Straw-inspiring: houses made of the humble bale
Paul Miles in the Financial Times: 4.12.2021
Humanity has reached a milestone. There is now more weight of man-made stuff on the planet than the
total mass of all living organisms, from plankton to whales, estimated by scientists at Israel’s Weizmann
Institute of Science to be 1.1tn tonnes. If we keep on at this rate, the weight of things such as
infrastructure, houses, phones and plastic bottles will be triple the biomass on earth by 2040. Most of
this manufactured mass is concrete.
The construction industry is responsible for nearly 40 per cent of global carbon emissions, according to
the International Energy Agency. There has rightly been a lot of attention directed at reducing
operational emissions of a building, by lowering the use of fossil fuels through rigorous ultra-low energy
Passivhaus standards, for example.
Yet there has arguably been less attention paid to embodied carbon: the emissions produced in the
mining, manufacture, transport and assembly of the likes of concrete, glass and steel. But there are
many building materials that don’t need mining; they grow naturally.
Governments worldwide are beginning to legislate on embodied carbon. The Netherlands has been a
pioneer since 2013 with mandatory “lifecycle assessments” that address environmental impacts —
including embodied carbon — of proposed developments over 100 square metres. In July this year,
Colorado passed a bill intended to minimise embodied carbon in new public buildings and roads.
In the UK, an amendment to building regulations has been drafted by architects and members of the
construction industry. “Part Z” would limit the embodied carbon of major building projects. The
proposal has widespread industry support.
On a smaller, homely scale, what can we do? Instead of constructing houses with bricks — the
manufacture of four pallets of which results in about one tonne of carbon emissions (even more if the
bricks are imported long distances, for example to the UK from Asia, as an estimated 25m are annually)
— we could build with materials that pull carbon out of the air.
Plants are one of the oldest building materials in the world — straw, a byproduct of food production —
has one of the lowest embodied carbon levels of any building material. While one cubic metre of
concrete is responsible for 246kg of carbon dioxide emissions, the same volume of straw, suitable for
use in construction, has locked up 129kg of the gas.
Straw is also long-lasting. A building that is believed to be the world’s oldest surviving straw bale home
was built in 1903 in the US. Today, the US has some of the largest contemporary straw bale homes in
the world, making use of the by product of a variety of local crops.
“We have designed and built homes of rice straw in California, oat straw in Alaska, rye straw in Oregon
and wheat straw in Colorado,” says Anni Tilt of California-based Arkin Tilt Architects, which specialises
in straw bale and rammed earth construction.
Over more than 20 years, the practice has built nearly 50 homes. Rather than quirky hobbit structures
that many associate with straw bale construction, some of the sprawling properties designed by Arkin
Tilt are sleek, expensive-looking homes. “There’s renewed interest in straw building due to the potential for carbon storing inherent in rapidly renewable bio-based building materials,” says Tilt. “The building of structures can be part of the climate solution rather than part of the problem.”
There are three main types of construction with straw, says Barbara Jones, a veteran straw bale builder
and director of the School of Natural Building.
Load-bearing straw bale, known as Nebraska style, began in the US in the 19th century. In this method,
compacted straw bales, with a density of 120kg per cubic metre — one and a half times more dense than
an average bale off the farm — are used as “bricks” laid one on the other, held in place solely by the
weight of the roof. The walls are sometimes shaped and tidied with a chainsaw before being plastered or
clad inside and out.
“Two and a half storeys is as high as has been achieved with load-bearing straw bale,” says Jones.
“Compacted straw bales are very stable.” Research by Pete Walker at the University of Bath has shown
that one metre of wall can bear at least one tonne of weight. “There’s no reason why you shouldn’t be
able to build higher [than 2.5 storeys] but it’s not been tested,” Jones says.
Europe’s first two-storey load-bearing straw bale property — on the west coast of Ireland — was
completed in 2003, using the labour of more than 100 volunteers, most of whom had no previous
building experience.
