342 – The Art and Science of Natural Plasters – Part 1

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In today’s podcast, Paul, Jocelyn and Fred review Bart’s DVD on The Art and Science of Natural Plasters, for which Paul was the “Supreme Executive Producer.” This mostly because permies.com people supported the Kickstarter in a big way.

You can find the DVD at http://plasterscience.com

Paul says “Overall, I thought it was really good.” He hasn’t had a lot of experience with natural plasters, although he did a lot of “mudding” with the commercial white stuff for drywall when he was a kid, working with his father.

So, a plaster is aggregate and fiber, somehow bound together. The aggregate lends strength, the fiber lends flexibility and strength. The binder for cob is clay – sticky and kinda like glue. You can make a plaster with clay as the binder, it’s just got finer aggregate and fiber in it than cob. You can also have a lime plaster.

Jocelyn thought there was going to be more about natural paints, but it was mostly about structural plasters and finish plasters, although of course you can tint the plaster and you’re done. Paul says that finally at the end, Chris Magwood (the star of the video, a guy with 20 years of experience in this space) says that lime plasters are recommended for outside uses and clay plasters are recommended for inside uses. Lime plasters are more rain resistant, it’s more of a chemical bond that gets stronger with time. Clay never sets permanently, this makes it vulnerable, but it also makes it easier to repair. Local clay has the least amount of embodied energy.

A big difference between the two is that lime has a lot more embedded energy – you have to expend a lot of energy to create lime. Paul recalls watching how limestone was converted to “slaked lime” by burning it in a kiln. When you add water to lime, it gets hot, because the energy previously put in there is sort of coming back at ya. (Editor: This is chemistry, folks. In the kiln an endothermic chemical reaction is forced to happen with the addition of heat, and then later on, adding water leads to an exothermic chemical reaction. It’s a way to transport the energy to where you need it.)

Anyway, getting back to the clay plasters, for inside use. If you put a thick layer of plaster (3/4”) on the insides of your walls, you are increasing thermal mass, which has a lot of advantages. The clay will fill all cracks, so your home is more “airtight” but the material itself is breathable, so you don’t have to poke holes in your house like you do when you seal your house with plastic. Per Paul, it’s like living inside a terra cotta pot – no wind, but breathable.

Clay plasters are what you’re going to use when you build a straw bale house, or when you build a cob house. Another framework for plaster is lathe (a bunch of thin boards). This reminds Paul of living in a plaster and lathe house and how you can’t hang a painting the usual way, because you can’t nail into plaster. One solution is to have a board worked into the wall, another is to hang your pictures from crown moulding. The moral is, plan where your towel racks are going to go BEFORE you finish the walls with plaster.

In the video, Chris Magwood does a good job of explaining how you use the different materials to make a good thick strong layer that sticks to the wall. They used a fair amount of straw, cut into small bits with a chipper. Jocelyn notes how they had a lot of good practical tips, and how Chris could plaster an entire wall in the time it took five students to plaster half a wall. Getting this video will be really helpful for anybody planning to use natural plasters because you can tap into all his experience.

So the plaster goes up in layers, one to create a base (a filling layer), one (with 50% straw by volume) that creates most of the mass, then a smoothing layer and then a finishing layer. There could also be a final pigmented finishing layer.

Paul remembers making drywall with his dad, where you have many steps, the final one being “getting out the splatter gun” and adding texture, and then they always painted the finished walls the same color of white. A natural plastered wall has a different aesthetic. You can have all the wonderful curves of the cob structures, you can feel the human touch.

Paul thought that the level of finish in the video was sort of beyond his own ambitions. In his wofati’s they’re still figuring out what sort of floor to have! Suffice to say, there’s a lot of gorgeous in this video.

Jocelyn remembers him saying that with natural plasters, particularly clay plasters, you have to make sure there is a “mechanical grip” between the layers. For example, you’ll need to score a cob or plaster surface to get a plaster to stick well. If you’re going to put a natural plaster on a drywall wall, you will need to make a glue, like a wheat paste glue, with sand in it, to provide a transition from the sheetrock to the natural plaster.

Paul remembers him talking about using mesh as a texture creating device. It looked to him like he used a metal mesh, but he also said you don’t want to use metal, it will expand and contract too much and mess with the plaster. He likes fiberglass or polypropylene mesh, Jocelyn recalls.

Paul diverts into a discussion about how chipmunks got into Allerton Abbey when they left off the door for a while and how they really don’t have a problem with bugs on the land. Ticks are rare. Still, in places with more bugs, the “airtight” aspect of a naturally plastered home will be great in terms of keeping bugs out. (Ed: this is a function of water. Bugs love water and heat, so places with summertime rain tend to have a hell of a lot more bugs.)

Paul wanted to see how he finished the window wells, but the video is over two hours long, so of course they couldn’t show anything.

Relevant Threads

The Art and Science of Natural Plaster Thread
Natural Building Forum at permies
Cob Forum at permies

Credit: Julia Winter

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