Episode Six: Exterior Finishes

Natural Earth and Lime Plasters

"Today we're going to play in the mud. It's time to cover the straw bales of our dream home with natural plasters. Like skin, our plasters are going to hold in its precious contents, protect them from the elements, and then breathe to maintain a healthy environment inside. We're also going to pour our windowsill. That's right, pour them. Our windowsills actually come in a bag, and we'll create a tight-fitting, leak-proof windowsill that will add a striking element of beauty to our homes."

- Building Green Host Kevin Contreras

Kevin's straw bale dream home is progressing nicely, and now it's time to start the plastering process. The challenge is to cover the exterior of the house and all the straw bales with a plaster that is breathable and low in embodied energy.

Kevin explains that advanced bale builders are covering entire straw bale buildings in earth plaster, a mud and straw mixture that is dirt cheap. Earthen plasters are incredibly green because they do not off-gas, there is very little waste, they are easy to repair, they are nonflammable and they resist mold growth. Kevin has also decided to add a final limestone coat to his home in order to give it more durability and a more classic, "finished" look.

Environmental health expert Alyssa Alvord explains that modern building systems have created very airtight structures, which actually promotes mold growth. Using earthen plasters to cover the straw bales is the easiest way to assure that they'll stay breathable and mold-free.

Contrary to popular belief, earthen plasters can be very beautiful. When done right, earthen walls are incredibly durable and aesthetically pleasing. To find out more about earthen plasters, Kevin visits Bob Bolles, codirector of Sustainability International, a group dedicated to demonstrating the economic benefits of sustainable technologies. Bob explains that earthen plastered walls are not as hard as concrete, but when taken care of correctly can be nearly as strong.

Natural Hydraulic Lime

Kevin then visits Michel Couvreux. Michel owns Transmineral U.S.A., of Petaluma, California. He is the only distributor of St. Astier lime plaster in the United States. This natural hydraulic lime has been extracted from the same mine in the Bordeaux region of France since 1851 and has been used to restore its greatest monuments. Hydraulic lime is as easy to use as concrete, but its production creates 80 percent less carbon dioxide than the production of concrete, making it a great green choice. Hydraulic lime is also highly breathable, meaning condensation will be able to move freely out of the straw bales and mold will not grow. Hydraulic lime is about 2 to 3 times more elastic than concrete, which is incredibly important because with straw bale building there is a certain amount of setting or movement. Coat the straw bales in anything that is not elastic and you get cracking.

On the inside and the outside of the bales, Kevin is eventually going to apply several layers of earthen plaster. Typically, only two coats would be needed but, because Kevin wants very straight walls, in some places he will have up to five coats. Once the walls are sufficiently coated and straight, a couple coats of lime plaster will be used on the outside and a couple of coats of gypsum plaster will be used on the inside.

Saving Green with Earthen Plasters

Earthen plasters ensure a less toxic home environment and they're also as affordable as traditional wall construction. Although the labor will be a bit more expensive and the lime and gypsum plaster will be more expensive than concrete, the rest of the wall building materials are so inexpensive the entire wall system is still low cost.

The Application

Before starting the application, Kevin invites straw bale expert Tim Owen Kennedy to visit the job site. Because the greenest way to make earthen plaster is to use soil from your building site, Tim tests the local soil to see if it will make good plaster. After mixing the soil with some water and straw, Tim finds that it is not suited for plaster. While wire mesh is being installed over all of the walls, Kevin talks to Jay and Michael about the windows. Because protecting straw bales from moisture is so important, windowsills are very critical in keeping the elements out. Using a wooden form below each window, allows Kevin to use a pourable limestone product that will provide a fully sealed, formfitting windowsill.

Because the soil from the building site is not suitable for making plaster, Kevin orders a truckload of the best clay product from Lake Elsinore, California for just 10 dollars a ton. When it arrives, Kevin starts out by mixing an earthen clay test sample and experiments with a spray applicator for the scratch coat, which is purposely very rough so that it will key in (or adhere to) the next coat. The scratch coat will take one to two days to dry and then additional coats will be added. Earthen plaster can be applied by hand, by spray applicator or by trowel.

As the first coat of earthen plaster is drying, Michael begins pouring the limestone sills. Having put down a concrete base, he tapes off the window and adds a layer of bonding glue to ensure that the lime will bond to the concrete substrate. He mixes the lime according to the instructions and can then pour it into the form. It will need to dry for a few days and then be sanded. After the earthen plaster has dried, it is time for the application of the lime plaster. First, Michael does a few batches to test the color. Using yellow oxide pigment, Michael has created the color Kevin wants for his exterior lime finish. Once the color is perfected, the lime plaster can be applied. It is applied one wall at a time, and then protected from the sun as it dries by a large piece of plastic hung from the roof. Once the wall has dried, it is given one more coat of lime plaster.

Kevin then visits coating and finishing expert Peter Colburn to learn more about exterior painting. Kevin is most interested in natural, low toxic paint and Peter explains that while some natural paints don't work as well as traditional paints, there are good, healthy solutions, like milk paint. However, because of the color mixed in with the hydraulic lime plaster, Kevin has completely eliminated the need for exterior paint on his house—a savings of thousands of dollars.