“Load-bearing straw bale building is quite labour-intensive and often involves volunteer labour from
friends and family,” says Jones. As for longevity, “we build to a design standard of 200 years, anything
less is unsustainable,” she says. “The most important thing [to ensure a house endures] is that the straw
is very dry [to prevent it decaying] and also very dense,” says Jones. “If later in the home’s life, you want
to put in a new window or door, you simply cut out a hole with a chainsaw,” she says.
A newer method of straw construction is to build a wooden frame and infill it with straw bales. In the
UK, one example is Leyburn in Yorkshire by architects Halliday Clark. Completed in 2017, this
handsome home clad in reclaimed limestone was the world’s first straw bale building to achieve the
Passivhaus Plus standard (Passivhaus standard with additional on-site renewable energy, usually solar
panels), thanks in part to the highly insulating properties of thick straw walls.
In France, a seven-storey block of flats has been constructed in a similar way, the straw bales squeezed
into individual timber caissons before being installed in an engineered timber frame. The 26-apartment
block Jules Ferry residence in Saint-Dié-des-Vosges was completed in 2013 at a cost of about €1,800 per
sq m after tax, about 15 per cent more than a conventional build.
According to architect Antoine Pagnoux of ASP Architecture, this premium is soon offset by small
energy bills. A similar 10-storey straw bale and timber tower block is presently under construction.
“France is at least 10 years ahead of the UK in straw bale building,” says Jones. “They have the land, the
straw, the farming co-ops and the political framework. People see what straw buildings are like and
want to live in them. If we’re to build affordable, high-quality straw houses for ordinary people here in
the UK, the way forward is the newest method of construction: prefabricated panels filled with
compressed straw.”
EcoCocon began production of its prefab panels in 2008 in Lithuania. The structural panels have wood
fibre board on the exterior and an interior of compressed straw that can be plastered.
“A two-storey house can be erected in two days,” says chief executive Bjorn Kierulf. So far 32,000 sq m
of walling — equating to some 260 homes — has been constructed using EcoCocon panels, mostly in
Europe. One sq m panel contains 48kg of straw and has sequestered 97kg of CO2, says Kierulf.
“Even taking into account the transport emissions to deliver the panels, an EcoCocon home will be
significantly carbon negative,” he says. The highest building so far is three storeys “but five or six is
possible”.
The panels meet stringent EU fire regulations, resisting burning for 120 minutes at temperatures of over
1,000C. “Compacted straw just chars on the outside,” says Kierulf. “There isn’t enough air for a fire to
take hold.”
The fear of fire, even if unfounded, is a hurdle in the UK. “After the Grenfell tragedy, insurers are jumpy
about anything that is different,” says Jones. There are estimated to be just 10 homes built of EcoCocon
panels in the UK.
The company is now expanding, opening a second production unit in Slovakia. There are plans for at
least 10 more across Europe in the next decade. “The main problem for scaling up is storage of straw,”
says Kierulf. “There’s plenty of material — in Europe 48m tonnes of straw annually could be used
without degrading the soil — but we need lots of storage to keep it dry.”
Another company producing straw-filled panels is UK-based Modcell. These panels differ from
EcoCocon in that they are manufactured from whole straw bales in a temporary production unit near
the building site.
Fifteen one- and two-bedroom homes constructed of the straw, timber and strawboard structural panels
will be placed atop the office block of homeless charity Emmaus in Bristol. They are expected to be
completed next year.
On 2016, the world’s first straw homes to sell on the open market went on sale near Bristol. The seven
homes of Modcell straw bale and timber panels clad with bricks (to satisfy council planning
requirements) have solar panels and air-source heat pumps.
“They attracted lots of attention and all sold on day one,” says Modcell founder, Craig White.
Research continues on the potential of new straw crops. The Institute of Biological, Environmental and
Rural Sciences (IBERS) at the University of Aberystwyth has studied elephant grass (miscanthus) which
grows prolifically even on poor-quality land. Unlike cereal crops, miscanthus doesn’t need to be sown
annually, thus minimising carbon emissions from ploughing and soil degradation. IBERS helped with
the construction of the world’s first miscanthus bale house in 2017.
“After COP26, with everyone talking about decarbonising, building with straw is the way to go,” says
White. “[Building with plants] is ‘carbon capture and storage’ in the built environment. The best thing
you can do with carbon is to build with it.”