Clay Plaster vs. Lime Plaster

When I consider what plaster to use, I always think of clay plaster first. I love clay plaster, it has so much going for it. In terms of minimizing environmental impact and carbon footprint, you cannot beat clay plaster. It is minimally processed, needs no additives, and can be sourced locally. In terms of beauty and attractiveness, clay plaster is hard to top. It draws you towards it and makes you want to touch it: I can’t explain why but it’s true.

Clay plaster has some magical qualities: it is one of the most vapor-permeable finishes, coming in at 11 perms per 2″ of plaster. This makes it ideal for any building system where you do not want trapped moisture — which should be the goal for every building system but is often ignored in conventional construction. Clay plaster is also anti-static which helps it repel dirt and dust. It doesn’t bounce sound or cause echoes as much as harder finishes. Clay plaster has hygric mass and can help balance indoor humidity. You can write on clay plaster with a Sharpie marker and erase it with a damp sponge. Magic.

Clay plaster has many benefits, but it does have some real limitations. Once it gets wet enough, it will erode. It is also not as hard or impact-resistant as some finishes. Outside corners in high-traffic areas often suffer chips and dings. Clay plaster is easy to repair but many people do not want to have to care for their walls. Sometimes a project or situation requires a plaster that can fulfill different duties. This is when I turn to lime plaster.

Lime plaster has a much higher carbon footprint than clay plaster, it is only slightly lower than cement plaster. Lime plaster is less vapor-permeable than clay plaster but is more vapor-permeable than cement (lime plaster is 9 perms per 2″ where straight cement plaster is less than 1 perm per 2″).

If you do it right, lime plaster goes through a chemical change as it cures that effectively turns it back into limestone on the wall. (If you do it wrong, it turns to useless dust on the wall.) Done right, it will not erode. It is harder and more impact-resistant than clay but that makes it more difficult to repair.

Lime plaster used to be very common in the US but was replaced by drywall and gypsum-based plasters in the last century. Then it became difficult to source lime of high enough quality to produce consistently good lime plaster. Although it is possible to make a good lime plaster from locally available Type S lime, it requires knowledge, skill, and a fair bit of babysitting to succeed.

On larger-scale projects, like the exterior of a strawbale home in a climate that gets sideways monsoon rains, we will make our own Type S lime plaster, don our gloves and safety glass, make the mix ourselves, apply the plaster skillfully before it begins to set, and then tend to it patiently until it has cured. It is not an easy process but it results in a water-resistant, durable, vapor-permeable finish.

Hydraulic lime is another option. It still requires knowledge and skill, but it sets faster and more consistently with less babying. You could import hydraulic lime from Europe, where the tradition of lime plaster has not been broken, but at a high cost financially and environmentally. You can make your own hydraulic lime by adding just the right amount of the right pozzolan if you know what you’re doing.

So you can see why I gravitate towards clay plaster whenever possible. For exterior projects, I encourage clients to build wrap-around-porches to take advantage of outdoor living and to allow for clay-plastered exterior walls.

On a smaller scale, like a bathroom, lime plasters are a less daunting. When applied well and polished, lime plaster can look and feel like marble. When finished in the tadelakt tradition, by burnishing oil-based soap into the plaster before it has cured, it can even be waterproof enough for shower walls. But these thin veneer plasters leave little room for error or inconsistency, so I have been reluctant to sell them to clients in the past.

But then I learned about Limestrong. It is a bagged lime plaster created by the talented plasterer Ryan Chivers, in partnership with other experts in plaster chemistry. In my mind, it is the lime equivalent of American Clay Plaster: a convenient, bagged, premeasured plaster with a tested range of pigments that allow you to create a consistent result time after time. And, like American Clay, you do pay more for this convenience.

Most importantly to me, Limestrong sources all of its ingredients from the US. Since lime itself has a larger carbon footprint than clay due to the amount of heat needed to process it, keeping the embodied carbon involved in shipping as low as possible is important.

Limestrong uses pumice as the aggregate. This not only makes it lighter to ship (and carry around the job site), the pumice creates a mild pozzolanic effect, making the plaster slightly hydraulic. Although this means a faster set time, it also ensures a more even cure. The only additive in Limestrong is cellulose glue, which is a non-toxic material that helps retard the set and acts as an adhesive to help it stick to difficult substrates like painted drywall.

In early March of 2020, we hosted a professional plasterers’ training for Limestrong plasters at our job site in Valley Center, CA. We learned directly from Ryan Chivers and his co-teacher Liz Johndrow about applying Limestrong over drywall. We also installed Limestrong over a clay plaster base coat on a straw bale wall. In a private, week-long training for our crew, Ryan led us through applying tadelakt to two showers. I still love clay plaster but am now happy to add the range of Limestrong lime plasters to our offerings.