More Straw Bale Building. Peter Mack
side, the unexposed side rose just 1.8°F (1°C). The maximum average increase permitted to pass the test is 144°F (80°C).
5. 2002: Bohdan Dorniak and members of AUSBALE tested individually plastered bales to the Australian standard simulating the heat of a bushfire front. Subject to a maximum heat intensity of 29 kilowatts per square meter, none of the nine plastered bales ignited, or even developed visible cracks. According to Mr. Dorniak, this qualifies them as noncombustible under the current Australian Bushfire Code AS 3959.
6. 2000: Flame Spread and Smoke Density tests. Katrina Hayes sponsored an ASTM E84-98 test on straw bales in 2000 at the Omega Point Laboratories. They passed the test easily; where the Uniform Building Code allows a flame spread of no more than 25, the test produced a flame spread of 10; where the code allows a smoke density of no more than 450, the bales produced a smoke density of 350.
Why Aren’t More Old Houses Molding and Rotting?
In earlier times, leaky windows, doors, walls, roofs, and floors kept relative humidity indoors below problem levels. Those same homes also required more energy to heat, because the heated air escaped the building through all these leaks, dissipating into the atmosphere. As better windows, doors, insulation, and building practices — especially the use of continuous plastic vapor barriers — began to make houses more airtight, the need grew to prevent moisture from migrating directly through the walls.
Direct Air Leaks
Moisture can also enter your walls through direct air leaks. The Canadian Home Builders’Association estimates that, over a single winter of heating, air leakage through a hole with an area of less than one square inch could allow up to eight gallons of water to pass through a wall! It is critical to keep the moisture content below 20 percent in the walls.
A Bit About Mold
Mold spores exist in the air all around us. With every breath we take we are likely inhaling at least one mold spore no matter where we live. These spores are the seeds of mold colonies and are released into the wind to settle in another area and start a new colony. Molds provide an important function when they break down plant matter, returning nutrients back to the soil just like when a pile of wet leaves are raked up and after a few days start to rot. Mold spores that settle into a moist, dark environment with a food source such as cellulose (wood, straw, cardboard) will start to grow into colonies in as little time as 24 hours.
The study on the health effects of molds came from observing animals bedded on moldy straw. Molds have two health issues: the first is allergies to the spores that we inhale, the most common symptoms being runny nose, eye irritation, cough, congestion, and an aggravation of asthma. The second is that mold colonies also release gases as they grow that can be potent mycotoxins, fungal metabolites that have been identified as toxic agents. Individuals with chronic exposure to the mycotoxins produced by Stachybotris chartarum fungus reported cold and flu symptoms, sore throats, diarrhea, headaches, fatigue, dermatitis, intermittent local hair loss, and generalized malaise.
The straw that is baled in a field and stacked to form walls for houses contains a high amount of mold spores that have settled out of the air and from the soil that the grains were grown in. All they need is a moisture level high enough to grow. A tarp lifted up from a pile of stored straw may reveal bales discolored with a black mold, likely Stachybotris chartarum, considered to be the most toxic to humans. The tarp may or may not have stopped rain getting in, but it trapped the moisture coming up from the ground. If the moisture levels in straw bales is above 20 percent then the mold spores present will start to grow. ☞
Inspection of mold in straw bale walls has shown that in most cases the moisture entered the wall during construction with the straw not being replaced prior to plastering. It can take up to two months for a perimeter wall to dry out, which is ample time to allow molds to form. Unplastered bales can withstand some surface wetting that will dry with air exposure. If during construction the walls are exposed to moisture then they should be inspected to see if the moisture has penetrated the core of the bales. A bale should be replaced if the moisture has penetrated more than three inches into the bale and above the 20 percent moisture content. Moisture meters with long probes are usually available from local farm supply stores. When handling any bales that have become moldy, wear personal protection in the form of respirators with HEPA filter cartridges, gloves, and goggles. Strong air blowers, used in the flood and fire industry, can be rented to dry surface moisture. Moisture can also enter bale walls from floods, a leaky roof, a large crack in the plaster, poor window and door placement, or plumbing leaks as is the case with any type of wall construction. All walls exposed to moisture regardless of the type of materials, can support mold growth and should be inspected and dealt with in a timely and appropriate manner.
This book is about best building practices of straw bale construction and following the recommendations on roofs, overhangs, and foundations that will ensure a healthy, mold-free structure. People who are considering straw bale building but who have allergies to dust and the general spores found in straw bales should realize that during construction these are present and should wear personal protection. After plastering, mold spores and dust are encapsulated and will not influence health or indoor air quality (IAQ). Straw bale walls actually improve the IAQ in homes due to the slow air diffusion of the walls.
Paul Battle is a certified Bau-Biologie Environmental Inspector in Ottawa Ontario. He specializes in the detection of home health hazards and consults on ecological home construction. Contact <www.thehousedoc.ca> (613) 297-2996
Moisture concerns are hotly discussed among bale builders. Most of the testing done to date confirms what conventional builders already know: cracks, openings, and penetrations into the wall pose much greater risks for moisture damage than does vapor migration through walls finishes
So What About Bale Walls?
The plaster coating on bale walls is an effective barrier against damaging air leakage. If properly detailed to tie in with conventional polyethylene vapor barriers installed in the ceiling and under the floors, a bale house can be made airtight.
Throughout this book, we will address proper detailing for creating a leak free bale home without using a plastic vapor barrier over the straw wall.
Barriers
Why Build without a Vapor Barrier?
In conventional building practice, moisture is prevented from migrating into the wall cavity through the use of continuous plastic vapor barriers. This addresses the very real concern of air leakage in stud-framed homes, but it is unnecessary and a structural compromise with bale walls.
Attaching a vapor barrier to a straw wall offers many complications. The barrier can only be attached to the top and bottom of the wall, making it hard to maintain a taut surface unless wooden attachment points are added to the wall. A vapor barrier prevents the plaster coating from attaching itself directly to the straw. This not only makes plastering much more difficult — and will likely require more metal reinforcement for the plaster — but eliminates the substantial structural benefits of bonding the plaster to the straw. Remember, bonded together, the two materials create a stressed skin panel far stronger than the sum of their independent elements. A straw wall without a vapor barrier is less time-consuming to build and eliminates the use of a manufactured product with a high embodied energy. Finally, straw, metal stucco mesh, and plastering tools all increase the risk of introducing punctures to the vapor barrier, rendering it less effective.
To Barrier
There is no doubt that a properly installed vapor barrier is an effective tool in preventing moisture penetration through your walls. Though moisture migration has not been shown to be a problem in straw bale walls, a builder might choose — or be forced by local building officials — to spend the extra money and time and forego the structural strength and environmental bonuses of building a straw wall without a vapor barrier. In this case, you might want to use