One of the really fun parts of straw bale building is throwing the mud on the walls. That’s right, you mix up some good, sticky mud with clay, sand, and water, adding some straw for extra authenticity, and then you slap it on the walls. Cool, huh?
We found out that yes, it is cool and fun, but it is not quite that simple. And also, we were pretty unprepared: recall that plastering was the part of the project that we had not planned. Unfortunately for our un-planned selves, Natural plastering is a whole Big Topic of its own, and there are even entire books dedicated to the topic. Different climates will require different plasters, for example plaster made with clay and sand is fine in dry regions but will wash off after a few hard rainstorms in New England. Even for New England, there are different ways to achieve similar results; how is a novice to choose? It also takes a lot of time. This is an account of what we did, including our mistakes and how we tried to fix them.
This is a longer than usual letter because the plastering was a Big Project
Basic method for making plaster
Whatever, you choose specifically, in all cases, the plaster is made up of four elements: a binder - which can be clay or lime powder; sand - a bulking aggregate, which can be coarse or fine depending on the layer; water; and optionally some organic matter like straw or manure that also help bind the plaster to itself. Mixing it is just like what you would do for baking. Put the dry ingredients together and mix well, then add some of the moisture and mix, alternating with the additives (like nuts or the chocolate chips, but in our case, the straw or manure), until you get the “right” consistency. For the wall plaster that is usually pretty sticky (but not too sticky!), but for the earth floor it was pretty dry. You learn after a while what consistency works well.
We did lot of experimenting to find the right mixing vessel. Apparently, a cement mixer can be used, although it takes practice to keep the plaster from balling up and we did not try. We used a hand-held electric power mixer, single paddle from Ridgid (we also used this tool to mix the bits of mortar and cement we needed for the foundation). We used 5-gallon buckets for a while but preferred a metal trash can so we could do double batches. You want something where your mix is not too deep because if it is you’ll have trouble getting stuff from the top and the bottom well mixed (we know, we tried). You can also put the ingredients in a shallow container like a big wheelbarrow and mix them by hand with a hoe or small shovel. We ultimately liked this method best because you have a lot of control and goes pretty quickly.
Before you start interior or exterior plaster
First, we trimmed our hay bales using so that there aren’t lots of bits of straw sticking out. Actually, Michael, our pro builder, did this with his little electric chain saw, so we don’t have first-hand experience to hare.
Then, we wetted and inoculated the straw bale walls inside and out with clay slip. Wetting the straw seems counter-intuitive, but don’t worry, the clay will suck the moisture out of the straw and having the moist under-layer will lengthen the drying time of the plaster and reduce cracking. Clay slip is a mix of clay powder and water with the consistency of chocolate milk. At the workshop, we used a sprayer that Michael had brought that made it easier and more effective to get the slip into and on the first layer of straw. If you don’t have the sprayer, you can also use a thick and medium soft paint brush, but it’s not as effective, and it’s slower, or your hands and fingers.
On the inside, at the workshop, the timber frame was also covered to protect it from the plaster. Folks got really creative and put up all kinds of plastic!
Interior walls plaster
The interior walls are the most straight-forward because they are not subject to weather. There was never any question about how we would do these. Our materials were: ball clay powder (we used so-called OM4), coarse sand and fine sand (for the first and second layers respectively), finely chopped straw, and water.
For the first, base layer, the plaster mix consisted of 1 part clay, 3 parts rough sand, 1 part water, and ½-1 part straw, all by volume. At the workshop, people were taught how to put the plaster on the walls using a trowel. I did not master this trick, and after the workshop for the remaining plastering, we used our hands. We went with a two-step process recommended in the “Essential Natural Plasters” and the “Complete Guide to Straw Bale Building” books. The first step is a thin layer where you really push the plaster into the straw using quite a bit of oomph. This ensures good attachment to the bales. While this layer is still moist, build up a thicker layer, which sticks easily to the material on your first pass. We built up about ¾ of an inch.
We needed special treatment for wood frames (the bucks) around windows and doors because plaster does not stick to wood. So, you need something to form a bridge. Two popular options are metal lathe and burlap. We used burlap on the interior and metal lathe on the exterior. Getting clay and burlap to stick is tricky and the learning process involved a lot of plaster falling on the floor. Eventually, we developed a three-pass process. Cut your burlap in a strip that will overlap the clay/straw wall about 3-4 inches and cover the entire wood frame. Apply a first thin plaster layer to the straw as above. We used a pretty wet mixture for this. Press your burlap into the plaster and put on a second thin layer that also covers the burlap over the wood. Wait for this to dry a little. For your third, thicker layer, use a regular plaster mix that should stick pretty well over the clay-enhanced burlap.
We scored all the plaster while it was still a little wet – nearly leather hard, still easy enough to press our scoring tool in to (we used a little trowel).
The first layer of clay should be totally dried before you apply the next layer. We did not always do that; sometimes applying the second layer when the first one was leather hard. In pottery, you attach two pieces (say a handle on a mug) when the clay is leather hard, and you moisten the two pieces. In either case, before you start, you should brush off the loose particles of clay and mist the walls before you do the second plaster using a misting nozzle. If the wall is totally dry, mist the night before, the morning of, and just before you apply the second layer of plaster. If the first layer was leather hard, we just misted the day of the second layer plastering.
