At Home with Natural Plasters

Apply earthen plaster to your walls for a beautiful, breathable finish that’s as pleasing to the eye as it is to the environment.

| August/September 2020

plastering

I began plastering straw bale buildings in Ontario almost 20 years ago. Bale building has its roots in the Sandhills of Nebraska in the early 1900s, and it had a global renaissance starting in the 1980s. The only reference in popular culture to straw bale building at the time I started was the story of the three little pigs, so we used to hear some variation of a big bad wolf joke on almost every job site. Our standard reply was, “The first little pig forgot to plaster his house.”

There’s a lot of truth to this, because most of the strength, weatherproofing, and air-sealing in a straw bale home comes from the plaster. When engineers are approving a load-bearing straw bale building, one of the most important numbers is the compressive strength of the plaster. And what about the famous insulation value of straw bales? Every now and then, an owner-builder tries to overwinter in an unplastered bale house. On a windy day in the dead of Ontario’s winter, you’ll find them huddled under a blanket, with the temperature in their house hovering above freezing. But the following winter, when their house is wearing its draft-proof coat of plaster, it’s the most comfortable and efficient home for miles around.

Traditional Techniques Meet Modern Methods

Plaster is typically made of sand, fiber, water, and a binder. The binder is what defines the plaster. The most common binders are clay, lime (either hydrated or hydraulic), and gypsum (plaster of Paris). Some plasterers use blends of any of these three binders in various combinations, but the ratio is important, because complex chemistry is hidden underneath the kitchen science. For example, clay and lime can make a nice plaster, but in the wrong ratio, they’ll just make a crumbly disaster, because the necessary chemical reactions only happen above a certain pH level.



house

Natural plasters have many advantages over modern plasters that use cement or polymer as a binder. The two most important are that they’re more vapor-permeable, meaning water is less likely to become trapped and rot the underlying materials; and they have a much lower carbon footprint. (Cement is the source of about 8 percent of the world’s carbon dioxide emissions.) For me, the connection to a construction heritage that spans thousands of years is also meaningful. You can’t quantify it, but if you’ve spent time in historic buildings, you’ve likely felt it too.



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