22 June 2020 · By Rob Wilson

Film director-turned-hemp farmer Steve Barron commissioned Practice Architecture to help him ‘grow’ a prototype house using his own crop. The result is Flat House, an ultra-low embodied carbon building the team is now looking to reproduce at scale. Rob Wilson interviewed Paloma Gormley, partner at Practice Architecture, to find out more. Photography by Oskar Proctor

Could you describe the background to the project? Who is the client and what was the brief? How did you put the project team together?

Practice Architecture was already working on a number of projects involving hemp and natural materials when we were introduced to Steve Barron by the owners of Hempen, a hemp farm in Oxfordshire. Steve wanted to ‘grow’ a house and establish a materials research facility. We worked with him while he looked for an appropriate site, testing soil quality and assessing the viability of development. 

The first 20 acres of seeds were sown in May 2017 and the crop was harvested at the end of that summer. The hemp had to be processed to separate the different elements of the plant – seed, fibre and shiv (the chopped woody core, used to make hempcrete) – and we set out to develop a design that integrated the different materials produced. Steve established Margent Farm as a business and began researching and developing bio-based alternatives to plastics, integrating the materials grown at the farm. A team began to assemble on the agricultural side, Hempen advising on cultivation and two local farmers joining to help grow and harvest the crop. 

At the same time Practice Architecture began drawing up and testing the prefabricated system from which the design of the house emerged. We contracted local engineers and M&E consultants to advise on the structure, photovoltaics and biomass boiler. As the project moved to site we worked very closely with the construction team, who were a combination of local builders and long-term collaborators.

What was your overall design approach? You’ve described the project as ‘ground-breaking’ in its levels of embodied carbon. Can you explain how this was achieved and the key technical innovation in the design of the house that enabled this?

The design is driven both by the choice of material and the construction rationale. Our starting point was the raw materials from the farm. The body of the house is entirely made of materials that have been grown. 

Plants sequester carbon as they grow, so if large transport distances aren’t involved, plant-based materials store more carbon than they produce. Hemp is fast-growing and better than commercial forestry at sequestering carbon – and its long tap roots help to regenerate the soil. Beyond the relatively light processing and transportation emissions, the materials can be understood to be carbon-neutral (or carbon-negative if you count the sequestration), which is a vast improvement on steel, concrete, or petrol-based insulations. 

Compared with conventional construction, the building is radically low in embodied carbon. It’s important to remember that carbon calculating is a fairly blunt tool when it comes to designing for ecology and we really need to be going much deeper in understanding the impact of the things we make. Crucially, bio-materials can be non-toxic, they don’t give off gas and can be returned to the ground in the form of mulch at the end of their life cycle. I’ve begun tentatively using the phrase ‘circular ecology’, an attempt to describe a construction logic in which buildings emerge from, and then reintegrate into, the landscape. 

Can you explain the choice of structural and prefabricated system and of the materials used? How did you source or manufacture the main elements of the house?

The building is composed from a panellised system that we designed with a mind to scalability. Hemp construction is limited by the seasons, because you can’t cast hempcrete out of doors in the winter. The panels are factory-made and dried off-site so construction can take place at speed and year-round. 

The system uses timber I-joists with a hempcrete infill and is based on the dimensions of a standard 8ft x 4ft sheet. Hempcrete, which is a mixture of hemp shiv and lime, provides wall mass; it is both insulating and thermally massive. All of the shiv used in the house was grown on site and we used the only timber I-joists produced in the UK. There is also a layer of Steico Woodfibre insulation that forms a woolly coat over the structure beneath the cladding. The building breathes, allowing moisture to pass through the wall freely and removing the need for membranes. We have avoided the many discrete layers that evolved from a petroleum-based construction culture and the wall build-up is very simple. The exciting thing is that hemp can be grown anywhere and the materials produced across the country. 

We worked with Will Stanwix, a hempcrete specialist, and carpenter Oscar Cooper to refine the details of the design and put the panels together at HG Matthews’s Brickworks in South Buckinghamshire. The panels are structural and self-supporting, with integrated sockets and service runs. Our sister organisation, Material Cultures, is developing the system for a broader market.

