Strawbale construction is a building method that uses bales of straw as structural elements, building insulation, or both. Strawbale building originated in Nebraska in the 19th century and flourished until the 1940s’. The 1970s’ brought a revival of this natural building method.
In Ireland there are structures including houses, garages, classrooms, community centres. One man, whose horse stables are built using straw bales, told me the horses are warmer than himself!
The method is used throughout Europe, the U.S. and Australia.
In the UK, the Building Control passed a straw bale structure for a building society.
Wiseman Designs received planning for a creche to be built out of straw bales in Ballincollig.
Unfortunately, the specific builder was not available to do so in the time needed. Another builder was found but was only familiar in common construction techniques.
Method
There are two methods to building with straw bales. The Nebraska method and the Post and Beam.
Nebraska is where the bales are load-bearing (they take the weight of the roof). They are generally, limited to 1- storey, although, the first 2-storey load bearing straw house in Europe was built in Mayo.
Willow, or hazel ‘rods’, are hammered in to each bale tying the courses together. The same for every course after, forming a ‘skeleton’ of rods, holding the bales in-situ.
Ideally, a ‘breathable’ lime or earth render should be used .
In the Post & Beam method, a frame, usually timber, is erected and the roof made weather tight. The straw bales are laid between the posts. Here the density of the bales is not as important.
Technique
Start with good dry ales. If bales are to take the weight of the roof they need to be baled tightly. When a whole course is laid, the bales are stamped down into place – this gets some of the wall settling done immediately. The walls need to be raised carefully. Anyone can lay a strawbale wall, it is easy and enjoyable to do.
Because of this, a thing called ‘bale frenzy’ can happen, where the wall goes up too fast and slope outward. They need to be checked regularly and kept vertical or sloping slightly inward. Cramming can push other bales out of plum or push them out of square. There are details where greater attention is needed but in the main that’s about as complicated as it gets.
Building a single storey strawbale show house in the Mallow Racecourse, we first laid the foundation and 2 courses of blocks in preparation. On the Friday of the bank holiday weekend, 15 people from around the country, gathered together for a course in building with straw. None had experience with building. A talk on the method was given on the Friday night in the pub. By Monday afternoon, the walls were up and the wall plate on. The only trades men on the site were 2 carpenters who created the timber box windows and doors the wall plates. The most dangerous tools on site were saws.
Reasons for building with Strawbales
- Building with straw is flexible, imprecise and is a forgiving material to work with.
- It is relatively simple and can be accomplished by people of all ages and skills.
- Owners can more easily participate in the construction of their own dwelling and make cost savings. It is also empowering.
- Extremely high level of insulation. The walls themselves are the insulating material. It is a monolithic construction. No cavities are used.
- Environmental reasons.
Straw is a secondary waste material from grain production and its’ embodied energy is negligible. It is usually burnt, with a resulting high carbon release. Straw is annually renewable and is usually available within short distances of the site. Strawbale construction consumes fewer natural resources than any other common method of construction and is one of the least damaging in environmental cost.
Fire Properties
- Loose straw burns easily.
- Tightly baled straw only tends to smoulder. The high silica content (3 – 14%) of straw impedes fire and, as it burns, a layer of char prevents further burning
- Tight baling of the straw severely limits free flow of oxygen
- Plastered on either side a straw bale wall can attain a 2hour fire rating.
Vermin
- Straw is a dead plant stem of grain crop, is pure cellulose and contains no food value.
Structural Stability
- Like all building, the details are important for structural integrity. Natural building is no different. There is no problem structurally with straw bale walls as long as they are constructed properly
- Straw bale building is accepted in the California Building Code. A siesmic area prone to serious fires. Bruce King, an engineer in California, was instrumental in this.
Problems
Overexposed Sites
Straw bales need to be protected from the rain. A breathable render will enable straw bales to dry out should they become damp. A deep roof overhang is important. The first course of bales should be 200 – 450mm from the ground level, negating splash back problems. Self-draining foundations should be used.
Sites in areas of prolonged driving rain may not be a good idea for building with straw bales.
Expertise
There is a tendency to think ‘this is cheap and easy’ and therefore not get professional help/guidance resulting in poorly built structures and consequently, dissatisfaction.
Finance
There can be problems with valuation, mortgages and insurance.