Hempen Archives - House Planning Help

Architect Paloma Gormley of Practice Architecture explains how versatile hemp can be as a building material, and how plastic use can be reduced on site.

Interview with Paloma Gormley

Paloma and Lettice Drake’s Practice Architecture made their name when they were approached to design a roof-top cafe above a car park while in the third year of their architecture degree. Not knowing where to start with appointing a contractor, they decided to build it themselves. From being incredibly inexperienced and using power tools they’d bought from a supermarket, they successfully constructed “Frank’s Cafe” which became an incredibly popular venue.

Frank’s Cafe and Campari Bar, Peckham. Paloma Gormley and Lettice Drake

Being closely involved with the construction process is a theme which has continued throughout their approach to architecture.

Hemp is a versatile plant

Industrial hemp is a plant within the cannabis family that has very low THC – the psychoactive element of the plant. It’s legal to grow in the UK with a licence and has a multitude of uses, with the fibre being used in rope and linen, the seed which has nutritional value, the bud which can be used medicinally, and the shiv which is the stalk of the plant that can be combined with clay to form hempcrete.

Hemp also sequesters carbon more than most other plants. As it’s growing it sequesters carbon into the body of the plant and also into the soil.

Paloma’s interest in using it as a building material began with a friend’s group, Hempen, that had set up a hemp farm in Oxfordshire. The friend had suggested using hempcrete as a building material for a three-storey London self build house they were working on, and it turned out to be a very practical material for the design. It was cast in-situ and infilled between a Douglas Fir frame.

Drying time issues can be overcome by using pre-fabricated cassettes

Paloma agrees that there can be an issue with drying and curing times when using hempcrete, depending on the binder that is being used. Some of the more reliable binders have a cement content, which isn’t ideal, but those that are pure lime can be more tricky to use as they can set too fast or slow or don’t hold.

While it can set in 24-48 hours depending on the binder, the drying times can be incredibly long. They can average at around three months for the building to completely dry out depending on temperature and air flows, meaning secondary finishes can’t be applied during this time.

Another limiting factor can be that it is not ideal to be casting hempcrete on-site during the winter period. There’s a risk that there can be too much dampness in the wall for too long before it has a chance to dry out, causing all kinds of problems.

One solution is to take the wet process off-site. This means the drying can take place somewhere else and then delivered to site as dry cassettes ready to use. The benefits are a more streamlined construction process, with it being cleaner, less complicated, no time spent waiting about and all twelve months being available for building.

Hempcrete Cassettes – Image Credit Paloma Gormley

Hemp is being used innovatively at Flat House, Margent Farm

Margent Farm in Cambridgeshire are an organisation that are growing a 40 acre hemp farm. They carry out R&D and are developing a number of hemp-based bio alternatives to plastic products.

The brief for the farmhouse on the site was that it needed to have a good connection with the landscape, have very low embodied carbon and to avoid plastic use wherever possible.

Plastic in its various forms is prevalent even on buildings with high ecological credentials, being used as polyurethane rigid foam insulation, vapour barriers, sticky tapes, expanding foam, nylon thermal bricks and gaskets. It can also be other things invisible to the eye such as the wiring, ducting, pipes and vents.

Hemp cladding has been used

It wasn’t possible to achieve the elimination of plastic completely, because of barriers in terms of cost and also in terms of alternative products just don’t exist. But overall, Paloma said they did pretty well.

They achieved it partly by returning to what is known as breathing construction. This takes moisture from the air inside a room, absorbs it into a wall and then breathes it out the other side.

“There are many reasons why that’s a really good thing. It limits condensation, you can have a self-regulating moisture content in a room because if the moisture content in a space gets too high, it just gets pulled into the walls and then evacuated, and it means that when something goes wrong, you know it’s gone wrong because breathing construction generally involves a much more direct relationship of material.”

Achieving a high level of airtightness hasn’t been a key concern on this project. The main priority has been to make a well thermally insulated, incredibly low embodied carbon building.

“So, you always have to have a level of background ventilation which we used to achieve by just having slightly leaky buildings. Now we’re making incredibly airtight buildings and then making intentional holes, which I think is a funny logic in a way, because so much generally plastic goes into making incredibly airtight buildings and then we’re punching holes in them.”

One of the products developed from the crop at Margent Farm is the cladding that they are using on the farmhouse. It is a hemp fibre with a sugar based resin derived from agricultural waste. They are combined, pressed together and baked in a heat moulding process.

The circular economy is informing design

Cost concerns meant that aluminium windows were chosen over timber, and although it is a very high embodied energy material, the way it can be recycled means it fits in well with a circular economy way of thinking.

Getting the foundations right was one of the biggest challenges on the project, and they opted for concrete foundations with a suspended timber joisted floor. Since then, they have been thinking of alternative options for future projects. One possibility would be screw piles, which are dependent on the type of soil you have and can’t be used in every situation. The thinking here is again about the circular economy, where these could be unscrewed and used on another project in the future.

Another option they’re excited by is compacted hardcore such as flint or stone which could be sourced very locally.

A cassette system was used for the walls, based around an eight by four sheet made in a factory as boxes, filled with hempcrete and dried. They were brought to site and erected within two days.

The roof was a straight forward joisted construction, with a wood fibre insulation.

The Flat House led to the establishment of Material Cultures

Material Cultures is a not-for-profit organisation that explicitly researches and explores these different materials in the realm of off-site construction. The aim is to make them more accessible, available and affordable. They are looking to start manufacturing at a larger scale, drawing on the model of the Flat House.

Material Cultures are also working on research projects with two universities and are developing another prototypical building with architecture students at University of the Arts London, all feeding in to the broader research.

“So, it feels very important to us that this project and these processes happen in a very reflective way, that we’re constantly learning and moving and developing thinking.”