Cork: a different kind of bark
The bark of the cork oak tree (Quercus suber L) grows in Mediterranean regions such as Spain, Italy, France, Morocco, Algeria and, most particularly, in Portugal, where there are more than 720 thousand hectares of cork forests.
This bark – more commonly known as cork – is a material with amazing qualities. It is strong, durable, provides thermal and acoustic insulation, and is resistant to fire, water and rot. The Cork Oak replaces its bark every 9 years, which means that it can be harvested without damaging the tree. The tree itself can live for up to 250 years.
Cork blocks are produced from left over pieces of the wine cork making process. The discarded cork granules are too resinous to be used for wine corks, but are perfect for making into insulation blocks. By heating the granules they expand to fill a mould, and their natural resin is released, binding the granules into a solid block form. The largest sizes currently available are 1000 x 500 x 300, and they can be cut to size or milled.
We wanted to test the material to see what it could do, and how it would perform against conventional building blocks. Our testing procedure was a little on the Heath Robinson side and we therefore offer no guarantees or recommendations, but we hope that this might inspire others to carry out more formal testing.
Water For our first test of these cork blocks, we poured a glass of water onto the surface to test the waterproofing. To our dismay and bemusement, we found that the water drained straight through like a sieve; though cork granules themselves are waterproof, the gaps left between them following expansion are large enough to permit water. Discussions with the manufacturer led to the discovery that the blocks can be compressed further, resulting in a block that is almost twice as dense (a 300mm thick block compresses down to 180mm). This high-density block is almost completely watertight, and proved capable of holding an upturned glass full of water for a week with only a minor amount of seepage.
Spraying water at the side of the block to simulate driving rain did not result in any water getting through either.
These two tests established that a solid cork wall could theoretically keep out water in the same manner (if not better) than a solid brick wall. Both are partially porous, and yet both can keep out rain; the key is to ensure that water does not sit on the surface, but is instead guided away using sills and drip details.
Fire A large piece of granulated cork was left over a candle for a prolonged period to establish the fire proofing. The material did take initially take flame (possibly dust on the surface), but within seconds self extinguished, and would do nothing other than smoulder. The flame did not catch to other parts of the block. We wish to make clear again that this is not an official fire test, and we would recommend carrying out formal testing before implementing this on any larger scale.
Rot Left over wine corks were placed in the compost bin the year before, and were still fully composed the year after. This seemed pretty good. A typical wine cork used for years in bottles also seemed to provide good evidence that cork does not rot or wick water over time.
Planning Permission and Building Control
For the sake of the experiment, we wanted to avoid the time and cost of planning permission so kept the proportions of the building within permitted development requirements (2.5m eaves height). Eaves height is defined as the vertical distance from the eaves to the highest point of ground adjacent to the building.
As the building was partially dug into a slope, it was possible to keep the official building height to 2200mm whilst maintaining a comfortable internal height of 2400mm. Through a quirk in the planning system we have therefore created a real life Cork-Tardis, which is bigger inside than it is outside.
The Building was exempt from Building Control, as the floor area was 13.2m2, i.e below the threshold of 15m2. We cannot think of any reason why it would not pass formal approval, though this would probably need some more formal testing.
The site chosen for the build already had an existing resident, in the form of a young sycamore tree. We decided that as it was here first, it had priority, and so we built our building around it, leaving the growing tree exposed within. Given that the rest of the building was made of bark, it seemed a fitting feature.
Our research into tree-integrated buildings found that trees do not mind their trunks being inside a building, though it is almost impossible to waterproof around a tree without water ingress and without damaging the tree. Most sources seem to advise against doing it all together, but we decided to sidestep the problem. Rather than completely waterproofing the tree, we opted instead for a rubber gasket which keeps out heavy rain from the building but still allows some water to trickle down the trunk, allowing the tree to self-water. A small square of floor has been left cut-away around the tree, and filled with granulated cork bark for natural drainage.
The cork studio is much lighter than a conventional build, and so we were able to build on top of the root protection area without damaging the tree.
