Is Cork the Perfect Green Building Material?

CC BY 2.0. Cork tree in front of factory/ Lloyd Alter

It's all natural, renewable, healthy and has zero embodied carbon. What's not to love?

When speaking recently at a Passivhaus conference in Aveiro, Portugal, I mentioned one of my favourite subjects, embodied energy, and noted that cork, most of which comes from Portugal, has the lowest embodied energy of just about any insulating material, and was in many ways the perfect product.

A representative of Amorim Isolamentos was present at the talk, and arranged for me to have a tour of their factory, an hour out of Lisbon, where they make cork insulation.

Cork factory

Lloyd Alter/CC BY 2.0

Alorim has been in the cork biz since 1870, making corks for wine. During the 1973 oil crisis people finally started seriously worrying about insulation in buildings, even in sunny Portugal, so they started producing cork insulation in larger quantities.

The process of turning cork bits into blocks of insulation was discovered accidentally by John T. Smith at his New York life jacket factory, where a metal cylinder full of cork chips was accidentally left over a hot burner. The next day he noticed that the contents had fused together into a solid chocolate-brown mass. He patented the process of making "Smith's consolidated cork", which has no additives or chemicals other than the natural resin called Suberin.

cork for bottles

Cork thick enough to punch out bottle corks/ Lloyd Alter/CC BY 2.0

Wine-quality cork comes from the lower part of the tree, and after the corks are punched out of the slabs, the rest is used for insulation. They also take the thinner cork and the stuff from the branches that is not suitable for wine corks. The trees are harvested every nine years and the entire process is rigidly regulated; chop down a cork tree and you go to jail. The industry employs 15,000 people plus another 10,000 in the 5.2 million acres of cork oak forests.

mountains of cork

Mountains of cork drying out/ Lloyd Alter/CC BY 2.0

Making cork insulation is a fascinating, simple yet sophisticated process. First the cork scraps and pieces are stored in mountains for six months.

wine corkes in the mix

Wine corks mixed in with new cork/ Lloyd Alter/CC BY 2.0

The company also buys back wine corks for recycling and throws them into the mix; it doesn't make a lot of economic sense, shipping containers full of old corks around the world, but keeps them out of landfill, which is of course the right thing to do.

cork dust burning

Cork dust burning in furnace/ Lloyd Alter/CC BY 2.0

The dust and the waste is all sent to the boiler, which makes the steam required for the process, so it is all running on biomass. This is supposedly carbon neutral but it is not pollution free, and I choked a bit on the cork smoke, but we are out in the country.

cork pellets in Lloyd Alter's hand

Lloyd Alter holding cork pellets/CC BY 2.0

The cork pellets, like these I am holding, are then fed into a chute and fed into forms, where under high pressure and temperature from the steam, the suberin resin fuses the pellets of cork together into blocks. There is nothing added; it is all natural.

You can see in the video the cart coming up to the press, the hydraulic ram pressing down, then the cork block coming up and moving onto the cart. It then moves off through a cooling chamber where it is sprayed with water, and then taken to a cooling rack.

cork cutting machine

Cork cutting and packing machine/ Lloyd Alter/CC BY 2.0

The cork blocks are then sent to another building where they are squared and sawn into sheets as ordered by the customer.

cork socks

cork socks/ Lloyd Alter/CC BY 2.0

There are many uses for cork beside just sheets. Pellets of a smaller size are put in socks and used to surround and then absorb oil spills. The socks float, soak up many times their weight in oil, just get squeezed out and used again.

cork powder

Cork powder/ Lloyd Alter/CC BY 2.0

One of the most interesting products is this really fine, 1mm cork that is mixed with plaster to make a light, insulating and breathing plaster coating. Cork is antibacterial and helps with air quality; I could see this being very useful in interiors on top of cork insulation instead of drywall.

Carlos Manuel in front of wall

General Manager Carlos Manuel with wall/ Lloyd Alter/CC BY 2.0

Here's General Manager Carlos Manuel in front of a sample wall built up from cork, mesh, and plaster mixed with cork powder.

It's amazing stuff with amazing properties.

Cork doesn't burn just chars a bit.

Cork over flame, doesn't burn through or catch fire/ Lloyd Alter/CC BY 2.0

Although cork is rated in the EU with a Class E rating, the same as plastic foams, it doesn't really burn. Here they are demonstrating a flame underneath, and General Manager Carlos Manuel putting his money, his cigarettes and even his head on top. Meanwhile, a piece of foam plastic burned through in four seconds.

Cork in water

Cork after being soaked in water for days/ Lloyd Alter/CC BY 2.0

Unlike a lot of other fiber insulations, there is no capillary action sucking up water if it gets wet. This is after days of floating and there is almost no absorption.

cork getting squeezed

Cork getting squeezed/ Lloyd Alter/CC BY 2.0

It is not incompressible, but doesn't compress much. The sides don't bulge out, which is important if one spot gets pushed in. When the pressure is removed, it pops right back.

50 year old cork from cooler

50 year old cork from old cooler/ Lloyd Alter/CC BY 2.0

This is really in so many ways, the perfect insulation, the perfect building material. It lasts forever; this pile of cork is recycled from a 50-year-old industrial cooler. It is totally natural and has an embodied carbon of almost zero. It is healthy, free of flame retardants. It is sound absorbing, antibacterial and easy to install.

Iberian Lynx

© World Wildlife Fund

The cork industry is local with trees all within 30 km of the factory, the trees are protected, the industry employs thousands and provides habitat for that cute Iberian lynx. It is hard to think of anything wrong with it, other than it isn't local and requires shipping, and the biggest problem: it costs about twice as much as plastic foams with the same R-value.

Carlos Manuel with Leo Park

Leo Park of Corkworld with Carlos Manuel/CC BY 2.0

It was truly a wonder, wandering a few dozen meters from tree to factory to warehouse full of plastic wrapped insulation ready to be shipped. It is all so yummy and green. But can they meet the demand? Does it scale? Can we afford it?

This is the fundamental problem we face in green building. We need to build and rebuild millions of housing units, but we need to do it in a way that doesn't cause a big carbon burp from concrete and plastics. We need healthy materials that don't cost the earth. That means using more wood, and more natural materials like cork. It means being willing to pay a premium for materials with all these benefits.

cork tree closeup

Lloyd Alter/CC BY 2.0

With new irrigation technology, Carlos Manuel tells us that he can have cork trees producing within ten years; they should start planting like mad right now.