Blog Post

Expanded Cork - The Greenest Insulation Material?

Introducing all-natural expanded cork boardstock insulation to the North American market.

Expanded cork insulation is available up to 12 inches thick and can be used much like polyiso. Click to enlarge.Photo Credit: Amorim Isolamentos

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.

The outer bark of cork oak tree can be harvested every nine years. Click to enlarge.Photo Credit: Amorim Isolamentos

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.


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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%–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).

A billet of expanded cork coming out of an autoclave. Click to enlarge.Photo Credit: Amorim Isolamentos

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, rapidly renewable as defined by the LEED Rating System, 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 7.0–7.5 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.

A 40 mm layer of expanded cork insulation resists burn-through for over an hour. Click to enlarge.Photo Credit: Amorim Isolamentos

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. 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%, 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 (membership required). You can also visit the company’s website or contact the company by e-mail:

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.

Published July 31, 2012

(2012, July 31). Expanded Cork - The Greenest Insulation Material?. Retrieved from

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October 7, 2019 - 4:19 pm

Suberosis appears to be specific to the inhalation of cork dust during material processing (e.g. mulching, cutting, forming, etc.). The research paper mentions that it is much like "farmer's lung", "furrier's lung" or other occupational respiratory tract illnesses that arise from the ongoing inhalation of dusts/particulates.

This article doesn't point to any particular health risks associated with cork insulation as a finished product. My understanding from other reading that I've done is that cork insulation testing indicates low-to-no VOC or particulate emissions (assuming no artificial binders or coatings are added).

It would be socially responsible to source your cork from ethical manufacturers who are protecting their workers (e.g. implementing adequate dust control measures, ensuring that their workers are wearing appropriate dust masks, etc.).

October 7, 2019 - 3:37 pm

You might be interested in looking up the monolithic load-bearing cork residence designed and built by Matthew Barnett Howland, Dido Milne and Oliver Wilton in Berkshire, England.  It's on the shortlist for the RIBA stirling prize for 2019 and there's a great video about it on Youtube.

February 18, 2019 - 9:45 pm

All these blogs are interesting. I am trying to decide which isulation would be the safest or which I would react to the least, I have many allergies: latex, soy, fiber glass, wool, VOCs, chemicals, etc.. So, my conclusion was that cork might be the safest except I am not sure anymore after I read this article    on Respiratory disease in cork workers (`suberosis'). This was kind of scary, but I wonder if it would be safe how it comes as insulation panels?

From what I've read, it may not be completely natural because of the binding process through steam, but my main concern - is it safe after reading this article above? 

August 28, 2013 - 3:27 pm


I've not heard of a SIP with cork insulation, though I don't see any reason why it wouldn't work. Those panels would be quite heavy, though, as the cork is about 7 pcf (vs. 1 pcf for standard EPS). And, of course, adhesive would be needed to glue to OSB to the cork. I could put you in touch with my contact at Amorim in Portugal if you'd like; I'm sure he would be very interested.

August 28, 2013 - 3:44 pm

Thanks for the offer but I don't have any actual project to use this on right now.  My main interest would be some eventual custom built house I keep tweaking the design of.  But that's years away.

August 28, 2013 - 3:24 pm

I just had a lunch-n-learn with a SIP sales rep.  I do like the performance of SIP walls and roofs.  But when I asked about alternative insulations he doesn't really have any.  He said they could do Grey EPS but no other alternatives.  I was asking to see if something other than brominated flame retardant could be ordered.  Since then I started to think about this article and I think SIP walls would be great if they came with expanded cork as the insulation.  - No flame retardant chemical- All of the product is now renewable.- Less smoke hazard during a fire.- Less structural compromise during a fire since EPS shrinks when heated.

Does any one know of any SIP manufacturers that offer walls made from expanded cork?

July 13, 2013 - 9:49 am

I have a former walk-in freezer insulated in 12" thick cork panels. It is crumbley. Is it worth salvaging the cork or starting from scratch with spray foam? Termites are a problem too. The floor is 2' thick cork with 6" concrete... the concrete has caved in over time... not sure if the cork has compressed. This place was built well over 75 years ago.

August 31, 2012 - 4:10 pm

Bill:  a well-followed publication like EBN and this blog are ideal places to discuss the various features of building products.  People will make purchasing or specification decisions based on what is written here so I suggested that some care be taken in describing products, especially if (per the Green Guides) a stated feature or claim could be very difficult to substantiate.