Where is the site and how does the design sit within its wider context? Were there any planning issues?

The building is located just outside Pidley, a small village in Cambridgeshire between Huntingdon and Ely. As a comical nod to the flatness of the surrounding landscape, the area’s gentle undulations have led to the establishment of Pidley Mountain Rescue. 

The small arable farm had an existing collection of agricultural sheds, including a large, open pole barn and long, yellow brick store. The buildings nestle into the landscape and are barely visible from the road. The house has been built with prior approval under Class Q permitted development rights, meaning we had to retain the volume and frame of the existing structure. Beyond this, the planning policy is reasonably loose and there weren’t any prescriptions about materials or openings. 

The CASS Studio [see AJ Buildings Library] that was built subsequently at the farm made use of the Caravan Act, which again is open to interpretation. Both buildings are reasonably light on the ground and invite a relationship with nature and surrounding context by blurring the edges between the two and through their fabric.   

Describe the house’s layout. How was this developed in response to the brief?

Steve wanted a modest and practical family home, somewhere that would help facilitate the work he wanted to do, the cultivation and processing of the crop, and their product development. He wanted a building that spoke to the principles of Margent Farm, that was experimental and that could act as a demonstration of the viability of hemp as a construction material. It was Steve who initially suggested prefabrication, seeing it as a route towards a mass market.

The development of the design was guided by Steve’s intentions and enabled by his trust and willingness to take risks and to experiment. The project felt very much in line with the way we work and we were really excited at the opportunity to continue to evolve our work with low-impact design. 

The existing barn frame had four bays and each one has had a different treatment. The first bay is the existing steel frame, interlaced with a wild climbing rose that shades the south-facing glazing at the hottest times of the year. In the next bay, the frame has been lightly clad to create a light-filled enclosure, dubbed the hot house. Heated by the sun and an open fire, it acts as a highly flexible indoor-outdoor space that links the house to the workshop and to the surrounding landscape. The southern elevation opens onto this space with windows from the kitchen and living space and a set of double doors that can be left open for most of the year. The living space is double-height, yet intimate and open-plan. The landing of the stair cantilevers into the room, creating a platform with views out through the double-height glazing. 

The first floor has the feeling of a bright attic space and is a study-cum-third bedroom. You are connected to landscape in three directions and low-set windows take you out onto a small roof terrace. 

Beneath are two compact bedrooms, a bathroom and toilet. The bath was redesigned to fit the space, taking its inspiration from the Japanese ofuro. The building becomes increasingly immaterial as you move through it, from waking in a bedroom, going through to the kitchen, then through the hot house and out to the fields. 

What are the key spatial and tactile senses that you want users to have of the building?

The house is composed of interconnecting rooms, each with quite different spatial qualities, from the low-ceilinged introspective compression of the bedrooms to the light airiness of the first floor. This spatial complexity creates a feeling of generosity and enables different modes of being and occupation. 

The structure of the building is expressed internally, making the construction highly legible. You feel in contact with it and you know what you are dealing with. I think that registers with us at a very primal level. 

The hemp has been left unplastered with only a light coat of clay paint, which gives a rough texture to the walls and makes them highly absorbent acoustically. The absence of reverberation creates an acoustic intimacy and sense of being held by the space.

Sharing some of the methodology of Tudor construction, the building also has a historic quality, not just in the expression of the interior but in the atmosphere created by being in contact with ‘real stuff’.

Are there any passive or operational design features which contribute to minimising the energy use?

The building is conceived with quite a different logic from Passivhaus. It is low-tech and high-performance. Thermal mass is favoured over complicated heat recovery systems and the importance of breathing construction over airtightness. Passiv strategies often lead to the integration of vast amounts of petrol-based insulations, membranes, piping, ducting, and regulating equipment. These materials don’t last, the systems break and are hard to maintain and you end up contributing to the production of mountains of unrecyclable and polluting waste. If we are to meet our carbon targets, we need to think in a more comprehensive way about what we are putting into our buildings.

