Straw Bale Construction.

 

Houses have been built using straw for thousands of years but due to the interest these days in sustainable housing the straw bale construction method has had a revival.  America particularly favours this form of construction but the UK is catching on too.  In 1995 Bob Mathews, the author of the book The Complete Manual of Practical Home Building, built a straw cabin and has lived in it ever since.  Straw is a surplus farm product and is also very cheap, it can be just 40p a bale but if you want it delivered this can rise to £1.50 per bale.  If properly constructed straw bale walls are super-insulated and straw bale houses are fire-resistant and waterproof and contrary to believe they are pest free.

 

The construction:- To build a typical 2,000 sq ft house, around 186 sq metres, you will need about 300 standard three-wire bales of straw.  To keep them in place and strong you will need to skewer them on rebar pins or use wood or bamboo stakes.  Once plumbing and wiring is in place the walls can be sealed and finished.

 

Straw bales need to be of a uniform size which equates to around a metre long and half a metre wide, they also need to be secured well with two strings.  The bales of hay need to have very few seed heads and have to be compacted well and be quite dense.  The weight of the bales should be between 16 and 30 kilos and they have to be dry, when building commences the bales have to be kept completely dry throughout the build for obvious reasons.  The centre of the bales always have to be kept dry so avoid getting the bales wet from the top or the bottom but if the outside gets wet then  this is not a problem as they will dry out naturally.

 

For larger structures a “non-structural” construction or “in-fill” system is used, this means that a frame is made first which will support the roof then the bales will be pierced with rebar as they rise.  The bales are attached to the frame to secure the walls.  “Structural bale construction” is a lot like Lego as the bales are stacked together in a running bond style.  Once construction is completed the exterior is stuccoed and the interior walls are plastered.  Remember the bales compress when they settle so you have to leave settlement gaps between both windows and doors. 

 

In theory a straw house which has been constructed to a good degree should last for centuries.  For the roof you will probably need something more permanent and so the roof needs to be at a steep angle for the drainage.  For the foundations it is best to use a rubble trench and you will need to use plastic or metal strapping to attach the wallplate using anchor bolts to the foundations.  Make sure that the bales do not meet the foundations and that the foundations go well below the frost line which can then be filled in with stones.  The elevations of the foundations need to be at least 23mm higher and leave a good overhand on the roof of around 450mm.

 

Once the construction is finished you then have to render, some use limewash which tends to be expensive but Americans tend to use Earth Plaster as it is durable and can be used both inside and out, it is non-toxic and it is available in many colours. 

 

When using straw bales you should not have a problem with rot, as long as the bales that you have used are dry.  Paint should be permeable so that moisture does not get trapped in the wall, a disaster when using straw.  Another fact about using straw is that there are less insects in the bales than there are with wood, once the building has been plastered there is no way of them getting in either.  Surprisingly straw bale construction is fire resistant but mortgage lenders do not tend to lend willingly for these homes and insurance companies feel the same.

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