on fire.
Spontaneous Combustion
Concerns about spontaneous combustion do not apply to straw bales. The significant microbial activity within hay bales can result in spontaneous combustion under extreme conditions of humidity, temperature, and storage. Straw bale walls, however, will not create or support spontaneous combustion.
Moisture
Moisture is the enemy of all builders, regardless of which materials they are using. Wood, brick, and even stone walls will deteriorate when exposed consistently to moisture. Bales, like any other building material, must be kept as dry as possible throughout the life of the building.Wet straw molds and eventually decomposes, creating an unpleasant odor, potentially harmful spores, and possible structural failures. Dryness is important.
Many excellent building practices have been established over the years to help solve key moisture problems, and it is important to apply these practices to bale buildings. There are two kinds of moisture concerns for bale walls:
• direct wetting or leakage of liquid water into the wall
• vapor penetration and air leakage into the wall
Straw builders must be aware of both these concerns in order to take adequate protective precautions.
Direct Wetting and Leakage
The most obvious source of moisture problems is the penetration of liquid into the wall cavity. This can happen in many ways, including windblown rain, drifting snow, splash-back from roof dripping, plumbing leaks, floods, and breaches of the wall’s protective layers such as leaky roofs and window sills.
These problems are real and important considerations in any kind of building, and our chapters on bale wall design, construction, and finishing place these concerns at the forefront in every regard. Liquid breaches in the walls are preventable, and back-up protection can be included to provide a further safety margin.
Vapor Migration through Walls
Think about blowing up a balloon. You force warm, moist air from your lungs into an airtight container, creating a higher pressure than exists outside the balloon. Nature’s incessant balancing act insists that the warm, moist air will do its best to leave the balloon and join the surrounding atmosphere.During the heating season,your house is essentially the same as the balloon. When you add heat to your living space, you fill your relatively airtight house with warm, moisture-laden air. Warm air naturally carries more moisture than cold, and you also add extra moisture by breathing, cooking, bathing, etc. That air will do its darndest to get out of the house and give its heat and moisture to the cold dry air outside. Moisture always drives from the warm side to the cool side.
Why Not Just Wave the Moisture Good-bye?
The warm, moist air that wants to travel through your walls does not stay warm and moist. At some point in its journey to the outside, it will begin to cool. As it cools, the water vapor it is carrying can condense back to liquid. The point at which this condensation occurs is known as the dew point. If liquid is deposited in your walls and allowed to remain there without drying out, it will reduce the efficiency of your insulation and eventually lead to molding and rotting. In hot southern climates, the whole process can happen in reverse, especially if you use air-conditioning.
Fire Testing
Straw bale architect Bob Theis composed this list of fire testing performed to date on straw bale walls as part of the movement toward a new straw bale building code in California:
1. 1993: Two small-scale ASTM E-119 fire tests at the SHB AGRA lab in Sandia, New Mexico — one test wall with plastered faces, the other bare bales — showed bales to be very fire resistant. The unplastered bale wall withstood the heat and flames of the furnace for 30 minutes before flames penetrated a joint between bales. The plastered bale wall was naturally much better, resisting the transmission of flame and heat for two hours.
2. 1996: A full-scale ASTM E-119 fire test at the University of California Richmond Field Station easily passed the criteria to qualify as a one hour wall. In the opinion of the experts present at the test, the wall would probably have passed as a two-hour assembly.
3. 2001: The Appropriate Technology Group at Vienna Technical Institute conducted an F90 test (similar to the ASTM E-119 test), which gave a plastered straw bale wall a 90-minute rating.
4. 2001: The Danish Fire Technical Institute tested a plastered straw bale wall with exposed studs on the fire side as a worst-case scenario and got these results: in a 30-minute test with a 1832°F (1000°C) fire on the exposed 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
