About a month ago I posted an article written by Terrell Wong, an architect specializing in sustainable building and design, about a straw bale addition to a home she had designed. She was frustrated because the city of Toronto denied it even though the city had built a straw bale building in High Park. One of the reasons the structure was denied was because the design doesn’t use a vapour barrier. The theory regarding a vapour barrier is that it is necessary to prevent water vapour from permeating walls and getting stuck there leading to mould and mildew problems which could eventually cause significant structural deterioration — not to mention indoor health problems. But straw bale building has been around for a lot longer than vapour barriers and homes built from straw have been around for hundreds of years in Europe and still stand today. What that indicates is that as long as you know what you are doing, straw bale homes are perfectly safe, healthy and durable, contrary to what someone unfamiliar with straw bale building might think.
After I posted the article, I was contacted by Mick Paterson, a project manager with The Fourth Pig, a co-operative sustainable building not-for-profit group based in Baysville, Ontario. He is currently overseeing a straw bale addition to a house in Toronto. They received approval from the city just as he contacted me, so his project was good to go.
I took the opportunity to visit when I was briefly in Toronto in June, to get a feel for a straw bale home and to ask questions that tend to come to mind when thinking about straw bale building. After the tour, I sent the team my questions and concerns — which I think are fairly representative of straw bale novices like myself. So, below are my questions and concerns, followed by Mick’s and his team’s answers.
1. The straw goes moldy after a while and can lead to such problems as black mould and wall collapse.
Fourth Pig: Moisture is the enemy of any type of construction. The straw bales would only become moldy once prolonged heavy exposure to moisture is seen. A straw bale home must have a breathable protective coat. Lime and clay based plasters provide protection from bulk moisture while allowing any absorbed humidity to escape from the wall.
2. Straw can’t act as an insulation material.
Fourth Pig: Straw can be classed as one of the oldest insulation types on the planet. Straw has been used in Europe for centuries for insulation in different forms but straw bales as a wall construction and insulation really took off in the US in the mid-19th century. Its r value is dependent on the bales compaction, orientation and construction detailing. The most agreed upon r value is 30 for 2 string bales or 1.4 to 2.6 per inch.
3. Its fire rating is low and therefore unsafe to use as a building material.
Fourth Pig: Both ATSM in the US and CSIRO in Australia show testing on straw bale walls to have a high resistance to fires with a 2 hour fire rating on plastered walls and 30 minute rating on an exposed strawbale. There are several examples of commercial straw bale buildings word wide. In Australia there is a veterinary hospital and in Colorado a Waldorf school chose straw bale as the only wall material for all 22,000 sq. ft. of its classrooms.
4. You need a vapor barrier to protect the straw from moisture.
Fourth Pig: Vapor barriers are not necessary for a straw bale wall as the plaster skin provides a similar function. As an added benefit lime and clay plasters allow the straw to absorb and release water vapor on both sides of the wall, preventing damage from accumulation. This happens in cold to hot climates, and dry to humid ones.
5. What is the best material to use for covering up the straw bale and why?
Fourth Pig: Natural plasters like lime and clay provide the most benefits for straw bale walls for their breathability qualities. Various factors such as design, cost, performance and historic longevity have shown the benefits of using lime and clay and avoiding cement and acrylic based plasters or covering the straw bales with drywall.
6. Can straw bale be used for houses that are more than one story high?
Fourth Pig: The limits to how high you may build a SB wall are the same limits all buildings face. With intelligent design a skyscraper could be constructed with a façade of strawbales. (We would like to retrofit a multiple storey building with strawbales!)
7. How long does a house made with straw bale generally last?
Strawbale walls and all buildings will last as long as the buildings inhabitants make them last. All buildings require regular maintenance and upkeep as do strawbale walls. With proper design and upkeep SB walls can last for centuries or more. There are strawbale homes in the US that are still standing and in great condition from the mid to late 1800s.
8. Is it more or less expensive than building a stick built house?
Straw bale walls can be cheaper than regular insulated wall construction but there is so much variation of wall finishes and detailing that that need to be taken into account when trying to compare apples to apples. You will not get an R30 insulation value out of a traditional stick framed wall.
For more information on building a straw bale home, or have more questions about the material’s durability, visit the Fourth Pig’s website, or contact them directly.
Feature Image courtesy of Mick Paterson. Rear view, straw bale constructed home addition, before exterior cladding