Perfect Plaster

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Water vapor moves from warm to cold: from a warm interior wall to a cold exterior wall, for example. Great differences in temperature cause vapor to condense into liquid water on the cold surfaces. Where moisture accumulates, building materials can be at risk for deterioration: Wood rots, straw molds, metal rusts and plaster erodes.

Although the overall goal is to keep building assemblies from getting wet, and allowing them to dry if they do get wet, different climates call for different vapor-barrier strategies. For specific recommendations for your climate, the Energy Efficient Building Association publishes climate-specific Builder's Guides ($40 each; available through their Web site at www.eeba.org ), which offer good vapor-barrier design methods.

Before building a conventional home, chat with local contractors to find out what works for them. If you're building a straw bale, cob or adobe home, you'll have to design your overhangs and foundation well, but the materials (straw bales, earthen plaster) used in these construction styles are permeable to water vapor and will allow your house to release water vapor freely.

EARTHEN PLASTERS

Earthen plasters are the most ancient of all plasters and continue to be used in many parts of the world today. Here in the United States they find a receptive audience among many natural builders, who are making homes out of straw bales and earthen materials, such as adobe, rammed earth and cob (see "Building with Earth," April/May 2002).

Earthen plasters require some experimentation. Unlike other wall finishes, such as gypsum plaster, earthen plasters are not sold in ready-mix powders, except in Germany, where they are available in bags from building supply outlets. Earthen plasters must be made from scratch: dirt, often with a little sand, straw and cooked flour paste (hardener) or manure added to create a more durable finish that won't shed sand or fine clay particles. In most locales, earthen plasters are made from subsoil containing a mixture of clay, sand and silt. The subsoil in some regions is perfectly suited for earthen plaster. In other areas, the subsoil may need to be amended with additional sand or clay to create a workable mix. Most mudders experiment with various mixtures to see which ones are easiest to apply and adhere well, and determine which mixtures offer the best long-term performance. Even so, it doesn't take a mad chemist to get a good mix.

With the right clay-sand-silt ratio, little more is required to make an earthen plaster other than screening, stomping and smearing. First, dirt is screened to remove pebbles, rocks and organic debris, such as roots. Then it's mixed with water and straw, and stomped or stirred. (Kids and adults love to join in: It's far more marvelous than mudpie making and is a satisfying stress reliever.) Once the mixture is thoroughly combined, the mud plaster is applied to walls by hand or with a trowel.

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