Lime and Lime Again

I asked Scott why we decided to use lime plaster on the walls inside of the south bumpout since we were using earthen plaster everywhere else in the house. He said, “Because we want to practice for the exterior lime plaster and because we want to put it in the most conspicuous place possible. Ha. Ha. Ha.”

Doug said this would give us the proper motivation to do it well.

And, as I wrote this post, Scott said, “We did it VERY well.”

Yes, there was a spot of plaster failure (easily corrected), but we were successful in practicing the lime plaster application technique as well as beginning to learn how to apply lime wash to color it.

First we mixed up a load of lime plaster. It consists of Type S lime, pumice, sand, soda ash, kaolin clay, water and poly fibers.

You can see the poly fibers. They are meant to hold the plaster together. In earthen plaster, straw plays this role.

West side of the south bumpout: we planned to plaster the lath and straw portion on the right side of this wall and the OSB (oriented strand board) above the door and below the right-hand window. Because the surface is so smooth, plaster won't stick to OSB easily. To help with this challenge, we stapled expanded metal lath to the OSB.

First Scott sprays water on the surfaces to be plastered. The tape and plastic are meant to keep the lime plaster off of the wood. Otherwise, the lime will change the color of the wood.

Scott begins the first coat of plaster on the wall.

Scott begins the second coat of plaster on the wall.

Scott finishes the second coat of plaster. The texture in the first coat is the scratch pattern that we added to the first layer. This provides the tooth (something for the plaster to grab on to) for the second layer.

The finished lime plaster looks just like sidewalk cement.

Now we will add color to the wall with a lime wash. I begin by measuring a given quantity of Type S Lime. (Note the dust mask and rubber gloves. Lime is not a benign material.)

After several experiments on our practice board, we chose yellow ochre.

It doesn't take much pigment to make a lot of color.

We mixed the wash to the consistency of skim milk. This was very important. On our practice wall, even though the directions clearly stated "consistency of skim milk," it didn't seem like "paint." We made the wash thicker. When it dried, there were hundreds of little cracks and it flaked off. This is called "crazing." When we followed the directions, it worked perfectly. Imagine that!

Before applying the wash, Scott used a sponge to remove loose sand grains from the surface.

Lime wash is semi-transparent when applied.

The wash becomes opaque as it dries. You can see the beginning of this in the upper left-hand corner of the picture.

We decided the color needed to be more robust so we add six more coats of wash. The lower right-hand corner took the longest time to dry. We still don't know why.

After six coats, the color was beautiful but it seemed a little too uniform and flat. We hoped to give the wall a little depth by sponging on one more layer of yellow ochre containing a little more pigment.

We didn't like the result.

So Scott added two more thin coats, one applied horizontally and the applied vertically. Now we love it! The only thing we need to do to complete the walls in the south bumpout is put on the last bit of window trim.

It A Matter of Lime

Water and straw do not go well together...at least not when that water is precipitation and the straw is your wall. Therefore, it is extremely important to design a straw bale house with features that keep rain and snow away from the straw walls.

Our house, with its eight-foot porch, demonstrates one way to do this. However, on two sections of the house, the north kitchen bumpout and the south sunroom bumpout, there is no porch. There is only an eighteen-inch eave overhang. This is perfect for passive solar design but not so good for shedding weather.

In order to protect these two vulnerable walls, we will use lime plaster rather than the earthen plaster used under the porch. Earthen plaster will erode when precipitation hits it. In contrast, lime plaster chemically sets up in such a way that the plaster becomes hard enough to shed water without eroding.

We actually have two such scenarios to deal with. On the south sunroom bumpout, where we have three, nine-foot by five-foot windows, we decided against using any straw at all. You might ask why. Of course, straw bales have excellent insulative value, but, as mentioned above, you must protect the bales from moisture. There is only room for one bale of straw beneath these huge windows. To us, it seemed that one bale would provide minimal insulation and, as an extra bonus, would present all of the challenges of moisture exposure. So, no bales there.

Instead, we framed this section with 2X6s and our timber frame. We covered the exterior and interior with oriented strand board (OSB) then insulated the air space between the OSB walls and the ceiling with icynene foam (see June 23, 2011 - More Insulation post). We stapled expanded metal lath to the OSB to provide a tooth for the plaster. Now we have great insulation, no moisture issues and an opportunity to use lime plaster on the inside and the outside of the south sunroom bumpout.

Scott made our exterior plaster with Type S lime purchased in bags from our local building supplier, Loa Builders. He added water, sand, polypropylene fibers, pumice (purchased  from Hess Pumice in Malad, Idaho), kaolin clay and soda ash.

There are two main challenges to using lime. One, it is extremely caustic so we needed to wear  rubber gloves, dust masks and eye protection. Two, lime plaster does not like wind or direct sun, so we needed to wait for a calm day. When two such days were in the forecast we set to work. The 18-inch eave overhang was enough to shadow the wall for nearly the entire day, so the wall was shaded as we worked. Once we finished the first lime plaster coat, we protected the lime from sun and wind by leaning boards against the lower part of the wall. The upper part was shaded by the eaves. For the next two days, Scott misted the lime periodically to prevent it from drying out too quickly. The result was perfect...no cracks and the first of two lime coats finished!

Scott applying plaster under the eaves

A section of wall with the first lime coat

Pete helping

The entire wall with the first lime coat finished