Monday, July 21, 2008

Mud Pies and Moisture Dynamics...What More Could a Gal Ask For?

Well, folks, it's been awhile! I'll be surprised if anyone is still checking this blog considering my lengthy cyber-vacation. But here I am, ready to tell ya'll about my latest class: Natural Wall Systems. In a later post perhaps I'll get into the "why's"behind natural building, but for now I'll just give a short definition and then delve into the project I worked on. Natural building means constructing with earth based, renewable materials that are mainly processed on-site by the builders themselves rather than far away in a factory. Remember that timber-frame barn I helped raise several weeks ago? (Heh, if you don't, just scroll down--it was my latest blog entry.) Here's that barn again, now chock full of dirt, straw, sawdust and other wacky materials that we would never think to put in our walls nowadays. Thank goodness for wackiness...I'd much rather be throwing around mud pies than cuddling up to a pretty pink batt of fiberglass. Mmmm.

In the photo above you can see three different wall systems. The system on the doorway wall is called "light straw clay." We gathered clay from a variety of local places (quarry, riverbank), mixed it with water in a big 55 gallon drum, and then used a pitchfork to toss the soupy clay (also called clay slip) and loose straw salad-style. The slightly wet mixture was then dumped in between a double-stud wall that was enclosed with scrap pieces of plywood. After folks tamped down the straw-clay mixture, the forms were removed, revealing beautiful wall sections that held their shape. The picture to the right was taken a week after the wall was finished. There must have been grass seed in our straw bales! Luckily this is fine--there isn't enough oxygen or moisture in the wall to sustain the grass for long. Some natural builders even purposefully include grass seed in thier wall systems; they know when the walls are dry and ready to plaster when the grass is dead.

We used a second system called "wood-chip clay" on the back wall of the barn as well as each of the four outside corners. We mixed a variety of grades (sizes) of woodchips obtained from local sawyers in a concrete mixer with the clay slip. Then we simply dumped the mixture into the stud wall. We didn't tamp because the chips are heavy enough to fill the space without creating any huge air pockets (as would have happened had we not tamped down the light straw clay). Because the chips can't hold thier form in the wall, however, we lathed the inside and outside of the double stud wall (lath are the thin, narrow strips of wood) in order to hold the material in. This was by far the fastest of the three methods because of the use of the concrete mixer and because the wood chips don't compact like the straw does (thus the cavity fills faster). However, the mixer was real loud and used a bunch o gasoline--both tradeoffs in terms of sustainability...I'd love to see a bicycle powered mixer someday!

The main wall system we implemented was the straw bale walls on the east and west faces of the building. Leaving the bales intact, we squared the ends with a chainsaw and laid them longways, brick-style, in the wall cavity. We retied and separated a bale into two if we needed shorter bales. Bales were then attached to studs with long timberlock screws or polypropelene strapping. Once all the bales were in place we took a "persuader"--literally a giant wooden mallet the size of a mailbox--and pounded individual bales that stuck out until the wall was nice and plumb. Then we shaved the entire inside and outside surfaces with a chainsaw (you can use hedge clippers or a weed wacker), beveling window openings.
The key to a straw bale wall system is that it is tight--any holes or spaces are stuffed well before the plaster goes on. So that's what we did--the dark spots in the top picture are handfuls of a straw clay mixture wrung into knots and shoved in between the bales. A tight bale system means virtually no oxygen, which means no possibility of fire and no room for rodents. Yay straw!
After the natural walls were constructed, we sprayed them with clay slip. Well, before we sprayed the walls, we had a little fun with the sprayers ourselves. :) After the wall is wet down, we slapped a gooey, puddin' like consistency of clay and straw on. Usually a building will have two, three or more layers of plaster, using successively shorter pieces of straw and more refined clay and sand as you work towards the outermost layer. Often the last layer involves some sort of lime, especially in climates like the northeast where there's lots of moisture in the air. Lime is durable and also vapor permeable, meaning the wall can breathe in and out moisture. If you put a cement plaster on the wall and the wall hadn't dried out or moisture somehow got in, that moisture would have nowhere to go. The breathability of a lime plaster is an apt metaphor for the vibrancy of the natural building it covers--how meaningful to live in a building created right out in nature with one's own hands.

That's all for now! More before the summer ends.....?? Keep your fingers crossed.