Desiccant Activity
Note: the equilibrium moisture content values are from data published by Minke.
What is equilibrium moisture content?
Equilibrium moisture content is the amount of moisture stored by a material at any given relative humidity level. It is measured after a material has been maintained in a "dry" state for a long enough period of time to reach equilibrium, with no external source of water other than the humidity in the air. In general, equilibrium moisture content for all materials rises as relative humidity rises.
How does the equilibrium moisture content of straw compare with that of earth plaster?
Earthen materials are mineral-based, and as such, have an extremely low equilibrium moisture content. Staw is a plant-based material, and has a much higher equilibrium moisture content.
What is desiccant activity with respect to a natural wall assembly?
Desiccant activity is the ability of one material in a natural wall assembly to draw moisture out of another material, acting as a preservative for that other material by keeping it dry. We've all seen that little pouch of desiccant found in a vitamin container. It's the same process—the desiccant in the pouch is absorbing moisture and keeping the vitamins dry.
How does desiccant activity manifest itself in a straw-bale or straw/clay wall with earth plasters?
Because earth plasters have such a low equilibrium moisture content compared with straw, the drying potential is great. This is indicated by the arrows in the above chart. When earth plaster comes in direct contact with straw, the earth plaster acts to continually draw moisture out of the straw and evaporate it to the atmosphere, effectively acting as a preservative. Earth plasters are therefore the key to durability of all natural wall assemblies.
Earth plaster is an amazing finish material, having numerous advantages over more conventional wall finishes. Because earth plaster is highly permeable, it freely allows any water vapor in a wall assembly to escape. Because it is extremely hygroscopic, it is able to seasonally regulate indoor relative humidity at the optimum level for health and comfort. And last but not least, because it has such a low equilibrium moisture content, it acts as a preservative for the entire wall assembly through its desiccant activity. Earth plaster is the ideal finish material for all natural wall assemblies, and offers benefits for conventional wall assemblies as well!