An Introduction to the Role of Earthen Plasters & How They Work for Natural Buildings:
Earthen plasters are an essential part of any natural building. When we say ‘natural’, we mean straw bale, clay fiber, or cob to name a few. There is one key point that all of these wall systems have in common: whatever the finish it needs to facilitate the movement of moisture, and clay is the perfect companion. [For a discussion on choosing an exterior plaster, read through this article]
Clay plasters are the skin of these buildings. They work in a very similar way to the way the skin of your body works. You can sweat, take on water, dry off, keep cool or warm, and you probably appreciate it when you have some protection from the elements. But you also don’t feel comfortable when you are clad in a plastic raincoat that doesn’t breath, as you usually end up as wet on the inside as out. This is somewhat analogous to the differences between how clay plasters and cement stucco behave in relation to any natural wall system. Not only will the final outside layers determine how the building will respond to moisture in the long run, they also have an effect on the indoor experience of the building.
Clay helps to buffer moisture and sound, resist mold, is VOC free, as well as warm and calming; cement does not provide these qualities. For a wall system like straw bale that mostly provides insulation, clay plasters can truly add benefit by providing thermal mass.
Consider this: the relationship between thermal mass and insulation is inversely proportionate. Really, all that means is when you have good insulation value, you have poor thermal mass value. We live in a chilly, Northern climate, so we all know about the importance of insulation, for sure. But what about thermal mass? You can think of thermal mass as sort of a bank, for warmth as well as coolness.
Basically, thermal mass is the ability to store heat after the source of that heat is no longer present. If you have a fire place, surrounded by bricks, those bricks are acting as a thermal mass. If you have passive solar design with south facing windows, your floor and walls can act as thermal mass if made of appropriate material, like clay plaster. Thermal mass has the ability to take extremes in temperatures (the peak of daytime highs, and the valleys of nighttime lows) and moderate the available heat. The same applies to the cooler temperatures that we all appreciate during the summer months. What this means for you is more comfortable temperatures all the time.
Recently, Ashley has had the opportunity to apprentice with two very talented people, Bill and Athena Steen who are both very experienced straw bale builders and earthen plasterers’. In fact they are credited with the modern day use of straw bales in building, and were the first in North America to teach a natural building workshop, more than 20 years ago. They have a beautiful place in Arizona called the Canelo Project. If you are interested, Ashley conducted an interview with Bill Steen which can be read here. It was Bill, who chuckled to Ashley at one point, “Straw bale construction is really more about the plasters than it is about staking bales”.