Design Institute. To keep up with Alex’s latest articles and musings, you can sign up for his Twitter feed.
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