Our second plaster layer had the same ratios as the first one, but we used fine sand instead of coarse, and very finely chopped straw. We still found it useful to do the two-pass application – a very fast, very thin smudging on the first layer and then putting the bulk of the layer on more carefully. This second layer can be thinner than the first one, maybe ½ inch, but you also use it to even out the walls a bit. Finally, when a good piece is done, go over it with the palm of your hand to smooth everything out.
The result seems solid and looks great. The walls are sort of undulating from the hand application, which is a certain aesthetic choice, but I like it on this building, and it links back to the millennia old origins when the plaster was applied by hands, I am sure. That’s it, the walls are now ready for the lime paint!
Exterior walls plaster
The exterior walls are much trickier, and we made several mistakes. In New England we have regular extreme weather events, which will pound the side of a house with rain, hail, or snow. Our first mistakes were: orientation of the house – we have the less protected gable end facing the weather; overhangs that were too small; a stem-wall that is too low. Our other error (possibly) was to start with clay paster.
You can use all clay plaster on the exterior in New England if you plant to put a porch all around your house so that rain and hail and snow will never get to your wall. In fact, we experienced the effect of rain on clay plaster because just in the brief few weeks after putting the first layer of plaster on during the workshop, we had a few driving rain events, and a good portion of the plaster on the gable end facing the weather melted off. We could see straw poking through. I almost cried!
The solution is to use lime plaster. One lime option is hydraulic lime plaster. It is easy to apply and reliable. You can apply one thick coat over your straw walls which sets relatively quickly in a few days, and then a finish coat. The downside is cost: our closest hydraulic lime source is LimeWorks in Pennsylvania; from them the stuff costs $53 for a 55 pound bag plus double for shipping to Massachusetts. To plaster our little studio this way would have cost over $1500 in materials. Sad face.
A less expensive option is to use a combination of plasters from clay and hydrated lime (costs $16 per 50 lb bag at Home Depot). It is tempting to use clay plaster for the base layers because it is cheaper, easier and safer to apply and add a lime finish layer in the end. Many builders do apparently, and we started with this approach, adding a first layer of clay. However, when the first layer of clay was on the walls and dry, and I was doing some research on the second layer, I found out that lime on clay often fails (see this and this article, as well as p 248 of Nitzkin and Termens). Very sad face.
Some more research led to this article, and p 249 of Nitzkin and Termens, which suggest using a clay-lime mixed plaster for the base coats, and then ending with a lime plaster. This apparently works (we also purchased the book “Essential Natural Plasters” by Henry and Therrien and recommend it 100%!). The article and the books propose to use the clay-lime mix from the straw bales out, but given we already had the clay plaster up, we decided to try the mix over the clay plaster, using the logic that the mixed material might bridge both ways (to clay as well as to lime). This turns out to not be ideal as we found out. We had the clay-lime mix fall off the clay plaster even as we were working in a few places, so it’s really not ideal to mix the two plaster types. For anyone starting out we suggest just use the clay-lime mix for both of your base layers as proposed by the plaster experts Henry and Therrien in their book on pp 87-88. In any case, the lime-clay plaster is hard enough to withstand strong rain (hooray!) and it will get even stronger when we put the finish layer of just lime on.
Our clay-lime plaster is 1 part clay, ½ part lime, 4 parts fine sand, 1 part water, and approximately 1 part chopped straw. To try to get better adhesion to the clay base, we used the two-pass method that we also applied for the second coat of the interior. It’s more work, but we’re just not sure it will stick otherwise. The clay-lime plaster is quite hard when it is dry, much more than a clay plaster. If you do not add enough sand it cracks like a dry riverbed. It also needs to be firmly pressed into the clay underlayer or it will come away, it is best applied out of the sun, and the under-layer needs to be misted the night before and the day of the plastering to prevent the dry clay plaster from wicking all the moisture out of the new plaster layer. Also, don’t forget to brush the loose particles of clay off the previous layer before you mist and start to put on more plaster (we sometimes forgot to brush. Oops). And, remember to score your layers when you are finished so the next layer can adhere better!
We have found that the clay-lime mix did crack easily but that may have been because we were working in July and August with high temperatures, or we were working on clay plaster, or the mix was too wet, or…. We filled in the cracks at the end.
Besides special attention to the plaster, the exterior also requires flashing to make sure that water coming off the walls drips off. Michael was able to borrow a roofer friend’s tools for us to bend metal sheets, but you can also purchase it pre-bent. Over the flashing comes metal lathe so that you can attach plaster. Flashing is also used above and below the windows, and flashing tape (this is just to keep water out) on the sides of the door and windows. All is then covered with lathe.
All of this plastering on the exterior was an enormous amount of work, and I can’t believe we did not put it into the Plan! We worked on this from June to September. More than 20 people were involved for about 65 person days. It looks great though! Thank you to everyone!
If we were to do this again, could we do it more quickly? We could do only lime-clay, which would have made putting the second layer on quicker. And we could have learned to use trowels, which is what people usually use. But then, we would not have our lively, undulating nature-walls …
Finally, note to self: don’t leave the planning on a major part of your project until you are right in the middle of doing it! Seriously.