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 Mold and Alternative Building Materials
  Straw Bale  
  Straw bale homes can be very healthy places to live. Straw bale homes are no more susceptible to mold than convention homes. Mold spores are naturally inherent inside a bale of straw. If you take a core sample of a bale of straw and place it in a petri dish, mold will grow. Straw bales in a home will not grow mold unless the straw gets wet. Since all building material that gets wet except for glass and steel has the potential to grow mold, why worry about straw? Itís usually conventional homes making the news about toxic black mold, not straw bale ones.  
  Straw bales, like adobe walls, have a plaster finish that keep mold spores from ending up in the air as demonstrated with air quality testing. The natural plaster finishes applied to these walls breathe, allowing for moisture inside to escape and for some amount of natural ventilation. In 1997 a public library was built in Portland, Oregon using straw bale. Moisture sensors were installed in the walls. After six years of testing the sensors on the exterior walls never exceeded 12%. Wood and drywall in conventional homes may typically indicate 10-12% moisture content. As with conventionally built buildings, those involved with the project noted three design features that accounted for overall low moisture content in straw bale walls9:  
  • A rubble drain around the foundation that allows water to drain and can not wick up
  • Breathable stuccos using earth and lime
  • A good roof
  Adobe Homes  
  Contrary to belief, mold does grow on adobe. Adobe bricks are basically dirt. If dirt gets wet mold can grow and generate odors. What prevents mold from growing on adobe and affecting the indoor air quality is the plaster finish. Plaster is an amazing finish material. Itís almost similar in composition to what is used to make adobe bricks, yet by adding slightly more clay creates a hard surface that seals, sheds water, and breaths.  
  The cement stucco applied to conventionally built homes with wood framing built to look like adobe is different than plaster. Cement stucco does not breathe. Cement stucco has a different coefficient of expansion than adobe, cracks and requires wire netting to be applied. To prevent mold growth in stucco homes ensure that there is a way for water that penetrates the stucco to drain off. This can be done by applying two layers of building paper. The stucco will stick to the first layer and there will be a gap between the first and second layers for water to drain. Another option is to put a single layer of light layer of building paper over house wrap. Weep screens should be used at the bottom of the stucco walls to allow the water to drain off. Do not cover the weep screens with stucco and paint or cover them with dirt from landscaping. If you have a stucco home go out and look at the bottom of the wall. You should see weep screens at the bottom.  
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Revised: July 05, 2017.

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