By Chris Magwood
An important concept to understand when considering moisture and building materials is adsorption. Moisture in vapor form infiltrates any and all materials. The surface of most materials will offer individual water molecules an electrically charged attraction, and the water molecules will “stick” to all available surfaces. The makeup of plaster and of straw bales offers a vast amount of surface area for this adsorption. Plasters are full of micro-pores and straw has great deal of available surface area as well as micro-pores in the hollow stems. Together, these materials allow a surprisingly large amount of moisture to safely adsorb onto/into the materials without the water molecules accumulating in sufficient layers to become drops of liquid water. Bales and plaster can hold a remarkable amount of moisture in adsorbed form. “For a 8 pcf (pounds per cubic foot) bale, more than 1 pound of water (approx. 1/12 gallon or 0.46 liters) in vapour form can safely be stored per square foot of wall area” according to John Straube in Building Science Digest BSD-112. This explains why the walls can perform so well as “vapor open” or “vapor permeable” systems.