BASICS
In the early days, the stacked and plastered bale walls supported the load of the roof on their own (without wood posts). This type of wall construction, known as “load-bearing”, is still used to a limited extent on smaller 1-story homes. However, to achieve greater structural capacity and architectural freedom, the majority of straw-bale walls built today incorporate structural wood columns and bond beams (in various formats) to take vertical loads off the bales. These walls are sometimes called “non-load-bearing”, although the bales and plaster are still used to resist lateral loads from wind and earthquakes.
A traditional straw-bale wall assembly consists of only three things: exterior plaster, bales, and interior plaster. There are no plastic vapor barriers, house wraps, etc. The walls are naturally vapor permeable and extremely low in embodied energy. They can also help create a healthy, non-toxic indoor environment.
HISTORY
By the 1940s, the mass-production of cement and oil-based building materials provided an affordable, attractive housing “package” that’s still common today. Many of these homes strived to recreate the durability of natural materials like stone but used unchecked quantities of energy-intensive, toxic ingredients. Environmental awareness in the 1970s and ’80s led to a modern resurgence of straw-bale construction in the USA and worldwide. Today, there are straw-bale homes in all 50 states in the USA, and over 100 in Colorado alone. The introduction of Appendices S and R in the 2015 International Residential Code has further supported and encouraged the growth of straw-bale construction.
FAQ
The bulk material cost of bales is very low and comparable in price (per cubic ft.) to blown cellulose or fiberglass. The material cost of clay plaster can be extremely low (cheap as dirt, basically), while prices for lime and cement-lime plasters will be roughly equivalent to other similar stucco finish materials. As for “installed cost”, i.e. the price of the whole wall, fully built, this will of course depend on what contractor(s) you use and how much you do yourself, among other things.
Pests will try to live in any type of house, no matter what the wall system is. Good detailing that’s appropriate for your climate zone and geographical area will always help eliminate pest issues. In a straw-bale home, the straw is sealed on all sides by either plaster or wood substrates, making it difficult for a pest to access. And remember, straw is not typically a source of food for animals… Hay is for horses, and straw is for houses, we like to say.
Straw is made of cellulose, just like wood. When exposed to moisture, cellulosic materials will decompose. To keep wood-framed houses dry, we protect them with a good roof and a good foundation that keep moisture away from the wood. The same is done with a straw-bale house. When kept dry, straw (and wood) can last hundreds if not thousands of years. In 2012, members of COSBA were able to obtain a straw bale that was taken out of a historic 1920’s bale house in Nebraska (to make way for an addition). The bale appears as though it could have been installed a year ago. Straw-bale walls also have the quality of being vapor permeable, which allows them to dry out to the interior and exterior more easily than a conventional wall system.
Contrary to popular belief, straw bales will not burn up spectacularly when lit. They are too dense. Full-scale fire tests on straw-bale wall assemblies have been performed in accordance with ASTM Method E119-05a: Fire Tests of Building Construction and Materials. An earth-plastered wall successfully obtained a 1-hour fire rating, and a cement-plastered wall obtained a 2-hour rating. More information about the tests can be found at www.ecobuildnetwork.org.
Electrical cables can be run in between bale courses or in small chases below/above the wall, depending on your wall design. Some contractors also chose to use conduit. Electrical boxes can be nailed to small wood posts or stakes that are fastened to or embedded in the bales. In almost every climate zone, it’s generally not a good idea to run plumbing in exterior walls (to avoid freezing issues), and should be avoided in straw-bale walls to. Where it needs to be done, using extra casing and air sealing/insulation is not a bad idea. In “busy” wall areas like at a breaker box or mechanical room, consider switching to a more conventional framed wall system to improve access to cables and mounting equipment, etc. Also, note that locating showers next to exterior/straw-bale walls should be avoided.
The short answer is yes, they should, especially since straw-bale and light straw-clay construction systems are both now fully codified in Appendices S and R of the 2015 International Residential Code. There shouldn’t be any type of “special” permit process. COSBA can assist in educating building officials who are not familiar with straw-bale construction, which is a big part of our organization’s mission.
Recent innovations in straw-bale wall assemblies include various pre-fabricated systems as well as rain-screen and “straw-cell” assemblies. With these systems, it’s possible to have a variety of different exterior and even interior wall finishes.
GALLERY
