Case Study: The Lime Cycle and How Lime Stores Excess Air Humidity
Updated: Oct 20, 2022
Case Study: The Lime Cycle and How Lime Stores Excess Air Humidity
Dennis came to me in the midst of a full remodel of his downstairs space. He had bought a two story house the year before and was living with his family upstairs while redoing the downstairs. Extremely affable and endlessly curious, Dennis is very committed to leaving a better world for his children. And so Dennis wanted to do his remodel in the most ecological way possible within his time and financial budget. He also had noticed in his first year in the home that the downstairs space had serious moisture issues. The space was damp and uncomfortably cold in the winter and there was a black mold growing, particularly at the base of the walls. With two rambunctious kids, Dennis wanted to find a solution that would be ecological, address his excess humidity and mold issues, and be durable enough to stand up to his kids’ games.
I started off elucidating to Dennis how lime plaster is a far more ecological option compared to conventional cement-based plasters by explaining to him the lime cycle. Creating lime begins with limestone, a stone created by the bones and shells of ancient marine life. These bones and shells are worked on by the forces of time - compressive pressure that converts them into dense stone. Chemically limestone is expressed as CaCO3, or calcium carbonate. Lime producers take these stones, crush them into smaller bits and put them into lime kilns were the stones are cooked at a specific temperature for specific durations, depending on the type of lime product they are producing. In the course of being cooked, the Carbon and two Oxygen molecules (CO2) are baked off the limestone, leaving CaO, or quick lime. To create lime putty for use in lime plaster, the quick lime is submerged in water (H2O) and left to absorb that water (or “slake”) for no less than 90 days until you are left with a lime putty with the consistency of a very thick cream. From the water, lime reabsorbs Oxygen and two Hydrogens, making it Ca(OH)2, or slaked lime. Different cultures have different practices around how long to slake their lime, some believing that the longer the lime slakes, the higher its quality becomes in stickiness and workability. In bygone eras in Italy and Japan, a common wedding present was a barrel of long-slaked lime for the happy couple to use in the building of their home. The longer that lime had been left slaking, the more extravagant the gift.
The lime cycle is completed full circle and the lime putty turned back into limestone when the lime putty (combined with aggregates and sometimes plant fibers to control cracking) is applied to a wall or other surface and left to dry in contact with the air. During this time, the lime absorbs carbon dioxide from the air (CO2) and sheds water through evaporation (H2O) to complete its conversion back to limestone, CaCO3. It’s natural magic. When we apply lime to our buildings, we are crafting a natural stone!
So how is this more ecological than conventional plasters? First, lime production uses far lower temperatures over far less time when compared to cement production. This means it has a lower embodied energy than cement (the amount of energy consumed in the production of a given material.) Even more amazing is that the carbon knocked off in the baking of the lime is reabsorbed into its body out of the air as it slowly changes back into limestone. Cement does not reabsorb carbon from the air. Instead it cures by a chemical reaction with pozzolans and sometimes synthetic chemicals added to the mix. This has led some people to call lime a “carbon neutral” building material. Personally I disagree with that characterization as this label doesn’t take into account the carbon pollution generated by burning the fuel needed to bake the limestone. With that being said, I prefer to not let the perfect be the enemy of the good. Without a doubt, lime products contribute far less carbon pollution than do cement-based products.
And what about Dennis’s moisture and mold issues? Lime finishes remain hydrophilic and hydroscopic throughout their life. This means that they are water-loving and that water (in the form of vapor) can pass easily through its body. So when moisture enters the wall either as water droplets or ambient humidity, lime will absorb that moisture into its body, store it and slowly release it back into the air as that air becomes relatively less humidity. Put more plainly, the walls BREATHE.
Cement products do not do this. When water inevitably enters its body, it remains in the cement for far longer, if not forever. When you see a concrete building that has fallen into disrepair and the metal bars inside the concrete are exposed, you will notice that those bars are very rusted, perhaps even broken. That is because of the trapped moisture that was within the concrete rusting the metal. And those metal bars are integral to the structural integrity of the concrete! I believe in the coming decades we will see many huge buildings failing structurally because of the poor understanding of how these materials work, or perhaps better said, don’t work. The metal inside the building is disintegrating unseen and the structure will fail because designers have relied on that metal to hold up large, unsupported concrete expanses of the concrete structure. Why did the world ever turn towards cement in the first place? Because it is cheaper and easier for the designers and builders. Using lime requires more patience and understanding of the materials. It is a more delicate dance unsuited to the mentality of “I want it done yesterday!”
Dennis had mold in his house because of the combination of cement, synthetic paints, and a high humidity environment. Moisture stayed on the wall’s surface. That combined with ideal temperatures encourages the growth of harmful molds. After hearing what I had to say and seeing some examples of my work, we shook hands and agreed to go ahead. We decided on one cm of lime plaster on all wall surfaces and to remove all material from the bottom meter of the walls where the issue had been particularly bad. To that bottom meter, two cm of lime would be applied for increased humidity retention capacity. Because Dennis was concerned about being able to fix crayon streaks and other dings to the walls easily, we agreed that I would plaster the walls and show him how to paint the walls with a lime paint that can be retouched whenever he sees fit. Lime paint is also vapor permeable and can be made in many colors. As the lime cures over its first two months, it becomes harder and harder until it is stone. That is the main reason we went with lime instead of a clay plaster - lime is far more durable to blunt impact than is clay.
The results are beautiful and are addressing the problems Dennis came to me with. Wanting the feel of a natural earth-built home, Dennis asked me to create rounded corners and not to leave everything straight and smooth, to have some playfulness in the texture and outside edges. It was extremely fun to work in that way, striving for pleasing curves rather than rigidly straight lines. We even incorporated some niche spaces for keys and candles and the like.
I am a complete nerd for this stuff. If any part of it is not clear or you want to understand more, please don’t hesitate to get in touch. I will do my best to explain everything without prattling on endlessly, as I am want to do :)
Mano De Barro is a natural plaster company specialising in consultation and application of unique natural, ecological finishes of lime, clay and Tadelakt. Also offering educational workshops and complete construction using Natural Building methods, Mano De Barro is based in the Marina Alta, Costa Blanca region of Valencia and Alicante and serves the local communities of Javea, Moraira, Benitachell, Teulada, Marina Baixa and across Spain and Europe.
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