Underhill House is being built with a combination of natural building and high-tech green techniques and materials. Our exterior stucco lime plaster wall surface is one such convergence old and new.
The prep work was state- of- the-art steel mesh and highly adhesive sealing tape (see Underhill House Gets Plastered: part one the prep).
The plaster, however, is old school lime plaster – the kind that buildings have been made of for thousands of years. Lime has been the binder of choice for the most beautiful, old European buildings, and archeologists can date lime plaster use back 6000 years in Turkey. Lime basically held the built world of beams and bricks together and often provided an attractive, weather-proof exterior coating.
Lime plaster is made from limestone, a sedimentary rock, formed out of the skeletons of ancient marine micro organisms. There is a lot of limestone in Wisconsin, and our lime came from not too far away in Manitowoc.
When burned at high temperatures, limestone releases water and carbon dioxide, and its main ingredient, calcium carbonate, is turned into calcium oxide – aka quick lime. When this powder is soaked in water (a process known as slaking), the calcium oxide reacts with the CO2 in the air to become calcium carbonate again, which meanss a lime stucco house is covered in a thin, smooth shell of stone, which will keep getting harder as the chemical transformation slowly continues.
Lime plaster fell out of vogue when Portland Cement came along in the 1800’s and beat out lime plaster with a take-no-prisoners ferocity . Builders jumped at the chance to work with a material that set up much more quickly. Anyone who has watched a modern building go up knows how speed is the prime driver.
Portland cement is not completely different from lime plaster. It is made up of clinker, which is small lumps of limestone and clay (alumino-silicate) ground with the addition of gypsum (calcium sulfate). It become Portland cement by heating in a rotary kiln till the minerals are molten at 2600 degrees F. (Making lime only needs a mere 1800 degrees F.)
The limestone content of Portland cement is generally 80 percent or higher.
Nonetheless, we have been told that lime plaster is a better cover for straw bale because of its hydrophobic properties. It will repel water and keep the straw inside nice and dry. Lime tends to be a negatively charged material that drives water away from itself in its final stage. Concrete attracts water, which can lead to problems in straw bale walls over time.
Among lime plasters advantages are that:
Lime is lighter than concrete.
Lime tends to be more locally sourced.
We also like the fact that you can maintain lime plaster much more sustainably than concrete plaster.
Lime plaster has more innate flexibility than Portland Cement. Cement is formed of microscopic spikes that interlock to create a powerful shear set initially because it’s really locked up solid, but when concrete cracks (and concrete always cracks) it is more catastrophic. Think of iron, which will bend a fair amount, whereas aluminum will snap. This is similar to what happens with lime versus concrete.
When lime cracks it’s not ruined. You can lime wash every couple of years to fill those cracks and mend them. That regular lime wash is what gives those European buildings their patina.
That hard, flexible shell is the cool thing about lime plaster, and also why it is not used in commercial building anymore. Lime plaster must harden very, very slowly if it is going to harden.
Why does the process take so long? The conversion of calcium oxide back into calcium carbonate is the desired end result in making a lime stucco wall finish. You end up with a 7/8 inch sheet of honest to goodness limestone rock on the outside of your house. But converting the calcium oxide back to calcium carbonate requires the uptake of the dissolved carbonate that exists naturally in water that is in equilibrium with the carbon dioxide in the surrounding air. It’s a great process but there is only about one molecule of carbonate in every million molecules of water. That lonely molecule gets incorporated to form limestone, but more carbon dioxide has to enter the water for continued conversion. If the wall dries out, this slow process stops, and the wall cracks.
That means the surface must be misted 8 or 9 times a day for each of at least three layers of plaster, and each layer must dry for several days.
Doug and I agreed to take responsibility for the misting. Wednesday after the stucco plaster was applied, we were tutored in how to tell when it was getting too dry and how to get it just moist enough to go on curing properly.
I have learned how to put my hand on the plaster and tell if it needs misting. When it feels dry, we work our way around the west side of the basement wall, and along the south wall on three levels and part of the east wall on the second and third levels, which were plastered with the scratch coat on Tuesday
I am definitely feeling it in my arms! It’s been quite a while since I played on a jungle gym, and this one (made of bright yellow scaffolding) is three stories tall.
We are enjoying the process, but it’s going to keep our foot nailed to the wall for the next several weeks as the fresh layers of plaster are added and each layer needs days of misting to baby it into the nice hard surface we want our house to have.
We have set up an office to the barn where (in between misting) Doug can preparing lectures for his fall genetics class at University of Wisconsin-Platteville, and I can work on my assorted writing projects.
Going over and over the plaster walls is making me feel very familiar with and connected to the surface of our house in the same way I came to know and love the surface of the timbers we helped to peel, sand and paint.
Categories: Underhill House