The importance of protecting natural plaster during installation is something that cannot be over stated. Unlike modern stuccos that use rubberized coatings and other additives, natural plasters rely on a slow dry time and long curing process. This is especially true for lime plaster. Because upwards of 95% of my projects use Natural Hydraulic Lime (NHL), I will continue the discussion using that plaster as the focus. Before I go any further, I want to add that in my opinion, NHL plaster is the best plaster you can use on a straw bale house. It provides excellent strength, durability, breathability, moisture control, crack control and more. That said, it is harder to work with than conventional stuccos because of the need to protect it during installation.
You may wonder what I mean by the importance of protecting natural plaster. Protect it from what? How? The main issues that come into play are sun, temperature, rain, and wind. It is vital that the plaster not be exposed to direct sun or wind during application. Both of these elements will dry the plaster too quickly. When lime plaster dries, it also cures, a process that actually changes the chemical composition of the material itself. When that curing process is shortened, the lime crystals don’t grow to full length or strength. As such, the plaster is greatly weakened, sometimes to the point of failure. Check out the video below to see what I mean.
The plaster in this video was installed by a professional plastering company that has stuccoed many, many houses. They were not clear about the need for protecting the plaster from the Arizona sun and as a result, the plaster failed. This part of the country is not only hot, but also very dry. The addition of strong, dry winds across the face of the plaster helped suck all of the moisture out of the material in short order. The owner of the home and the plastering company are now working together to make sure this doesn’t happen again moving forward. They have protected the walls with tarps and are constantly misting the fresh plaster to make sure it stays hydrated during the curing process, most importantly, the first three days after plastering.
It is also important to protect the plaster from rain and extreme temperatures. Direct rain that hits freshly plastered walls can actually wash the lime out of the plaster. Simply having rain on site during plastering is not a bad thing. In fact, it helps to keep moisture in the air and thus slows the curing process. Just be sure it’s not in direct contact with the wet plaster. Further, you will need to cover the mixer so that the rain does not constantly add water to the plaster as it mixes. Temperature is something that needs to be controlled as well. You should not apply the plaster if the temperature will drop below freezing within 72 hours of application. On the other end of the spectrum, you should not apply plaster if the temperature will be over 100 degrees Fahrenheit during application.
If you take the proper precautions, the application of natural plaster will create a beautiful and long lasting protective layer over your straw bales. NHL plaster in particular, being my favorite, will give you many years of worry free protection and beauty. Although it takes time and effort to protect your natural plaster during installation, that effort is well worth the investment in the end.