Green Services Direct

Choosing natural building materials can boost your home’s green credential and help the planet.

Natural building materials have been used since ancient times and are still  widely used in construction today. But while the use of some natural materials in construction is commonplace, others are more niche. As awareness of climate change and the urgent need to cut emissions in the construction of buildings grows, so do reasons to look at more sustainable, alternative building materials.

What is a natural building material?

Natural building materials tend to have low embodied carbon and less energy used in their production.  Some can be regrown and are therefore renewable while others come from a plentiful source like soil.  Natural building materials are more likely to be biodegradable,  recyclable or reusable.

The Centre for Alternative Technology (CAT) says we need to consider the embodied energy or life cycle impact of building materials, including extraction, manufacture and transportation. “Think about what will happen at the end of a building’s life. Try to choose materials and techniques that will make recycling easy. Many common building materials eventually end up in landfill or an incinerator because they weren’t reused or recycled.”

Sustainable timber

Timber is, by far, the most widely used natural building material whether it’s exposed as part of the building design, like oak-frame builds, or hidden in the internal structure. Timber is durable, versatile and often attractive.  Sustainable timber can be used for cladding and joinery (roofs and walls) as well as internal fittings, such as kitchens and furniture.

Look for the FSC mark to ensure your timber has been sustainably sourced from responsibly managed forests.  The treatment of timber with toxic chemicals, such as preservatives and varnishes, is almost standard practice but is very rarely necessary, according to CAT. “To avoid timber decay, use good quality, well-seasoned timber and ensure the building provides protection or ventilation of all timbers.”

Hempcrete

While concrete is made with lots of  natural materials, a key ingredient is man-made cement responsible for about half of concrete’s giant carbon footprint

A greener alternative is hempcrete, made from fast-growing hemp, a relative of the marijuana plant, combined with lime and water.  As the hemp plant absorbs a high level of atmospheric CO2 as it grows and the production of hempcrete is quite low-tech, it is said to be carbon negative. This is despite the production of lime, a relatively high embodied carbon material and energy used in transport.

Hempcrete is durable, long-lasting, an excellent insulator and extremely lightweight – making transportation easier. However, it’s  no match for conventional concrete in compressive strength, so it can’t replace concrete in foundations. Hempcrete is largely used in blocks for walls and floors or to increase insulation for existing walls.

Straw bales

Forget the three little pigs, straw is a surprisingly strong natural building material. Straw bales can be designed to provide load-bearing structural support to a building, including the roof. And they can also be used as an infill insulation material (exceeding the current UK Building Regulations) with a separate load-bearing frame, typically timber. Building with hay bales is quick and easy.

In Bristol, seven innovative straw townhouses were snapped up when they were put on the market in 2014. Timber frames were used to support the bales of hay. Savings in fuel bills of up to 90% were expected for those living in the energy efficient homes, according to research by the University of Bath.  Fire resistance and weather resistance were also tested and found to be excellent.

Professor Peter Walker, who led the EU funded research, said: “As a construction material, straw is a low-cost and a widely available food co-product that offers the potential for ultra-low carbon housing throughout the UK. Building with straw could be a critical point in our trajectory towards a low carbon future.”

Cob

A cob home is essentially made with mud. The basic constituents are sub-soil, straw and water.  Cob is a cheap and natural building material which can be used to sculpt a highly individual and energy efficient home. Devon is estimated to have about 20,000 cob homes and an equal number of barns and outbuildings – more than any other county in Britain. Cob buildings were also common in parts of Wales. This is because the composition of the soil often has ideal quantities clay, silt, sand and gravel for use for cob construction.

Conventional cob walls are incredibly labour-intensive and slow to build as each layer has to dry out before the next can be added. The walls are typically built on a stone plinth, 300-450mm above ground level.  In Devon, there is a saying ‘all cob needs a good hat and a good pair of boots’ achieved at ground level by the stone plinth and traditionally at roof level by generous eaves overhung by a thatched roof.  This is to protect the earth walls from rain.

Simple cob homes can often be built without engineers or architects, as non-engineered construction. Cob can easily be recycled, has a low carbon footprint and embodied energy. On the downside, obtaining a mortgage from lending institutions may be tricky though they can be built to meet current Building Regulations.  

 Rammed earth

Another way of using earth in building is what is known as ‘rammed earth’. For centuries, Native Americans in the US south-west, in Arizona and New Mexico, used rammed earth construction to build their houses to cope with scorching summers and freezing winter nights. Rammed earth is also used in traditional Chinese and African construction. The process involves compacting loose, moist subsoil in thin layers between shuttering or formwork. Mechanical compaction forces clay molecules to bond with the aggregate, giving the wall its strength.

The Wales Institute for Sustainable Education (WISE) had designed its centre to be eco-friendly. Natural materials were used throughout. The circular theatre boasts the highest rammed earth walls in the UK at 7.2 metres. The walls are 500mm thick, include 320 tonnes of earth and support the roof.  Earth blocks were also used for internal partition walls on the ground floor, providing excellent sound insulation between classrooms.

In the UK, earth by itself tends not to be good material for external walls as it will degrade in our weather conditions. Some techniques add cement to make stabilised earth. But even adding low proportions of cement to the earth mix – less than 10% - would give it a similar carbon footprint to a standard lightweight concrete block, warns the CAT.

Green building does not always require the latest high technology. Architects and others looking to build ultra-low carbon homes are taking inspiration from ancient building methods to achieve comfort and sustainability. Many eco-building methods are accessible to amateur builders with short courses on how to build with straw, hempcrete and cob.

Useful sources: Centre for Alternative Technology, Straw-bale Building UK,