Hygroscopicity
Note: the hygroscopicity data is from Minke.
What is hygroscopicity?
Hygroscopicity is the ability of a material to absorb, store, and release water vapor. The presence or absence of hygroscopic materials has a direct impact on indoor relative humidity levels.
Why is hygroscopicity important?
Hygroscopicity is important because hygroscopic materials are able to moderate humidity levels throughout the seasons, maintaining a relatively constant indoor relative humidity level by absorbing water vapor during periods of high humidity and releasing water vapor during periods of low humidity. After temperature, indoor relative humidity level is the next most important climatic factor affecting health and comfort. A year-round relative humidity level of 45-50% is ideal.
Are the beneficial effects of hygroscopic materials tangible?
The beneficial effects of hygroscopic materials should not be overlooked. This is another of the major advantages of natural wall systems when compared to more conventional wall systems. Earth plaster is a highly effective hygroscopic material, as seen in the following example—after a steaming-hot shower, mirrors in a bathroom with conventional wall finishes will typically fog up; in a bathroom with earth plasters, the mirrors won't fog up. The water vapor generated by the steaming-hot shower is rapidly absorbed by the earth plaster and then slowly released over time, maintaining a nearly constant relative humidity level. It's hard to believe that earth plasters would be an appropriate finish material in a bathroom, but bathrooms are actually an ideal location for their use!
How hygroscopic are natural materials in general, and how does earth plaster compare to concrete stucco in terms of hygroscopicity?
Nearly all natural materials, and especially timber, earth, and straw, are highly hygroscopic. Earth plaster is significantly more hygroscopic than concrete stucco—during the same time period, earth plaster can either absorb or release nearly five times the quantity of water vapor as concrete stucco; earth plaster is also ten times faster at absorbing or releasing the same quantity of water vapor as concrete stucco. Earth plaster is therefore clearly superior as a finish material for natural wall assemblies.