On Straw as a Natural Building Material Straw

By Matt Muldoon

Straw bale building uses a sustainable resource to quickly build breathable, super-insulated structures which require little or no non-solar energy to heat if designed right.

Straw has very firm credentials as a sustainable material. Firstly, straw sequesters more carbon in itself than the carbon expended in cultivation and baling. This makes it carbon-negative from the outset. If straw is not transported too far, then it remains carbon-negative throughout its lifecycle.

Not only that; buildings whose walls are built from straw bales insulate extremely efficiently; way beyond what is required under the building regulations (if you’re interested, a typical 450mm thick bale has a u-value of 0.13W/m2K). This implies further carbon savings over the lifetime of the building, as less energy will be expended in heating.

However, there’s very little point in having super-insulated walls if you don’t have a super-insulated roof, as it’s through the roof that most heat loss occurs. Straw bales are an ideal way to insulate a roof if you have room for them. If not, you must make sure that you insulate the roof with another natural product such as sheep’s wool or hemp.

Like cob, straw is a healthy, breathable material and functions in tandem with breathable plasters and renders. A straw bale home with breathable plaster and render will self-regulate its humidity levels to provide a healthy living environment.

Straw is also (surprisingly) durable. Many of the buildings from the Nebraska settlers’ first attempts are now more than 100 years old and still standing firm. So, as yet, the upper limit for straw bale durability has not been established.

Again, straw is undergoing a massive surge in popularity. This is because of its firm sustainable credentials, its huge insulating capacity, and because of the fact that straw bale buildings tend to be soft-edged, attractive looking structures. It’s also relatively accessible to the untrained self-builder.

Straw functions well as a load-bearing material in most domestic contexts. It can easily be used as a load-bearing element in a two storey house. However, if you want to go much beyond this then it’s possible that straw will no longer function as a load-bearer. In this case, you need a structural frame built of a material such as wood, and the bales then function as infill.

Although straw bales insulate against heat loss, they do not store heat in that same way that earth does. A thick layer of internal lime or earth plaster will help, but if your design requires you to store heat then you should incorporate another element with a high thermal mass such as cob or limecrete.

Straw bale building was invented in the 19th Century by settlers in the plains of Nebraska. They needed an alternative building material where timber was scarce, and ended up with a sustainable builder’s dream.

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By Matt Muldoon of the Natural Building Company

On NBM – Introduction here

On Earth as a NBM here

On Lime as a NBM here

On Hemp as a NBM here

On Turf as a NBM here

To check Matt Muldoon and his company out, click here.

Read our article on him and TNBC here.