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Eco-materials

Strawbale Construction

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As a waste product from Agriculture, haybales make a great sustainable building material. Haybales have excellent insulating properties and make for a very pleasing, organic finished result. Thatched roofs are common around the world so it was initially  suprising to me that using haybales for walls is not more common. This is largely possible due to the fact that the mechanization that resulted and straw-baling formation only came about in the 1850’s!

Original haybale construction in Nebraska

The first documented haybale building was a school in Nebraska dating back to 1902. When the cows started to nibble at it, the builders decided the plaster the walls. Following this, numerous haybale buildings sprouted up in the area.

 

Haybale walls tick the sustainability box for a number of reasons:
– Renewable: Crops are grown every year and fast growing
– Waste product: Apart from animal food, hay is a waste product from crop farming. A field of 1 hectare can produce between 4 and 6 tonnes of straw annually. An average size house requires around 10 hectares. Given that the UK has around 2 million hetares of crops, that is a lot of house material!
– Low-embodied energy: Haybale construction should generally be used in areas where the hay can be sourced locally. This results is very short delivery / transportation which keeps carbon emissions down.
– High insulation: the great insulation properties will reduce energy bills and improve thermal comfort as indoor temperatures are regulated.
– Non-toxic and vapour permeable: bale walls have no off-gassing and allow a healthy passage of air which prevents mould growth.

Haybales are treated much like over-sized bricks. Organised in a running bond, the bales are tied together using re-bar of bamboo or timber, or covered with a mesh. This can then be plastered with a lime, cement or clay based render.

 

Plastering of walls protects the straw and neatens the finished wall
Using mesh as a support for the plastering

A solid foundation is generally used, onto which the bales are placed. A moisture resistant membrane should be placed between the bales and the foundation.

Example detail of wall at foundation and window
Deatil of ‘Hoks Strawbale house’ which achieve a LEED Gold certification

The bales can be used as the structure of the building, or as the insulative infill to a steel / timber frame structure. A field bale will generally support 900kg per linear meter but it is possible to get highly compressed bales for greater structural loads, that can support up to 6000kg per linear meter.

Bale infill to timber structure

‘Balehaus’ is a project of the University of Bath, where prefabricated panels with hay infill where fitted together and rendered with a breathable lime-based plaster. More about the ModCell concept here.

BALEHAUS by Modcell

Haybale walls have very low thermal conductivity making them a great source of insulation. Thermal consuctivity of wheat is about 0.06 W/mk. For a bale wall 475mm thick this would achieve \ U-value of 0.123W/m2K. This exceeds the passivhaus requirement of 0.15w/m2K for walls.

Temperature test on a warmer day
Temperature testing on a cold day

Because the haybale walls are plastered, this prevents the unwanted concerns of rats and mice / fire damage / water ingress. It is important that a good covering of plaster is used – around 25mm thick. If looked after, haybale walls can last for centuries.

Strawbale construction is perfect for cold enviroments

The NBS has now incorporated a guideline specification for Strawbale construction: http://www.thenbs.com/topics/Environment/articles/StrawBaleConstruction.asp