Some of the most frequently asked questions regarding straw bale buildings are those related to the type and specifications of the bales that should be selected. The following 5 points will hopefully address many of the questions first time straw bale and owner builders have when confronting the question of bales.
There continues to be much debate surrounding some of the issues such as the type of straw (fiber). Remember, straw is the stems of grain and cereal crops – not to be confused with hay which is grass and legumes which is used as animal feed. The recommendations set out here are based on real world experience from our design team and builders and the performance of the buildings we design and build.
- Moisture content. The drier the better! Anything below 18% is acceptable. Bales in the 12 to 16% range are ideal. Anything above 18% and you run the risk of rot and mould. Bales that have been wetted and dried are not prime candidates for bale construction. They will often appear grey or even blackened and will often smell ‘mouldy’ in spite of being dried and below 18% moisture. Reject them!
- Type of Fiber. Our experience has shown that all type of straw perform equally well, excluding canola. This includes wheat, barley, oats, rye and flax.
- Fiber length. This is important. Many modern combines (the machines that thresh the grain) have straw choppers which reduce the long lengths of straw fiber to much shorter lengths as the straw is discharged from the rear of the combine. The best straw bales and the only ones recommended are bales containing long straw fiber. (unchopped fiber)
- Size. A standard size bale should be 14 inches high, 18 inches wide and between 36 to 40 inches long. The modified post and beam construction that we design for our houses and buildings are design to accommodate this size of bales.
- Density. The weight of an average bale should be 7 lbs. per cubic ft. A bale that is properly tied (with polypropylene twine) should leave not more than a space to slide the flat of your hand under under the twine when lifting the bale by one string. Bales lighter then 7 lbs. per cubic ft. tend to be loose and contain more air space than is desirable to obtain optimum insulating value.
There are numerous producers across Alberta and Western Canada that supply straw bales suitable for straw bale construction. We will be posting the names and contact information for these suppliers in the spring of 2014.