Getting plastered: Earthen clay for the interior walls

1.27.2013


Insulation: installed. Ceiling: drywalled. Shou sugi ban siding: on hold until warmer days. In January, attention returned to the Matchbox's 140-square-foot interior—namely, getting its walls prepared for (and coated with) lots and lots of earthen plaster.

Why use plaster as a tiny house wall treatment? A few reasons:
  • It's natural. Earthen plaster is about as green as it gets: just mix a bit of dehydrated clay, some water, and a little natural pigmentation, and the plaster is ready to apply. Moreover, it's virtually waste-free—any plaster that drops or spills in the process of putting it on the walls can simply be thrown back in the bucket once dry to rehydrate and reuse.
  • It's breathable. A well-sealed tiny home is in some ways its own worst enemy. Because (in the absence of a ventilating fan) condensation has no means to escape, a tiny home right after a shower or well-cooked meal is likely to feel a lot like a steamroom. In the case of ordinary wall treatment, that humidity will just sit—or worse, sink into the walls and rot from within. Plaster, on the other hand, can act as a natural dehumidifier: sucking moisture in when humidity is high and gradually releasing it when humidity is low.
  • It's easily repairable. Crack or hole in the wall? With a wood wall treatment (or even painted drywall), fixing it in a seamless way would be a lot of work. But to fix some scuffed plaster, one can simply mix a little clay and water in a teacup and trowel over the affected area with a credit card for a consistent repair in a matter of minutes.
  • It's beautiful. While the Matchbox will be foregoing the vibrant hues in which plaster is commercially available for a more subdued smoky gray, plaster is nonetheless—in whatever shade it may come—a really attractive option for interior walls. Its rough imperfections give it a calm, earthy feel, beautifully capturing the flicker of candlelight, the glow of a desk lamp, or the rays of a rising sun.
A first coat of earthen plaster on the Matchbox's walls.
That said, plaster is a lot of work—much more of a task, time- and money-wise, than, say, just priming and painting some drywall. Key steps:
  1. Get the materialsAmerican Clay is certainly the most trusted brand when it comes to earthen plaster, so I opted to use their materials for the Matchbox. A national distributor, I was fortunate enough to find a good selection of American Clay products just a few miles away at the Amicus Green Building Center in Kensington, MD—a truly wonderful place with tons of sustainable building materials and friendly, informed, and helpful staff. Jason and his team at Amicus were able to get me set up with everything I'd need to start plastering: five 50-pound bags of clay (the loma variety, for a slightly rough finish), five packs of pigment (Wild Horse Smoke, for a mid-range grey), a few gallons of low-VOC primer and a few pounds of primer sand (for helping the plaster adhere to the drywall), and a trowel and hawk set (for, well, troweling on the clay).
  2. Prepare the substrate. Drywall is a suitable substrate for plaster, but it first needs to be spackled (filling in all screw holes—which, in retrospect, may be unnecessary), sanded (both the dried spackle and the mudded joints), and primed. Very importantly, the primer must be mixed with sand (one pound of sand per gallon of primer) to give the wall a rough-enough texture for the clay. To ensure the sand is both evenly mixed and fully applied, one will also need a paddle-bit drill mixer and a foam (not nap) paint roller.
  3. Mix the plaster. Pour about two gallons of water into a five-gallon bucket, then slowly add about half a bag of clay, mixing (with the aforementioned paddle-bit mixer) all the while. The mixture should have the consistency of soft-serve ice cream, but doesn't have to be perfect just yet.
  4. Add the pigment. In a separate container, mix one bag of pigment with a little bit of water—just enough to create a slurry. Shake well, then slowly add the pigment (again, mixing while pouring) into the five-gallon bucket.
  5. Get the right consistency. If the solution is too runny, add more clay; if it's too thick, add more water. Keep toying with the ratio until the plaster begins defying gravity (sticking to the bottom of the paddle-bit, for example).
  6. Trowel! Carefully pour a hearty helping of plaster onto the hawk, scoop some from hawk to trowel, and press the trowel against the wall to smear the clay. Angle the trowel in successive sweeps, spreading the plaster ("credit-card-thin" recommended) across the surface. The plaster will appear very dark, but will indeed dry a much lighter color.
  7. Do it again. When the first coat of plaster is done, put on a second coat.
  8. Compress with a nearly-dry sponge. In order to protect the clay and maximize its dehumidifying capabilities, the walls need to be "compressed" after the second coat, pressing a slightly-damp sponge against the surface and making slow circular motions on every inch of the wall.

Troweling the first coat of plaster atop the sanded primer (above and below).
      

    

After quite a few days of work, I'm just rounding out step six, with all drywall installed and primed and a nearly-complete first coat throughout the house.

Days 25 & 26: Drywall, more drywall, and a little mudding (16 hours)
Day 27: Lots and lots of (largely unnecessary) spackling (6 hours)
Day 28: Finishing the first coat and painting the ceiling (8 hours)
Day 29: First coat of plaster (10 hours)
Hours to date: 256

Care to help plaster? Let me know. :)

Meanwhile, Boneyard Studios enjoying a bit of snowfall.
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"I did not plaster 'til it was freezing weather, and it was my ambition to transfer the plaster from the board to the wall neatly and rapidly. I remembered the story of a conceited fellow, who, in fine clothes, was wont to lounge about the village once, giving advice to workmen. Venturing one day to substitute deeds for words, he turned up his cuffs, seized a plasterer's board, and having loaded his trowel without mishap, with a complacent look toward the lathing overhead, made a bold gesture thitherward; and straightway, to his complete discomfiture, received the whole contents in his ruffled bosom. I admired anew the economy and convenience of plastering, which so effectually shuts out the cold and takes a handsome finish, and I learned the various casualties to which the plasterer is liable. I was surprised to see how thirsty the bricks were which drank up all the moisture in my plaster before I had smoothed it, and how many pailfuls of water it takes to christen a new hearth." — Henry David Thoreau, Walden; Or, Life in the Woods 

2 comments:

  1. Thanks for sharing your journey with us! You have at least a couple of fans in the Real Estate Assessment Center (REAC).

    (By the way, you have misattributed the quote to Ralph Waldo Emerson.)

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    1. thanks!

      ooh, super-embarrassing about that misattribution. i just copy-and-pasted the quote from the post below for formatting's sake and didn't remember to change the author after putting in the new quote. though emerson does make a cameo in "walden," you're right; that quote is all ol' HDT. :) thanks for catching that; fixed!

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