Tuesday, February 9, 2010

Earthen Plasters - 2

Hi Folks,

Here's a little more about the natural earthen plasters used on our straw bale home project.  Quite a bit of the plaster work has been done by David Ludt and Nick Jackson, and for the past few weeks we've been slowly drying out the walls with the constant application of heat from the Elm wood stove.

We started with an adhesion or base coat that's rough and textured with quite a bit of chopped straw.  This plaster is based on the pioneering work of Athena & Bill Steen, so it is sometimes called the "Steen" coat:

Rough-Textured Adhesion Coat

The adhesion coat is allowed to dry until it begins to shrink, crack, and separate from other surfaces at the edges.  These cracks will be filled with subsequent plaster applications for a solid wall structure, and for good air sealing.
The timber frame is protected with cardboard throughout the plastering process.  The beveled window openings are still rather "hard-edged", and strips of drywall have been attached to the window framing to bind to future coats of plaster.  Additional drywall strips are then used to box in the top of the window structure, for good adhesion, as well as good air sealing with clear caulking.

When the adhesion coat has dried sufficiently, the second, or "Scratch" coat is then applied.  This varies in thickness to smooth out the contours of the walls, and the window openings are sculpted somewhat, and rounded over.


Air sealing is enhanced by sealing around the windows with expanding closed cell foam caulking, the application of beveled wooden strips around all the openings, as well as wallboard (sheet rock) strips, which are sealed with clear caulking.  The drywall is then covered with the scratch coat and finish coats.

Window recesses and scratch coat -- another view.
Note the caulking between layers of dissimilar materials.

The scratch coat then dries over a few weeks time - depends on the temperature and humidity.  We've kept the temperature between 32 degrees and about 55 degrees, and the relative humidity has been quite high (75% - 80%), due to all the moisture evaporating from the plaster.  This high humidity causes condensation on the cold surfaces of windows and the un-insulated roof upstairs, which will be remedied when the roof is ready for cellulose insulation.

As the scratch coat dries, it cracks and separates from other materials.  These cracks will be filled with the finish coat, and then sealed with the layers of clay paint on the inside.  On the outside of the house, the finish coat will be a lime plaster, and numerous pigmented lime washes will seal any cracks that develop.

Here you can see large areas drying, and darker areas still moist.  Areas with stuffing between the bales hold more moisture, and take longer to dry.


Cracks form as the plaster dries.  Cracks tend to form around inflexible structures, such as the saplings used to bind wall sections together or the rough 2 x 4 framing supporting the window boxes, as seen here.  Stem wall and wall outlet rough-in wiring can be seen at the bottom.  The stem wall will eventually be covered with drywall and plastered -- the space behind the drywall will be filled with dense packed cellulose insulation.

As the moisture is driven out of the plaster, it takes less heat to maintain the temperature, but we're still burning quite a bit of wood.

OK -- Time's up!  I've got to build up the fire and do some more wiring!