Despite its excellent durability, thermal, humidity and acoustic performance, cork remains underused in construction in the UK. It is also biodegradable, low in embodied carbon and has an attractive tactile quality.

South London Convenience Dive the architects cut its teeth in bio-sourced materials with its Camberwell Cork House, completed last year. The project is a single-storey ground floor extension to a London terraced house with cork cladding and insulation. Director of Delve and co-founder Alex Raher told us about the essentials of working with cork, including product specifications and cost, customer concerns, wall construction, complying with building regulations and the realities of using cork a natural material.

Why did you use cork in this project?

The brief focused on color, sustainability and eco-friendly materials. So we looked at natural terracotta floors, natural clay paints and then offered cork to customers. They really liked it and recommended us Study on cork by Surman Weston as one of their first inspiration images.

Is cork expensive?

It’s priced relatively well, around £150 plus VAT for 4 million.² of cladding panels and £80 plus VAT for 1m² of insulating panels.

Which provider did you use?

At the time there were only two cork suppliers available in the UK, Mike Wye and DLM coating. We could have used either one, but opted for Mike Wye for logistical reasons. They were incredibly helpful and had a very good technical team. We relied heavily on them when designing the wall builds and ensuring we achieved the right U-values ​​and breathability.

What products did you use?

The facade is covered Cork panel for facade MD and the insulation is cork board insulation. The cork itself comes from Portugal, where it is extracted from corks every nine years.

What are the aesthetic, environmental and technical characteristics of cork?

Cork has a beautiful tactile quality and is 100% natural, completely recyclable, sustainably grown and contains low embodied energy.

It is breathable, naturally water-repellent and, when used as part of an all-cork composition, has excellent thermal and acoustic performance. It has some limitations for use at higher levels or near the limits, due to fire performance and surface flame spread. It must be carefully considered in terms of the suitability of its application and whether flame retardant treatment is necessary.

What are the key data?

• Board size: thickness (20mm-100mm) x 1000mm x 500mm
• Density: 140 – 160 kg/m³
• Thermal conductivity: 0.043 W/mK
• Compressive strength at 10%: 220 KPa
• Water absorption by partial immersion: 0.17 kg/m².
• Fire protection: Euroclass E

Describe the construction of the wall

• Facade cork panel MD 40 mm (lamda 0.043)
• SecilTek Isovit E-Cork Fiber Reinforced Adhesive Fastener
• 140 mm expanded cork insulation panel (lamda 0.040)
• Insulated masonry fasteners
• SecilTek Isovit E-Cork Fiber Reinforced Adhesive Fastener
• Durox Supabloc 7 blocking 100 mm
• SecilTek Isovit E-Cork Fiber Reinforced Adhesive Fastener
• Internal insulating panel in expanded cork 80 mm (lamda 0.040)
• Insulated masonry fasteners
• Internal board and skimmed finish

What questions have you been asked by customers?

They wanted to know how the color of the cork would change over time, as they wanted the extension to be more gray than brown. We had to convince them that cork would turn “gray” with exposure for a year or two. And it is – we’ve been back ever since.

They also wanted to know the sustainability criteria, the origin of cork and its thermal performance.

Did the material present any difficulties in complying with building regulations and obtaining planning permission?

There weren’t too many problems getting planning permission as the neighbors were very supportive. However, we had initially considered options for a cork-clad loft conversion and dormer extension, which the planners decided against. The house is in a protected area and they were concerned that the material was very atypical for the area, so we removed it from the application.

Building regulations were more difficult because approved inspectors had never used the material before and didn’t really understand the structure of the walls. So there was a lot of back and forth to convince them that it worked and using the supplier’s technical sheets, and thermal calculations with Build Desk. If the cork skylight extension had been planned, we would have had to treat the material to improve its surface flame propagation performance.

Were the contractors safe to use the equipment?

We brought in the main contractor’s team, who had never used the product before. But they were confident about the installation with advice from the specialist supplier and support from us.

What was the key learning point from this project?

Cork is a fragile material and must therefore be treated with care, even after installation. The insulation is quite rough and ready to use and can easily be attached with the right fixings. But the facade panels are quite fragile since they measure 1 m high, 0.5 m wide and only 40 mm thick. A few of them broke during installation so we bought a few extras.

Interestingly, customers have noticed that in high traffic areas, if people lean on the facade, the corners can crumble a little. But that’s only part of using cork: you have to be careful with it and you’re going to get some chipping. However, the panels are replaceable if significantly damaged.

What advice would you give to architects considering using cork?

You should carefully consider the suitability of the application, including the entire construction and performance of the wall, rather than just the cork in isolation. It is a unique and natural material and care must be taken in how it is installed.

Also rely heavily on suppliers, as they are very familiar with the product and performance benefits.