Based on your comments, perhaps we are not talking about the same “study.”  I reference the consumer perception study done by the FTC for their October 2010 proposed mods to the Green Guides at


For example, the FTC states:  “The study further examined whether consumers believe that environmental claims suggest anything about any negative environmental impact that may come from the product.  Twenty-seven percent of respondents interpreted the unqualified claims “green” and “ecofriendly” as suggesting the product has no negative environmental impact.”  


Federal Register notice at page 43 -


Also, here is a longer quote from the FTC on the subject of general env claims like environmentally- and eco-friendly: 


“The consumer perception evidence and some comments reaffirm the current Guides’ advice that unqualified general environmental benefit claims convey a range of meanings. For example, [made from recycled materials, recyclable; made with renewable materials; biodegradable; made with renewable energy; non-toxic; and compostable].  Averaging across these seven attributes, 52 percent of respondents viewing an unqualified “green” claim stated that the claim definitely or probably suggested that the product had these specific green attributes. The percentages are similar for respondents viewing an “eco-friendly” claim.  Moreover, 27 percent of respondents interpreted the unqualified claims “green” and “eco-friendly” as suggesting the product has no negative environmental impact. . . . Given these findings, and because FTC law requires marketers to substantiate every express and implied environmental benefit that consumers reasonably could take from such a claim, unqualified general environmental marketing claims remain very difficult, if not impossible, to substantiate. Very few products, if any, have all of the attributes consumers appear to perceive from general environmental benefit claims. In addition, given that all products have some environmental impact, it is doubtful that a marketer could substantiate that a

product has no or negligible negative environmental impact. The  Commission, therefore, proposes revising the Guides to more directly caution marketers not to make unqualified general environmental benefit claims.”


Federal Register notice at page 44-46, emphasis added.  Is there another study we should also be looking at?  Thanks.

Bruce Ray

Johns Manville

August 30, 2012 - 1:15 pm

Bill:  in its October 2010 proposed changes to the Green Guides, the FTC noted consumer impression research that led them conclude an unqualified claim of “environmentally friendly” would tend to convey the message that the product had no adverse environmental impacts and a very broad array of environmental benefits.  They concluded that such claims may be impossible to substantiate.  So we agree there.

As for the kitchen criterion, I think we would agree that any “natural” product should have been only minimally processed else natural products would include gasoline and U-235 nuclear fuel.  The issue is where to draw the line and some have proposed that minimal processing means no more processing than you could do in a typical kitchen.  See, e.g., the Natural Resource Ingredient Center here -  ("In the early '80s the FTC came up with a great definition for Natural - never adopted. They said that an ingredient may be called "natural" only if it contains no artificial or synthetic ingredients and has had no more processing than something which could be made in a household kitchen.")  It sounds like you have also thought about this.  What approach do you recommend? 

You’re correct to question our product data sheet for our AP Foil-faced Iso-foam sheathing product.  Iso-foam does have the highest R-value per inch of the three types of foam currently in use for exterior sheathing:  XPS, EPS, and Iso.  We will change the product data sheet to make that clear.

The language on reducing indoor air quality problems is based on the California Air Resources Board IAQ Guideline entitled “Formaldehyde in the Home,” at and US EPA’s comments to the USGBC on a LEED proposal recommending reduction of formaldehyde exposure.


Bruce Ray, Johns Manville

August 30, 2012 - 4:52 pm

You are claiming the study showed people believed "environmentally friendly" would mean "no adverse environmenatl impacts".  There is no such claim or result from any study.  The actual wording from the study was "no significant environmental risk."  Little changes in words convey a significantly different meaning.

Minimally processed I would also agree with.  Baking and steaming are not a significant process dispite how much you're trying to spin it.  Autoclave is a type of pressure cooker.

Try focusing on enforcing language that has been adopted.

You're getting away from the fact that none of this applies to BuildingGreen.  They are not the manufacturer who's selling the product.  Just someone interested in it and writting about it.  They have no gain or loss if sales of cork go up or down.  This seems like a silly place to argue over what "natural" means.

August 30, 2012 - 11:53 am


Here are some thoughts.

Even if you assume that the cork insulation meets all applicable technical, legal, performance, and other standards for insulation (including, as applicable, the FTC Home Insulation Rule at 16 CFR Part 460, the emissions limits in SM v1.1, f/k/a ES-1350, mold resistance, etc.), it is very difficult to objectively compare the sustainability of different types of insulation.  Until there is an objectively based standard (such as the CRI ANSI standard for carpeting) we are pretty much left to compare various single attribute claims and weigh those against the cost – again, assuming all are equal on performance. 