In levelling the ground we also disturbed some other residents – namely an old television, telephone, shoes, a 1990’s can of Fanta, and a general rubbish pile which appeared to have been hidden under a mound of soil in veritable land-fill style. Though it was perplexing to think how a previous land owner could have brought him/herself to bury rubbish in a hole in the garden, we felt that it is no worse than the contemporary past-time of throwing rubbish ‘away’ and polluting other people’s soils. We sorted/recycled and removed all of the rubbish leaving the soil as clean as we could. It felt quite fitting that a zero waste building. would take the place of a former landfill.
We thought that we would put the no-rot theory to the test on a bigger scale. As a result, the cork blocks are laid directly on the ground, without a concrete screed or a damp proof membrane. Our only concession was to spend £20 on a few bags of sand, without which it was difficult to get an even surface.
Ground floor slab
The cork raft was tessellated from standard 1000mm x 500mm x 180mm slabs of cork. Our initial plan was to fix the blocks together, but found that providing the ground was made flat, the blocks held together well without gaps. The blocks are made perfectly square on the edge, and provide a degree of friction to one another, making for a cohesive slab. The blocks are solid enough to stand or jump on, warm to the touch, and very slightly springy, making them much more comfortable to walk or sit on than a timber floor. The material is washable, and the dark colour and material properties means that it is resistant to stain.
Most cork floors are sealed using polyurethane varnish. Not only is this high embodied energy and toxic, it turns a natural material into a wasteful composite that cannot be readily separated or recycled (see cradle-to-cradle below). We have left ours natural. The denser block is the best as a hard-wearing floor surface, however, we opted for the lower density cork which is better for insulation, with a rug to prevent surface wear.
The walls were made of the higher density cork blocks, cut to size and overlapped in a stretcher bond. We found that the most effective blocks were 1000mm x 250mm x 180mm (2 of these can be pre-cut from a standard 1000m x 500 x 180mm block) as these were easier to handle than the standard blocks, more resistant to lateral loads,and could be fixed together with 300mm insulation screws (see below).
We used the cork like giant steps to create access decks as we worked. They provided a solid stable base to work from, and could be used as steps, given that the width of a block is 180mm. We built the walls to chest height before adding new blocks to stand on, which meant that we always had a safe hand rail to stand up within. There is no need to access the outside face of the block during construction or afterwards for maintenance, so the blocks can be built right up against the boundary. As we only had a limited number of blocks, We borrowed a small access plaftorm for installation of the last roof pieces, but otherwise did not need to use any scaffolding. If you were building multiple units, you could easily pool a few spare blocks to stand on.
The longest standard fixings we could find were Spax 300mm Wirox screws. These are wide threaded, and pull into the high density cork really well. They only just about gripped in the low density, though the flange-head allows you to pull them tight against another material (whether timber or high density cork). The fixings were installed vertically down the centre line of the wall, so that they were perpendicular to the direction of heat transfer and therefore did not create a thermal bridge. We used a laser to set out the building, but that wasn’t strictly necessary as the blocks are all square. The best advice we would give is to make sure the ground floor slab is levelled and then check the wall for verticality with a spirit level as you go up.
As a result of the permitted development rules and building location, we were required to install a flat roof, which we also decided would be more cost effective, and easier to join.
A simple series of left over timber joists was used to provide support for the roof blocks, and also helped stiffen the building. The roof blocks were made of low density insulation, as it was the best thermally, and would not need to resist moisture due to the Butyl lining. We were able to screw through the blocks and into the frame, which also provided a degree of racking to the structure.
Windows were a difficult choice. Glass is recyclable, but is heavy, expensive and not inherently good at providing insulation. Double / triple glazing units are good for thermal performance , but sealed with butyl, which makes them difficult to recycle. In the end we opted for multiwall polycarbonate, which is a petrochemical product, but is ‘pure’ and can be recycled fully at the end of its life. When fitted with a rubber edge seal and galvanised angle drip detail (which can be taken apart at the end of its life), the windows are heavy enough to ‘self weight’ onto the roof without water getting in, but light enough that they can be lifted by hand, eliminating the need for hinges and catches.
The door was a single piece of pine plywood, with 40mm thick high density cork screwed to the outside face in a ‘secret nail’ fashion, so that they are not visible on the surface, and also so that the whole door is able to disappear into the pattern of the cork wall. Water has not permeated through to the inside face in the in use testing to date (Summer, Autumn, Winter). The door handle is made of an old corkscrew.
