Why use straw-bale?
Straw is an annually renewable natural product, which grows through photosynthesis, taking energy directly from the sun. There is a surplus of 17.7 million metric tonnes of straw in North America every year (Bowyer et. al, 2001). The use of straw translates into less pressure to use other environmentally damaging materials. It can be decomposed after the end of the life of building.
Increase in energy efficiency is a prominent design challenge of housing industry during the 21st century. Straw-bale houses utilize surplus straw, which is made of material that takes up carbon dioxide and converts it into oxygen during its lifecycle. Construction industries and associated transportation contribute to production of over 50 percent of all greenhouse gases (Amazon Nails, 2001). Thus straw-bale buildings contribute towards reducing greenhouse gas emissions.
High insulation value:
The walls of straw-bale structures are approximately 450 mm (18 inches) wide. These walls have a higher insulation value than standard house walls filled with fibreglass, cellulose, rock wool, or other fibres. They typically attain an insulation value of R40 to R45 compared to R20 of conventional walls. The mass of straw and stucco/plaster coatings takes hours to warm up or cool down. This behaviour of straw insulated walls leads to energy savings. According to research by Canada Mortgage and Housing Corporation (CMHC), straw-bale houses use over 20 percent less space heating energy compared to conventional houses (CMHC Technical Series 02-115).
Straw-bale walls have excellent sound absorption value. Such wall systems have been used for construction near airport runways and busy highways in the USA and Europe for sound insulation (Amazon Nails, 2001).
According to research by CMHC in the provinces of Nova Scotia, Alberta and B.C., straw-bale walls generally incorporate higher perm (more breathable) interior and exterior protective layers. A straw-bale wall has a limited tendency to adsorb and absorb moisture, and diffuse it to either the exterior or interior of a structure.
Based on CMHC's study, 66 percent of the researched houses had moisture readings within the acceptable limits i.e. <14 percent moisture (CMHC Technical Series 00-103).
Low fire risk:
Straw-bale walls are two times more resistant to fire than a frame wall as per fire tests results. The CMHC has performed fire tests on straw-bale buildings, and they are supportive of such structures.
According to research conducted by CMHC, an equivalent stick-built house uses approximately 50 percent more wood than a load-bearing straw-bale house (CMHC Technical Series 02-116).