Image Credit: Amorim Isolamentos The outer bark of the cork oak tree can be harvested every nine years.
Image Credit: Amorim Isolamentos A billet of expanded cork coming out of an autoclave.
Image Credit: Amorim Isolamentos A 40 mm layer of expanded cork insulation resists burn-through for over an hour.
Image Credit: Amorim Isolamentos An example of cork used as an exterior cladding. This is dramatic, but not an application I particularly recommend--due to cost of replacement.
Image Credit: Amorim
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. (See “Avoiding the Global Warming Impact of Insulation” and “Polystyrene: Does it Belong in a Green Building?”)
So I was thrilled to learn about expanded cork boardstock insulation made by the Portuguese company Amorim Isolamentos and just now being introduced into the North American market. Francisco Simoes, of Amorim, visited our office in Brattleboro in June and told us all about it.
Familiar to wine drinkers as the traditional bottle-stopper, cork 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 tree’s 200-year life.
In Portugal, the world’s leading producer of cork, these oak trees are federally protected, and many cork forests are certified to Forest Stewardship Council (FSC) standards. Harvesting is done by hand, much as it has for over 2,000 years. While cork oak forests in Portugal are expanding, cork’s market share for bottle stoppers is dropping as plastic stoppers and screw-off caps become more common — motivating the company to look for new markets.
Cork as a building material
I have long been a fan of cork flooring, floor underlayment, and acoustical wall coverings. These materials are made from residual cork that remains after punching cork bottle stoppers from the bark — which consumes only 25% to 30% of the bark.
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. (It was 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% natural and rapidly renewable as defined by the LEED Rating System. It is durable yet ultimately biodegradable, produced from sustainable forestry operations, and a byproduct 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 7.0–7.5 pounds per cubic foot and compressive strength of 15 psi (with 10% compression). It is intermediate in its permeability to moisture — with a 40 mm layer having a permeance of 2.2 perms. Although the expanded cork insulation gives off a smoky smell, a test report I examined showed the material to pass France’s stringent requirements for a dozen volatile organic compounds (VOCs) with flying colors. 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 that are used in the most common boardstock insulation products. A 40 mm-thick piece of the boardstock insulation held over a torch will resist burn-through for an 60-90 minutes, compared to less than 10 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 layer, much like polyisocyanurate.
Cost and availability
North American distribution channels are just being set up, so pricing is far from certain. (One U.S. distributor of cork insulation is Small Planet Workshop.) But Simoes told me the price to a distributor will be about $0.70 per board-foot, not including shipping, markups, or the exchange rate. If those mark-ups come to 50%, the cost per board foot would be $1.05 and the cost to achieve R-19 would come to about $5.50 per square foot 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 building project later this year.
You can read my full review of Amorim Isolamentos’ expanded cork insulation board at BuildingGreen.com (membership required). You can also visit the company’s website or send an e-mail: email@example.com.
Alex is founder of BuildingGreen, Inc. and executive editor of Environmental Building News. To keep up with Alex’s latest articles and musings, you can sign up for his Twitter feed.