Some consumers (and specifiers, architects, etc.) are more concerned about air pollution and some about water pollution.  Others are focused solely on carbon.  Still others are most concerned about the conditions of the manufacturing workers and whether they are earning a living wage.  But pretty much everyone is concerned about cost.

To me, cork seems interesting but very pricey.  I wonder how difficult it is to cut and install.  For attic applications, it seems a bit heavy, which can be an issue if a home has thin drywall between the living space and attic.

The point about the use of “natural” is that the word has been shown to be a powerful consumer motivator.  And since many consumers and green building professionals look to EBN and this blog for info, it is important that you use some care in describing the products.  Otherwise, someone may just repeat your description in their own marketing or make a purchase or spec decision based on your description. 

A better and more accurate description of cork insulation would be “biobased.”  The manufacturer could clarify but it sounds like the raw cork is exposed to both high temps and high steam pressure in a sealed industrial autoclave.  If it was just heated, it would be done in an oven, not an autoclave.  Perhaps someone can weigh in here but it still seems like this goes beyond minimal processing and should not be considered natural.

Sustainability and LEED APs can weigh in but when a product is available from only one place on earth thousands of miles from the US market, calling it sustainable could be a stretch.

Good discussion.  Thanks.

Bruce Ray, Johns Manville


August 13, 2012 - 1:11 pm

I guess you always view the world through your own lens.  So, since I am with insulation manufacturer Johns Manville, I always tend to read more closely any article related to insulation.  I have some comments to pass on for discussion purposes


1.       You use the terms “greenest insulation” and “environmentally friendly building materials.”  We should all keep in mind that the Federal Trade Commission considers unqualified, general environmental claims to be essentially misleading per se.  This is because general claims are very difficult or impossible to substantiate.   Most consumers believe that “environmentally friendly” means that there is not a single adverse environmental impact, which is pretty much impossible to substantiate.


[“The FTC’s consumer perception study confirms what the current Guides already state — unqualified claims that an item is ‘‘environmentally friendly’’ or ‘‘eco-friendly’’ are likely to convey that it has specific and far-reaching environmental benefits. Very few products, if any, have all of the attributes consumers seem to perceive from such claims. Therefore, these claims may be impossible to substantiate.”  75 Fed. Reg. at p. 63552 (Oct. 10, 2010).]


When you say “greenest,” this fairly means that there are objective “green” criteria for all forms of insulation and that you have compared or scored all forms of insulation and cork has come out on top.  For example, some insulation generates water pollution and some air pollution.  A comparative or superlative claim means that there is an objective scoring system to appropriately and quantitatively compare these tradeoffs.  I think such a system does not (yet) exist and therefore such claims are not substantiated.


2.       You call cork insulation “all natural” but based on the description, I think that claim is not substantiated.  There is no single regulatory definition of natural and the FTC in the proposed Green Guides mods declined to provide definitive guidance.  75 Fed. Reg. at p. 63,586.  However, there are two frequently cited criteria for “natural:”  1/ the material or product must “come from nature” and, 2/ the material must be minimally processed, which is pretty much limited to the processing you could accomplish in a typical kitchen. 


Cork undoubtedly meets the first criterion as it comes from a tree.  Yet the description seems to disqualify cork as being minimally processed.  The article references that the cork insulation is manufactured by “putting cork granules in an autoclave and exposing them to superheated steam at 660 – 700 degrees F for 20 minutes.”   This sure seems like a high degree of processing requiring special industrial equipment and resulting in some complex chemistry.


 3.       IAQ.  The most important test for the US market is the Standard Method v1.1 (f/k/a ES-1350) using the residential scenario.




Bruce Ray

Johns Manville 

August 15, 2012 - 2:01 pm

Hi Bruce,

1) I understand your concern with using vague terms, but everyone does this, including your own firm in their literature and website.Terms like “sustainability”,“enhance the quality of life”, “environmental respect”, and “preserve the natural environment”.These terms are also difficult or impossible to substantiate. Also, I don’t know anyone who would assume a product marketed as “environmentally friendly” does not have a single adverse environmental impact. I doubt any product exists that can’t be picked apart in some way as having at least one negative impact, including tap water. (ever read about the dangers of dihydrogen monoxide?)