Internal plywood shutters were fitted to provide additional control of light, and also as an additional buffer against temperature. When they are closed, they are thicker and therefore much more effective than curtains or blinds at controlling light or temperature. The sliding mechanism for the shutters rails was created using re-purposed galvanized plaster stop-beads. The beads have predrilled holes which makes them easy to fix to the timber and they run against one another quite smoothly.
The building has been built in a manner inspired by the cradle-to cradle principles set out by William McDonough, though it has not (as yet) received endorsement from him. To paraphrase cradle-to-cradle, each element of the building is natural / compostable material, or if man made, built using a 100% pure material than can be easily recycled. No glues tapes, or other bonding materials have been used to connect the materials, only mechanical fixings which can be taken apart at the end of their useful life.
Internal finishes, paints, tapes, coatings, chemicals, varnishes, sealants, VOCs
The full cost of the building was £6057 Ex VAT. The floor area or GIA was 13.2 metres, which gives a full cost of £459/m2. This prototype was built in evenings/weekends/holidays and so there was no labour cost, but to give some estimate, if we were to do the build again from scratch (i.e. without prototyping along the way) it would be possible for 1-2 people to build it within one week.
|Cork blocks cut to size (Cork Link)
|Levelling sand (Jewson)
|300mm x 6.0 Spax wirox flange screws (Screwfix)
|Butyl Pondliner – used as a roof membrane (Butyl Products)
|35mm Multiwall polycarbonate for windows (Birchwood Trading)
|Gate hinges – used as door hinges (Jewson)
|Galvanised Equal angles – Used for drip edges and window flashings (Metals4u)
|Galvanised plaster stopbeads – cut and used as sliding rails for shutters (Jewson)
|Pine plywood for door and internal shutters (Jewson)
|Leftover LVL timber for window/door frames and ceiling joists (Steico)
|GIA floor area
|Cost/m2 (Ex VAT)
In Use Testing
The best way to find out about a building is to spend time in it, and one of the Bark directors has kindly volunteered for some prolonged in-use testing. Almost a year in, the building has survived summer, rain, and the ‘Beast from the East’. More detail to follow in the future.
The furniture shown in the photographs is made of U-Build boxes. Click here to find out more.
With thanks to
The idea for using Cork as a structural material came from the inspirational Self Builder Paul Troop, who used a less dense version as ground floor insulation in the house he built from scratch in Bicester 2018.
The book Cradle-to-Cradle started our thinking process into the creation of a building that could be 100% recycled or composted.
Pro-bono advice on structure and assistance with the build was offered by good friend André Siwek.
Family, friends and partners helped with the preparation, especially Chris, Shelagh, Claire and Lydia.
Advice about Cork material properties was provided by Charles Cutler of Cork Link, who also supplied and cut the cork blocks to size (and provided the cork information above). Click here for their write up about the project.
Lenny Codd who captured photos of the finished building.
Friends André and Claire, also pointed out that the entire internal (and external) surface of the building was a cork board, and provided a box of celebratory drawing pins for the official opening in March 2018.
Some ingenious members of Studio Bark, who made a celebratory cork top-hat from an old roll of cork flooring, for the opening.
And finally, thanks also go to Franz the cat, a neighbouring feline who was caught posing on the wall and in the final photos.
Though the building was conceived as zero waste, we were saddened to find that the cork and window materials had both arrived in plastic wrapping. We understand why people use plastic film, but it seems about time that a better widespread solution is found. In the meantime we would be willing to accept a few bumps and scuffs to omit the plastic, and are working with our suppliers to find alternative solutions. We spoke with Corklink, who explained that they can supply the cork in cardboard boxes in the future, or if we order a container full, they can just load the blocks with no packaging. This is the kind of discussion that can lead to better solutions all round, saving on money and waste.
Other cork creations
In discussing this project we have become aware of others working to show that cork could we used more widely as a construction material.
This company is making novel bricks out of cork which fit together a bit like lego.
A team from UCL is working on a different interlocking thermal envelope made of cork.
Surman-Weston’s Cork Study is clad in cork and lined internally with a timber frame and plywood. We think this provides a really interesting contrast against the cork, and greater flexibility in terms of the structure.
We wish these teams all the best and follow their work with interest.
If you are planning your own cork project and would like some design support from us, please contact [email protected]