Desiccant Activity
When two materials of different equilibrium moisture content levels are bonded together, a degree of desiccant activity between the materials is established. The following diagram shows the equilibrium moisture content of both straw and earth plaster over a range of humidity levels, and the degree of desiccant activity between them…
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. Straw is a plant-based material comprised mostly of cellulose and lignin, and has a much higher equilibrium moisture content. Wood is also comprised of cellulose and lignin, and has a similar equilibrium moisture content to that of straw.
What is desiccant activity with respect to a wall assembly?
Desiccant activity is the ability of one material in a wall assembly to draw moisture out of another material, acting as a preservative for that other material by keeping it dry. A familiar example is the small pouch of desiccant found in some vitamin containers — the desiccant in the pouch absorbs moisture and keeps the vitamins dry.
How does desiccant activity manifest itself in the straw-bale or straw/clay wall assemblies of The Naturally Passive House?
In The Naturally Passive House concept, earth plasters are applied to the interior face of either the Edge-Plus straw-bale wall or the Lath-Plus straw/clay wall. Because these earth plasters have such a low equilibrium moisture content compared with straw, the drying potential is significant. This is indicated by the arrows in the above chart. When earth plaster is applied to a straw-bale or straw/clay wall and thereby forms an intimate bond, the earth plaster acts to continually draw moisture out of the straw and evaporate it to the atmosphere, effectively acting as a preservative by keeping the straw dry. Earth plasters are therefore very effective at maintaining dryness within Naturally Passive House wall assemblies.
Do other Passive House wall assemblies offer similar desiccant activity?
The insulation and finish material combination of straw and earth plaster is unique because of its intimate bond and significant drying potential. Other Passive House wall assemblies with fiber or foam insulation finished with drywall simply do not offer the same desiccant activity.
Earth plaster is an amazing finish material, having numerous advantages over more conventional wall finishes. Because earth plaster has no flame-spread index or smoke-developed index, it is noncombustible. Because it offers significant thermal mass, it helps to regulate indoor temperature extremes and improve comfort. Because earth plaster is highly permeable, it freely allows any water vapor in a natural 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 because earth plaster 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 interior finish material for the Edge-Plus and Lath-Plus wall assemblies, and offers benefits for conventional wall assemblies as well!