When your company’s website says “The best way to reduce indoor air quality problems is to eliminate and reduce sources of formaldehyde.” are there any tests or studies done to validate this claim?Or by stating, “Thermal Efficiency –polyisocyanurate foam provides the highest degree of insulation efficiency available,” I’ll just assume you’ve had this compared to all other forms of insulation including aerogel.

2) I have no idea where you got the kitchen requirement as a factor for using the phrase natural. Maybe for food claiming to be natural. The only reference for products I can find relates to the first point.{The FDA, however, has employed an informal policy regarding the term "natural.’’ Specifically, it: has considered "natural’’ to mean that nothing artificial or synthetic (including colors regardless of source) is included in, or has been added to, the product that would not normally be expected to be there.}

The FTC states: “Natural- Claims Marketers that are using terms such as natural must ensure that they can substantiate whatever claims they are conveying to reasonable consumers. If reasonable consumers could interpret a natural claim as representing that a product contains no artificial ingredients, then the marketer must be able to substantiate that fact.”16 CFR Part 260.

August 9, 2012 - 2:19 pm

I would agree that cork is a rapidly-renewable insulation that performs very well.  Since the late 1800s until the late 20th century, cork insulation was used extensively in the US for industrial applications like cold storage.  Its use began to wane as demand exceeded supply and it was eventually replaced with foam plastics like EPS, XPS, PIR and SPF.

Here is a great and free textbook on cork insulation:

I do have a few questions about cork insulation:

(1) If cork insulation becomes a product of choice for green building projects, is there enough supply to maintain the already high cost of 2-3x that of foam plastics?  Winemakers are already transitioning from natural cork to synthetic (foam plastic) stoppers due to restricted supply of natural cork.  If there is a limit to wine cork production, wouldn't it also constrain the supply of corkboard insulation, which is made from cork stopper scraps?

(2) How does cork insulation perform in terms of air permeance (air barrier material) and water vapor transmission?  Air barrier and vapor retarder performance is inherent in most foam plastics, but would require additional materials if cork insulation is used.  What is the environmental impact of these additional materials?

(3) While natural building products will undoubtedly lower the environmental impact of the building, can a builder or designer be assured of getting consistent quality from these natural materials?  There used to be a material specification (ASTM C640-83 Specification for Corkboard and Cork Pipe Thermal Insulation) but that was discontinued about 20 years ago. 

(4) Do corkboard products meet the current versions of the U.S. model building codes?  Are thermal performance, durability and fire safety issues properly addressed?  Issues of life-safety and product durability certainly must take precedence over environmental impact in any building design.

Rick Duncan

Technical Director, Spray Polyurethane Foam Alliance

August 9, 2012 - 10:40 pm

Let me respond to each of your questions:

1. Production of expanded cork insulation is never going to come close to that of foam-plastic insulation. It will remain a niche product that appeals to those committed to the highest levels of environmental protection. Natural cork's loss of market share to synthetic wine bottle stoppers and screw caps increases the importance of creating new markets for natural cork to prevent cork forests from being converted from the biologically rich ecosystems that they are to other uses. One of my interests in promoting cork insulation is to help protect those cork forests. Despite occasional rumors about a die-off of cork forests, that is not occurring; there is no shortage of cork. The loss of market share to synthetic products, as I understand it, was more the result of effective marketing by manufacturers of the synthetic products.

2. In the full product review in EBN I addressed vapor permeance of expanded cork insulation. A 40 mm layer achieves 2.2 perms (I-P units). I don't have data on air barrier performance, but because the cork panels are fairly small (2' x 3' typically), taped joints of the cork or a separate sealed air barrier would be important in achieving airtight assemblies.

3. I hadn't been aware that there used to be an ASTM standard for cork insulaton or that it had been discontinued. Thanks for that information.

4. As far as I know, expanded cork insulation meets all relevant standards in Europe, including Class E fire safety performance. I do not know if those European standards will be enough to satisfy all code officials in the U.S. I will note, however, that when exposed to a torch (see photo in blog) a 40 mm layer of cork insulation will resist burn-through for over an hour, compared with less than 10 seconds for polystyrene, yet those products have the same fire rating--I consider that an indictment of our fire safety testing methods. As for durability, I've seen expanded cork insulation that was installed in the 1920s in a home in Brattleboro that is holding up very well. I believe the durability to be exceptional.