Plastering a Straw Bale House
One of the things I love about my job is completing the straw bale installation of a house or a wall, and seeing the rich yellow color of the bales, the stucco netting tightened, the windows and doors fit into place. The organic ‘feel’ of the uncovered structure makes me almost hate to plaster it. But I know that once a straw bale house or wall has been constructed, I need to protect it from the elements (rain and snow), from the natural world (rodents, insects, molds) and from mechanical harm (a car backing into it or the vibrations from a door repeatedly slamming shut). This has presented me with many years’ worth of experiments in how to do so.
Here are some of the plasters I’ve used:
This is a combination of portland cement, lime and plaster, mixed for several minutes then applied by hand or blown on with a stucco pump. Stucco sticks well to the bales wrapped in stucco netting, it dries quickly, and is very strong, lasting up to a couple decades before re-stuccoing is needed. But it is an expensive option. Cement and lime are caustic, and can irritate or burn the skin and lungs. They are produced using environmentally-questionable methods. Cement stucco is rigid, which means that during the annual temperature swings it resists the normal expansion and contraction all materials go through, and therefore is prone to cracking.
In the two-plus decades that I have been working with bales, there has been an explosion of next-generation products designed to avoid the stucco-cracking problem I stated above. All kinds of names for these products have popped up: Merlex, Dryvit, EIFC, Sto, Finestone, and so on. Most if not all of these brands are plasticized, which allows the stucco to flex during the expansion-and-contraction of weather differentials and also shed moisture very well—but unfortunately, plastic and plasticized products do not allow the vital transpiration of moisture through the straw bales that keeps straw healthy. The few times I tried synthetic stucco I had disastrous results. I highly recommend no-one uses it on straw bale construction.
Straw in stucco
Although this may sound strange, I have used both chopped straw particles (chaff) and longer strands in both adobe and cement mixes. When finishing a straw bale wall, I always cap off the top of the wall with a very thick 70%-straw-30%-stucco mix cap. This not only holds the wall together but the straw-stucco cap acts as a ‘thatch’ and sheds water similar to European-style roofs and in fact when I apply the same cap to the bottoms and sides of windows in a house, the same water-shedding capacity becomes evident.
For both walls and houses, after I’ve applied the straw-less first coat of either stucco or adobe to the bales I mix finely-chopped straw pieces into the wet batch then apply it in a thick coat over the first coat. The chopped straw adds body to the stucco, strengthens the mix, and helps fill in the inevitable holes that a straw bale wall has.
Applying plaster to straw bales or, ‘stuccoing’ is probably the hardest part of the whole straw bale building process. But it can also be highly rewarding to watch your house or wall become even more aesthetically-pleasing as the plaster or stucco transforms the structure into a durable and thick-walled home. As with many elements of natural building, plastering is a process of trial and error. Experimentation is necessary for success. Creating a good mixture that is suitable for the climate and applying the mixture properly will always contribute to a longer lasting plaster or stucco for your beautiful straw bale walls.