More Straw Bale Building. Peter Mack
by Rob Jolly, 2000
Q: Can straw bale techniques really make a difference to the environment?
A: The embodied energy for the conventional frame house was 509,000 KBtus. The embodied energy for the low-impact straw bale house is 41,000 KBtus, or about one twelfth that of the frame house. — Investigation of Environmental Impacts, Straw Bale Construction, by Ann V. Edminster, University of California, Berkeley, 1995.
Q: Will a bale home be less expensive to heat?
A: Straw bale construction, along with appropriate building conservation technologies and simple passive solar design, could provide up to a 60 percent reduction in building heating loads over current practice. — US Department of Energy (DOE) straw bale assessment program, 1995
Fire
Straw bale walls have been tested for fire resistance. In these tests, straw bale walls have proved themselves far superior to standard wood framed walls.
Why Don’t They Burn?
Straw bale walls are naturally fire resistant.While the dry straw that makes up a bale is easily combustible when loose, the compact nature of a bale does not trap enough air to support combustion. A good analogy can be drawn by comparing the combustibility of a single sheet of newsprint to that of an entire telephone directory. The single sheet will ignite and burn quickly, but if you drop a phone book on your fire, you’ll probably put out the fire. The plaster coating effectively seals the already fire-resistant bales inside a noncombustible casing. A fire would have to burn through the plaster in order to reach the straw. When plaster is combined with the thickness of a bale wall, fire resistance is enhanced. Far from being an issue of concern, the fire testing performed around the world shows the fire-resistant nature of bale walls to be another impressive advantage of the system!
Don’t Ignore the Real Fire Hazard
All of the above is true of baled, plastered straw. Unbaled, loose straw, however, is extremely combustible, and the large amounts of loose straw that accumulate during construction are a serious fire hazard. Smoking, welding, grinding, or any other spark- or flame-producing activities should not be undertaken in the vicinity of loose straw. During construction, always be sure to have fire extinguishers and enough water available to deal with potential fires in your loose straw. Also, keep loose straw raked away from the walls to minimize the risk of any accidental fire spreading into the unplastered walls.
This is not just an idle warning. Several bale homes have burned to the ground while the straw was exposed and the site covered in loose straw, ignited most often by work site activity like soldering pipes and metal grinding.
4.2: This single, plastered bale shows how the plaster penetrates into the bale, gripping the straw. The two “columns” of plaster carry most of the loads imposed on the finished walls.
Loose straw (especially organic straw) makes excellent garden mulch. Rake it up, keep it under control, and don’t let it catch 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