Common straw bale FAQs and information…
Straw and hay are not the same thing
We build with straw. Straw is the dry stalks of cereal plants, such as wheat, oats and barley, after the grain has been removed. It has a low moisture content and very little nutritional value. In its makeup, it’s quite similar to wood.
Hay is dried grasses and legumes, e.g. pasture grass, alfalfa. It looks green and smells sweeter than straw. It has higher nutritional value and so is used as a feed for animals.
If you think of straw as wood and hay as food, the importance in building becomes clear. Straw, kept well, will last forever, whilst hay is much more likely to break down and compost due to moisture and the nutrients it contains.
Straw baling machinery was invented in the mid to late 19th century in the USA, and straw bale buildings soon became popular, especially in Nebraska, because trees were scarce and straw was a readily available by-product of growing wheat. The bales were easy to build with – just like using really big bricks. Some of these simple straw bale buildings from over a 100 years ago are still being lived in today.
Not only can a straw bale home often cost a little less money up-front than a brick-built home, by far the biggest financial savings come over time with energy efficiency. A straw bale home can significantly cut down on annual heating and cooling costs because it is so highly insulating. Straw bale far outstrips even the most stringent building regulations that have so far been adopted in the UK. If the regulations are made even tighter in future – and let’s hope so – hopefully we will see a lot more straw bale houses…
Enough straw is created in the UK each year to build about 600,000 houses. Currently we only build about 100,000 houses per year. That’s a lot of bricks and concrete we could be avoiding creating and using. Although a lot of straw is harvested for various uses, a great deal of it is just ploughed back in (it often used to be burned, but that is no longer allowed).
Health and breathability
Do you ever wonder about all the chemicals that get built into a ‘modern’ home? Straw bales buildings with lime render outside and lime or clay finishes inside let a building breathe. Moisture from inside a building move out through the walls, keeping the internal climate healthy and ridding you of damp. In brick buildings with modern plaster and paints you may have seen evidence of damp – for example mould on walls or above skirting boards and the need for dehumidifiers. Straw bale buildings don’t have damp proof courses, instead they use a capillary break which stops water moving up a wall, but lets any moisture in a wall slowly drain out, by gravity.
What about water?
Straw bale buildings, like cob, need ‘a good pair of boots and a good hat’. Straw bales in buildings start at around 300-450mm off the ground. Beneath this might be foundations of car tyres rammed with gravel, or more traditional foundations of block and brick. Foamglas is a good material for foundations because it uses recycled glass and is insulating. Straw bale buildings are lighter than brick built buildings and they are flexible, so foundations do not need to be as rigid or as deep. Eaves are often deeper – perhaps up to 50cm – to keep some of the rain off the walls, but it’s now thought this may not be entirely necessary unless in quite exposed locations.
Are straw bales a fire risk?
A straw bale is surprisingly fire-resistant. First of all there is not a lot of oxygen present in a bale because the straw is so tightly packed. Secondly, in a building, the bales are covered, normally with a thick layer of lime or clay render, adding even more protection. Rendered straw bale is now frequently accepted by building control in the UK as being ‘non-combustible’. In some international tests rendered straw bale has been given a fire rating of up to four hours. After bush fires in Australia straw bale walls have been reported as one of the few things left standing.
What about rodents etc.?
Rodents may try to live in straw bale wall, just like most other kinds of wall. The main thing is to make sure your bales have as little seed as possible in them, as the straw itself is not an interesting foodstuff for rodents or other animals. And attention to detail during construction and rendering will mean that next to no straw is left exposed, meaning fewer ways to get in.