I am not against the use of petrol per se, it is a valuable resource that we need to treat with respect and ideally use only where there is no other material that can do the job.  

Describe the key challenges of the build. How did you seek to minimise construction waste?

The house was relatively simple to construct, the primary difficulties on site coming from the need to integrate and align with an existing structure. The panels were lifted into place on site in two days and from there the remaining construction was carried out with a simple scaffold. The panels had very little tolerance and required an amount of ‘persuasion’ to get tightly into place. Despite removing the need for finishes, building with exposed structure comes with its own challenges and the need for precision, but it is well-suited to prefabrication. There are consequences for the sequencing of the programme. For example, the structural joining timbers were installed as part of the internal fit-out and finishing phase of works. The building was designed with a ‘digital twin’, meaning almost every element was modelled in 3D. It was used by the construction team on site to supplement traditional drawings and meant we could be very precise in material orders. 

All of the remaining materials on site were used in the construction, a short distance from the house, of the CASS Studio, a small building designed and built by students led by David Grandorge and me. 

This has been described as a ‘prototype’ project. What do you see as key steps to making this system of build possible or economical on a much larger scale?

We established our sister organisation, Material Cultures, to take on the task of scaling the project up. Through it, we are interrogating the intersection of natural materials and off-site construction. We have set up a testing base at HG Matthews Brickworks, where we are able to experiment at 1:1 scale. We are working with UAL and UCL, running research units exploring the technical and cultural possibilities of these principles. The Flat House system of cassettes is ready to be used at a larger scale. These can be made in large quantities and applied to much taller residential or commercial buidlings. The panels are structural and self-supporting and were installed independently of the existing steel frame at Margent Farm. Designing outside the constraints of Class Q, the formal language of the system is unrestricted. Equally, the interior of the building could be seamlessly plastered for a clean-lined interior. 

Strip away the steel barn and hot house and you can read Flat House as a prototype model for an urban or rural home that could be detached, terraced or extended to form flats. The construction costs for the house at Margent Farm are very low considering it is a one off experimental project and these would significantly reduce with economies of scale. All of the materials are affordable and readily available; hemp is already being grown in the UK in reasonably large quantities, mainly for horse bedding.

We are in the process of developing more systems and models for different applications and looking at how this integrates with regional manufacturing. 

It all feels very exciting and I think there is growing sense of urgency for us to be responding to ecological challenges in earnest. All we need now is a local authority or developer who is up for the challenge to dip their toe in. 

The house was designed to demonstrate how a low-tech approach and bio-based materials can be combined with off-site construction to create a scalable low-impact, beautiful architecture.  The orchestration of natural materials creates a building that regulates humidity, temperature and air quality without the need for ducting or equipment.  

It draws on centuries-old material technologies and construction principles, rationalising them for contemporary building techniques. When sourced locally, natural materials are inherently low-carbon, as less energy has gone into their production than contemporary highly processed materials. 

It is becoming increasingly clear that we cannot address sustainability in architecture purely through reducing operational carbon.

While the house has been designed for residential use and with the constraints of Class Q permitted development, the system has the potential to be used for any type or scale of building.

Paloma Gormley, partner, Practice Architecture

I purchased the 53-acre Margent Farm because, outside of my work in TV and film, I wanted to make a meaningful contribution to the fight for sustainability.

The idea was to build a farmhouse from the hemp crop we grew. I commissioned Paloma Gormley to design the house and to keep to the footprint and some existing framework of the cattle yard. Having worked with it previously, she confirmed hemp was a good material from all points of view. 

The site is three-quarters of a mile from the nearest house and I decided staying off-grid was a good part of the story. We are powered by solar panels and heated by a biomass boiler. We collectively decided to prefab the walls of the house off-site. This meant the material could dry after process and we would do as little damage as possible to the site. 

My main goal became to help inspire the larger-scale growing of hemp, hopefully to eventually replace some of the unhealthy practices of construction.

