Delve Architects on the nitty-gritty of working with cork

Despite boasting excellent sustainability, thermal, moisture and acoustic performance, cork remains underused in UK construction. It’s also biodegradable, low in embodied carbon and has an appealing tactile quality.

South London practice Delve Architects cut its teeth on the biobased material 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. Delve director and co-founder Alex Raher spoke to us about the nitty-gritty of working with cork, including product specification and cost, the clients’ concerns, wall build-ups, meeting building regulations and the realities of using a natural material.

Why did you use cork in this project?

The brief was about colour, sustainability and environmentally friendly materials. So we looked at natural terracotta floors, natural clay-based paints and then suggested cork to the clients. They really liked it and referred us to Cork Study by Surman Weston as one of their early inspiration images.

Is cork costly?

It is relatively well priced at around £150 plus VAT for 4m² of cladding boards and £80 plus VAT for 1m² of insulation boards. 

Which supplier did you use?

There were only two cork suppliers available for procurement in the UK at the time, Mike Wye and DLM Cladding. We could have used either but went with Mike Wye due to logistics. They were incredibly helpful and had a very good technical team. We leaned on them quite heavily when designing wall build-ups and to ensure that we were hitting the right U-values and breathability.

Which products did you use?

The facade is clad in MD Facade Cork Board and the insulation is cork board insulation. The cork itself is sourced from Portugal where it is extracted from cork trees every nine years.

What are cork’s aesthetic, environmental and technical characteristics?

Cork has a beautiful tactile quality and is 100 per cent natural, totally recyclable, sustainably grown and contains low embodied energy.

It is breathable, naturally water repellent and, when used as part of a full cork build-up, has excellent thermal and acoustic performance. It does have some limitations for use on higher levels or close to boundaries, due to the fire performance and surface spread of flame. It needs to be carefully considered in terms of its suitability of application and whether fire retardant treatment is required.

What is the key data?

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

Describe the wall build-up

• 40mm MD Façade Cork Board (lamda 0.043) 
• SecilTek Isovit E-Cork fibre-reinforced adhesive fixing 
• 140mm expanded cork insulation board (lamda 0.040)
• Insulated masonry fixings
• SecilTek Isovit E-Cork fibre-reinforced adhesive fixing 
• 100mm Durox Supabloc 7 blockwork
• SecilTek Isovit E-Cork fibre-reinforced adhesive fixing 
• 80mm expanded cork internal insulation board (lamda 0.040) 
• Insulated masonry fixings
• Internal board and skim finish

What questions were you asked by the clients? 

They wanted to know how the colour of the cork would change over time as they were keen for the extension to be more grey than brown. We had to convince them that the cork would ‘grey’ with exposure over a year or two. And it has – we've been back since.

They also wanted to know about the sustainability criteria, where the cork was sourced and its thermal performance. 

Did the material present challenges in meeting building regulations and obtaining planning permission? 

There weren’t too many issues obtaining planning permission as the neighbours were quite supportive. However, we originally looked at options for a cork-clad loft conversion and dormer extension which the planners pushed back on. The house is in a conservation 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 challenging because the approved inspectors hadn’t used the material before and didn’t really understand the wall build-up. So there was quite a lot of back and forth convincing them that it did work and using the supplier’s data sheets, and thermal calculations using Build Desk. If the cork dormer extension had made it through planning, we would have had to treat the material to improve its surface spread of flame performance.

Were the contractors confident in using the material? 

We used the main contractor’s team, who hadn’t used the product before. But they were confident about installing it with guidance from the specialist supplier and support from us.

What was a key learning point from this project? 

Cork is a fragile material so needs to be treated carefully – even after installation. The insulation is quite rough and ready and can easily be secured with the right fixings. But the facade panels are fairly flimsy as they’re 1m tall, 0.5m wide and only 40mm thick. A couple of them did break during installation so we bought a few extras.

Interestingly, the clients have noticed that in higher traffic areas, if people lean on the facade, the corners can chip away a little bit. But that’s just part of using cork – you have to be careful with it and you're going to get a little bit of chipping. However, panels are replaceable if they do get significantly damaged.

What advice would you give to architects thinking of using cork?

You need to carefully consider the suitability of the application, including the whole wall build-up and performance, rather than just the cork in isolation. It's a unique and natural material and care needs to be taken with how it's installed.

Also, lean heavily on the suppliers because they know the product and performance benefits really well.