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Natural Building Materials

ABOVE: A recently built cob house in Worcestershire.

Building homes in natural materials is no longer the preserve of the green enthusiast. Tim Pullen reports on a new wave of construction methods that provide excellent performance with a sustainable edge.

The term ‘natural building’ is used to describe houses that have a minimal environmental impact — straw bale, cob, rammed earth, earthship and even earth bag houses. The UK self-builder is ideally placed to build a house out of these materials, although for some the concept is deemed a bit hardcore. But along with the traditional methods, there are modern, more builder-friendly ways to do it.

Natural Masonry

Perhaps the oldest building method still in common use – and that can be considered masonry – is adobe or unfired clay bricks. Adobe is usually seen in drier climates, but the DTI, BRE, Bath University, Hanson Brick and The City of London University are researching the material. Adobe bricks are still not common in the local builders’ merchants, but they are available. Claytec offers a light weight clay/straw brick and a compressed clay brick. Errol Brick also offers an unfired earth brick. All of these are non load-bearing and best suited as a skin for timber frame construction.

A relatively new option is the Ziegel block, being marketed by Natural Building Technol - ogies Ltd as the ‘Thermoplan’. Some 40% of new homes in Germany use Ziegel blocks (‘ziegel’ being German for ‘brick’) and this fired clay block provides a thermally efficient and easy-to-use construction system. It uses a thin mortar bed joint and a tongue-and-groove design which allows for dry vertical joints. They are said to be three to four times faster to lay than a conventional cavity wall and give a Uvalue of 0.23 – around 35% better than Building Regulations Part L requires – with no other insulation.

Timber Frame

It can be argued that timber frame – using timber from a sustainable source – is inherently more natural than masonry. It has lower embodied energy and offers the potential for better thermal performance. NBT Ltd has taken it a step further by combining timber frame with natural insulation materials in its Pavaclad system. A single-skin, 140mm timber frame wall with 140mm natural insulation between studs and 100mm wood-fibre insulation on the outside – kind of a natural SIPs system – gives a U-value of just 0.18. That is almost half the heat loss for a Part L compliant wall, with the same overall wall thickness.

Hempcrete

Hempcrete is a mixture of hemp shiv (the waste from hemp-fibre production) and a lime binder. Sold in the UK by Tradical Ltd under the trade name Hemcrete, the product is available as ready-made blocks for conven - tional construction, or more usually as mix-on-site and cast in-situ. Hemp shiv and lime are mixed in set proportions to a stiff consistency and tamped into a formwork that surrounds a timber frame. The advantages of this are that the timber frame takes the structural load of the roof, the hempcrete protects the timber meaning that no timber treatment is required, and the finished wall has high thermal performance as well as providing thermal mass (the ability to store heat energy).

Finishes

The thing that all these natural building methods have in common is that they offer high levels of airtightness with high levels of ‘breathability’. The wall needs to be airtight but vapour permeable (breathing). This is to allow any moisture which gets into the wall structure to escape to the outside. Using synthetic finishes – gypsum plaster and acrylic paints – stops the wall breathing. Lime plasters and natural paints are now available from many sources, meet all the necessary criteria and are an essential element of a natural build.

Natural building methods are still some way from being mainstream, but with the number of modern systems now available it is fast losing its niche status. A house built using natural methods will be cost comparable to a conventional house, will often be built quicker, will out-perform and could well out-last the conventional house. But using natural building methods needs a bit of deter mination. Resistance could be encountered from the contractor. Being unfamiliar with the materials and methods, they are ultimately being asked to work outside their comfort zone — never a happy place for a builder. He might need a bit of a cuddle if he is to build your natural house well.


"I built an extension using hemp."

Architect Ralph Carpenter (modece. com) built a 140m2 two storey extension to his house in a village near Bury with solid walls made of hemp, lime and timber. The timber – which is largely reused – forms a framework into which the hemp is poured and mixed with hydraulic lime and water. Hemp has excel lent sound insulation as well as a very high thermal inertia, mean ing that it slows down both heat loss and heat gain and so is able to regulate the flow of heat into and out of walls. “Its heat transfer is seven or eight hours instead of about one hour in the case of mineral wool and some other forms of insulation,” he says. “This form of construction produces a very comfortable form of environment to live in.” In addition, a hemp/lime mix gains strength as the lime absorbs the carbon dioxide in the atmosphere as it sets.

The Traditional Methods

Straw bales are a sustainable resource which create breathable, highly insulated structures. Straw is an abundant agricultural waste product and when bales are used to build or insulate buildings, they are commonly finished with plaster. The plastered walls provide excellent thermal mass, structural strength and fire resistance. Straw can easily be used as a loadbearing element in a two storey house but if you want to go much beyond this then you need a structural frame built of a material such as wood, and the bales then function as infill.

Rammed earth – or cob – has been used as a construction material for at least 10,000 years. Beautiful and versatile it is also an abundant resource that is incredibly durable, breathable and thermally efficient. Cob is simply clay, sand and straw mixed into a workable mass. In many areas of the UK, the raw ingredients for cob can be dug straight from the foundation trenches on site. This means that the bulk of your construction material is free.

A Natural Choice for Self-builders

Matt Muldoon of the Natural Building Company explains the implications for self-builders

Most natural buildings are built by their owners. Self-builders are inherently design concious, and if you’re prepared to get your hands dirty, you can build a unique home for a low cost. But what are the practicalities?

Natural building emphasises human labour over highly processed materials. You can pick up the raw ingredients of your house – straw, mud, hemp, wood – relatively cheaply; it’s the specialised skills that eat up the budget. If you can find a natural buil - d er who is willing to teach (and most of them will), and you have the time, then you can carry out the bulk of the work yourself and keep costs down.

If you require a mortgage then approach one of the specialist lenders who understand the materials, some of who will give you a discount on a ‘green mortgage’ based on the energy performance certificate (try Ecology Building Society: 0845 674 5566). There is a definite lack of precedent to allow self-builders to assess potential resale values, but hemp and cob tend to do better than straw bale.

Building Regulation compliance needn’t be a headache. Often the local authority will be flummoxed due to a lack of data on the materials and insist on unnecessary structural additions, but there are alternative private agencies who understand natural building and are qualified to carry out the inspections.

Matt Muldoon runs the Natural Building Company, providing natural building contracting and training. 07942 368485 www.naturalbuildingcompany.com.

TOP: Pavaclad is a single-skin 140mm timber frame wall packed with natural and mineral insulation, which offers a U-value of just 0.18 BOTTOM: Thermoplan is a fired-clay ’ziegel’ block with a tongue-and-groove design allowing for dry vertical joints. It offers a U-value of 0.23