Strawbale

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Mention building a house from strawbales and most people have a vision of the children’s nursery rhyme with a wolf ‘huffing and puffing and blowing the straw house down’. Of course the reality could not be further from the truth and there is now a groundswell of interest in building with strawbales.

Background

Let’s go back to basics and consider why you would want to use strawbales in the first place. Their origin as a building material dates back to the late 1800’s when the white settlers in the plains of Nebraska, USA needed somewhere to live (timber was scarce and as they were growing grains, there was an abundance of straw). Around this time, mechanical bailers came into use and hey presto, they had just created giant building blocks. Although seen as a temporary measure to satisfy the immediate demand for shelter, they soon discovered that strawbale kept them remarkably warm in winter, cool in summer as was a great sound insulator too.

Over a hundred years have passed since those first houses, so why is strawbale of interest today? Amazon Nails, a UK-based eco-building company mention the following reasons in one of their publications:

  • Sustainability – Straw is an annually renewable product, fuelled by the sun. About 4 million tonnes are produced surplus to requirements in the UK each year.
  • Energy Efficiency – Over 50% of all greenhouse gases are produced by the traditional materials and methods of the construction industry and the transportation associated with it.
  • Highly Insulating – straw is super-insulating at a low cost. With a U-value of 0.13W/m2K, this means it is two or three times better insulating than contemporary materials. Current building regulations require walls have a U-value of 0.45 or less.
  • Sound Insulating – strawbale is acoustically super-insulative. This means that not only are the sounds of the inside kept in, but external noise which may otherwise be obtrusive is kept out. The interior acoustic qualities also lend a sense of peace to the occupants.
  • Low Fire Risk – plastered strawbale walls are less of fire risk than traditional timber-framed walls. Although many people may be surprised to hear it, strawbales are so tightly packed that there is little flow of oxygen within the bale, therefore they are actually very difficult to burn. Tests conducted by buildings regulation and safety officials in New Mexico, USA have confirmed the findings.
  • Low Cost – Because straw is seen as a waste product, it is comparatively cheap to build with. To purchase the strawbales for the walls of a typical 3-bedroom, two-story house would cost in the region of £600 to £1000, whereas bricks and blocks would be over £10000. In addition to the material savings, because building with strawbale doesn’t require the same skills as bricklaying, there is a saving in labour costs, and furthermore, once constructed heating costs can be up to 75% less than comparable modern-style housing.

Methods

So there are some compelling reasons to build with strawbale, so let’s look at the actual methods. There are several methods of actually constructing with strawbale, although two are most dominant, Nebraska (also called Loadbearing) and Infill (in conjunction with a framework).

Nebraska is so called because its origins are back in the plains of Nebraska. It’s also called Loadbearing because the bales work like big bricks, they are stacked on top of each other and directly support the weight of the structure, including the roof. The advocates of this system prize its simplicity and speed of construction, although to people who don’t understand strawbale construction, it can present a mental barrier in terms of using what appears to them as an unstable material to support the rest of the building.

Infill is based around a frame (normally wood) which carries the weight of the roof. The strawbale acts as an infill material that is simply placed in between the supporting structure for insulation. The advantage of this method is that it is more easily understood by others – ie they can see the framework as a solid structure and concerns they may have about the use of strawbale are lessened. A disadvantage is that greater skills and time are required to construct the framework.

Protecting the Walls

Regardless of the construction methods used, one thing is clear, if strawbale is left exposed to water (rain etc) and not given the opportunity to dry out, it will rot and eventually disintegrate. To stop this happening, it is important that the walls are built on a reasonably high foundation, that the roof has a good overhang and that the walls are plastered with a breathable material. Like building with Cob, the building should have a ‘good hat and boots’, along with a decent raincoat too! Lime plaster is the most common material used to cover the outside of the walls, being flexible and breathable it is a perfect companion to strawbale. Inside, lime plaster or clay plasters are most often used.

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Comments are appreciated – blog @ ecraftsmen.co.uk