I’m always on the hunt for the latest, most interesting, and most environmentally friendly building materials, and I have particular interest in insulation products -- partly because many conventional insulation products have significant environmental downsides. So I was thrilled, recently, to learn about expanded cork boardstock insulation made by the Portuguese company Amorim Isolamentos and just now being introduced into the North American market.
Most wine drinkers are familiar with cork as a material. It is a natural product made from the outer bark of a species of oak tree that grows in the western Mediterranean region of Europe and North Africa. The bark is harvested after trees reach an age of 18-25 years and it regenerates, allowing harvesting every nine years over the trees’ 200-year life.
In Portugal, the world’s leading producer of cork, cork oak trees are federally protected, and many cork forests are certified through Forest Stewardship Council (FSC) standards. Harvesting is done by hand, much as it has for over 2,000 years. Cork oak forests in Portugal are expanding, according to Amorim’s Francisco Simoes, who visited our office in Brattleboro in June, while cork’s market share for bottle stoppers is dropping as plastic stoppers and screw-off lids become more common -- motivating the company to look for new markets.
Cork as a building material
Until learning about cork
For cork flooring and these other products, the cork granules are glued together with a binder and then sliced into the finished products.
Expanded cork insulation is quite different. The same cork granules are used, but they are exposed to superheated steam in large metal forms. This heating expands the cork granules and activates a natural binder in the cork, suberin, that binds the particles together. (In an in-depth product review about expanded cork insulation in the August issue of Environmental Building News I describe the fascinating history of this process, which was actually invented by accident in New York City in the late-1800s.)
After producing these large billets of expanded cork, they are sliced into insulation boards in a wide range of thicknesses -- in both metric and inch-pound (I-P) sizes. In I-P units, thicknesses from a half-inch to 12-inches are available -- with dimensions of 1’ x 3’ or 2’ x 3’.
The material is 100 percent natural, biobased, durable yet ultimately biodegradable, produced from sustainable forestry operations, and a by-product from the cork bottle-stopper industry. Though there is significant shipping energy required to bring it here, shipping by ocean-going vessel is relatively energy-efficient. It’s hard to imagine a greener building material.
Cork insulation performance
Expanded cork insulates to R-3.6 per inch. It has a density of 7.0 to 7.5 pounds per cubic foot and compressive strength of 15 psi (with 10 percent compression). As for moisture transmission, a 40 mm layer has a permeance of 2.2 perms. Relative to indoor air quality, a test report I examined showed the material to pass France’s stringent requirements for a dozen volatile organic compounds (VOCs) with flying colors -- though the material does have a strong smoky smell. Cork also has superb sound-control properties.
From a fire-resistance standpoint, it meets the European Class E designation (the standard met by other rigid insulation materials) without the need for flame retardants. A 40 mm-thick piece of the boardstock insulation held over a torch will resist burn-through for an hour to an hour-and-a-half, compared to just a few seconds for expanded or extruded polystyrene, which meets the same Class E designation. (The flawed manner in which we determine fire-resistance properties of materials is the topic for another article.)
Cork insulation has been used as a rigid insulation material for decades in Europe. It is not uncommon to install an 8- to 10-inch layer on exterior walls and a 10- to 12-inch layer on roofs. The first Passive House built in Austria (in 1995) used a 350 mm layer (nearly 14 inches) of the material. It is typically used as an exterior insulation later, much like polyisocyanurate.
Cost and availability
North American distribution channels are just being set up, so pricing is far from certain. But Simoes told me the price to a distributor will be about $0.70 per board-foot, not including shipping, mark-ups, or the exchange rate. If those mark-ups come to 50 percent, the cost per board foot would be $1.05 per board foot square foot and the cost to achieve R-19 would come to about $5.50 for cork, vs. $1.10 - $1.60 for polyisocyanurate insulation and $2.00 - $2.25 for extruded polystyrene.
That’s a significant upcharge for cork, but you end up with one of the greenest building materials anywhere. I’m so excited about expanded cork insulation, in fact, that I’m hoping to use it on an upcoming project later this year. For information, visit www.bcork.amorim.com/en.
You can read my full review of Amorim Isolamentos’ expanded cork insulation board at BuildingGreen.com (membership required).
Alex Wilson is the founder of BuildingGreen, Inc. in Brattleboro. Send comments or suggestions for future columns to firstname.lastname@example.org.