Expanded Cork – Environmentally Sound Rigid Insulation

cork oak tree

Quercus Suber, AKA the cork oak tree pictured here in sunny Portugal. (image: Hannes Grobe)

Cork is an incredibly versatile material, including for use as insulation. Here’s how cork insulation is made, and why it’s the ultimate green choice.


All insulation has some earth friendly properties in the way it stops or reduces air infiltration in your home. It lets warm air stay inside in winter and summer heat stay outside where it belongs. Insulation saves money and natural resources. Wasted energy for heating and cooling is not environmentally friendly.

That’s the end of the green road for rigid insulation, though. There are three kinds, and all are petroleum based. So no matter how high an R-value (polyiso has a kickass R-value of 6.5 per inch!), they still have a dirty supply chain.

From growth through installation in a home, cork uses little energy. Its entire cycle from growth to expanded cork insulation is 100% natural and carbon negative, making it the most eco-friendly insulation.

The cork oak tree

Cork comes from the bark of the cork oak tree, Quercus suber. All trees are carbon sinks, taking CO2 from the air, and converting it to oxygen. The cork tree holds much more carbon than it uses for oxygen, approximately 4.8 million tons per year. That’s a lot of sequestered carbon!

Cork oak is native to regions of the Mediterranean Sea. Almost 2/3 of the world’s cork is grown in Portugal.

Cork trees are Forest Stewardship Council (FSC) certified, which requires that forests and plantations are responsibly managed. The rights of indigenous people, local workers, and local communities must be respected, too.

Cork forests are also federally protected. Cutting them down is illegal. Even cork tree farmers need to get approval to dig up trees that are diseased or past their prime. The cork oak is the national tree of Portugal.

Harvest and manufacture

When a tree is about 25 years old, the bark is harvested by hand with locally made axes. This method has been used for 2000 years, and the skills are passed down to each new generation.

A tree is harvested every nine years throughout its 200-year lifespan, which amounts to about 15 times. It’s the only tree that can have the bark removed and survive. That’s sustainable!

Only 30% of harvested cork is used for wine bottle corks. Other cork products, such as insulation, bulletin boards, and boat decks, are made from the waste. Using a byproduct scores another point on the eco-friendly scale by keeping waste out of the landfill.

By watching some videos of an expanded cork insulation manufacturing plant, Amorim Isolamentos, S.A., I learned that there is no waste in the plant. The cork is cleaned and sorted. Cork dust and unusable scraps become biomass that powers almost 100% of the building and the equipment that makes expanded cork.

The remaining cork granules are heated with steam in an autoclave for 20 minutes, and they expand 30%. The heat releases a natural binder in the cork that holds the expanded granules together. No extra glue is needed, so there is no off-gassing of the final product.

The blocks, or billets, are cooled and stabilized with water. All the exhaust from the plant is water vapor. Nothing toxic comes from their chimneys.


Pallets of cork insulation are wrapped in recyclable plastic and shipped to its final destinations, most of which are far from Portugal. The fuel and CO2 emissions of shipping seem to kill its green properties.

But wait!

Cork products are loaded onto cargo ships, which is actually the most fuel-efficient, overseas transportation method. Many products are shipped at once, making the large carbon footprint of the boat very small for each product. Shipping via cargo plane would have a very negative impact.

In the home

Expanded cork insulation has an R-value of 3.6 per inch. Sheets can be ordered in the thickness needed. It is suitable for walls, floors, ceilings, and exteriors under siding.

It is naturally fire resistant. Testing showed it would frequently take three hours to burn through a 1” thick sheet.

Cork is all natural, so there are no toxic chemicals to off-gas. It is also mold and mildew resistant. It scores big for good indoor air quality.

Cork also has good acoustic qualities, buffering noise and vibrations.

Expanded cork insulation qualifies for LEED credits, because of its renewable, recyclable, and low emissive properties.

The most sustainable insulation

I’m in love with cork! From its growth habit to how it powers the processing plant to its performance in a building, there’s nothing to not like about it!

The CO2 emissions created are in the short transport distance from the field to the factory, from the factory to the distributor, and from there, to the building site. The environmentally conscious growth, harvest, and manufacture of cork, though, more than offset that, making expanded cork a carbon negative product in the end.

No petroleum-based insulation can accomplish that. Cork gets my vote for #1!

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