As a building, it beautifully suits my purpose for compact living in a large, amorphic, multipurpose, inside-outside live and work space alongside the existing brick barn. This is all achieved with its healthy natural environment, brilliant insulation and inspirational aesthetic. 

During the design and construction I started to research and develop a series of products that use hemp and natural binders. After consulting with Paloma we decided to be brave and use a unique corrugated cladding composite material made from the hemp fibre grown on site. This is thermally compressed with a natural resin and we are excited to see how it weathers over time. I couldn’t be more happy with this oasis of positive action that Paloma has helped us create.

At the farm we are continuing to develop new hemp composite materials for all kinds of products, including trays and coasters for hospitality uses and balms and oils to help Save the Fucking World.

Steve Barron, director, Margent Farm

I feel very lucky to have been part of the team and journey that was the Flat House. Having the opportunity to participate in the design and build of a structure that is attempting to create a synergy of restorative carbon-negative construction techniques with the provision of beautiful, healthy homes feels like a step towards a future that we might be able to be proud of.

I continue to be inspired by the direction of travel that this project and others like it are going in, breaking a paradigm that assumes depletion of resources and the environment as a necessary part of human progress and provision of needs and shelter. I massively valued the process of working through material selection and construction methodology on this job with a mind to speed up construction time and material efficiency.

Prefabrication of large elements of the build off-site offered a great opportunity to extend the season that construction with these materials can take place in and reduced wastage of materials, money and energy use. It is a practice I am taking forward into future construction projects. 

I see a future of low-impact, low-cost, social housing being built at a rate that means housing shortages need never be a thing and having a home need not contribute to, but actually help prevent, a warming planet.

Oscar Cooper, carpenter and timber framer

The drawing shows a section through the ground floor, wall and first floor. The construction is designed to be simple to build, with minimal materials and no membranes. Petrol-based materials have been avoided wherever practically possible and engineered timber products are only used where the additional performance is required. While allowing for the precision required by prefabrication, we wanted to utilise materials in their raw form, with as little possible processing. The system is almost entirely cellulose-based and all of the timber is pine, which is sustainably and, where possible to discern, UK-grown.

The exterior wall is constructed from a 195mm-deep structural panel composed of planed timber and engineered I-joists. The I-joists form the vertical studs and lighten the cassette while reducing thermal transference. The panel is filled with hempcrete. Hempcrete is fire-resistant, provides wall mass and is insulating. It has been sealed with a very light coat of clay paint.

A thin cellulose breathing board provides both racking resistance and airtightness. The panels are over-clad with woodfibre insulation and a tongue-and-groove junction between batts gives it water-resistance. Battens support the corrugated cladding sheets, which form a rainscreen. The sheets are made from hemp fibres impregnated with a sugar-based resin and thermally compressed. The wall breathes, so moisture can pass freely from inside to out.

The first floor is supported by joists that rest on a timber ledger beam fixed directly to the cassettes. The first floor is stripped right back to a deck with no soffit nor insulation, with the thickness of the timber doing all the structural work.

Paloma Gormley, partner, Practice Architecture

Start on site April 2018 Completion March 2019 Gross internal floor area 97m² (house only), 60m² (hot house) Construction cost £250,000 Construction cost per m² £1,600 (including hot house), £2,200 (house only) Architect Practice Architecture Client Margent Farm Executive architect Paloma Gormley Structural engineer Fordham Consulting M&E consultant Eco Installer Project manager Kathryn Brown and Kate Minns  Approved building inspector Salus Main contractor Oscar Cooper and Brian Reid  Hemp construction Will Stanwix Hot house Henry Stringer Prefabrication Material Cultures CAD software used SketchUp Annual CO2 emissions –2.32 kgCO2/m²

Tags hemp Oxfordshire Practice Architecture Sustainability

I’m not sure I understand the passive vs breathable rationale here. The architect interview implies that passive houses must be built from petrochemical based materials when this is simply not the case. Yes you need a mechanical ventilation system, but the rest of the fabric can be built from anything. Some excellent alternative materials in the project, but if they had adopted some other passive house principles (i.e. good windows and thermal bridge free) the energy consumption could have been even less???

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