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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.