The benefits of using Clay plaster

Clay plasters are often associated with more ‘way-out’ construction or very niche historic building restoration. However, there are several properties of the material that makes it much more useful in modern contemporary construction than might initially be thought.

Humidity buffering

Unfired clay based plasters are very good at absorbing atmospheric humidity and can do so at a rate that makes them useful in areas with fluctuating high moisture levels, such as bathrooms and kitchens. As the moisture content of the air drops, the clay desorbs the moisture equally freely, enabling it to absorb more again when humidity rises.

Many modern buildings incorporate MVHR (mechanical ventilation with heat recovery)which is effective at supplying fresh air to buildings and also at keeping internal humidity levels low. However, the air flow rates are not fast enough to quickly remove cooking smells, which can linger for many hours after the cooking has finished.

In addition to the moisture absorption, clay building products can also help absorb smells from the air which can lead to rooms becoming ‘fresh’ again faster than with other plaster materials. Ventilation will slowly remove the odorous compounds from the clay plaster, allowing to repeat the same process again.

Sound absorption

Clay plasters are also much less reflective of sound. Whilst this may seem of little importance in a small room, the fashion for open living within buildings does create issues with sound reverberation. These issues are the number one complaint about newly occupied buildings as the behaviour of sound within a building is so rarely a topic during design and construction. Simply using clay plasters and paints is a simple way to reduce reverberation and clay boards are one of the most effective ways to prevent sound transmission through internal walls/floors available.

Tradesmen such as plasterers or builders are often wary of using clay plasters until they find out how easy they are to use. Using either site mixed or pre-mixed plasters they are easy to mix, apply and even if the finish isn’t perfect, they can simply be re-mixed and re-applied. This ensures very little waste and high levels of productivity. Additionally, the pre-mixed plasters can be applied by a spray rendering machine making their application very fast and very economical.

Clay plaster in retrofit

In spite of coming from a background of building with clay (cob construction) I have only recently re-visited how this extremely abundant and very sustainable material can help us in the retrofitting of buildings.

A chance conversation with Toby Cambray from Greengauge about a project where they needed to ensure that an internal wall insulation system applied to the walls could be removed without any trace got me thinking about the use of clay plasters.

One of our main suppliers had always recommended using a layer of clay plaster on the wall before applying their wood fibre internal wall insulation but I had always thought of it as being a little unnecessary and novel for use in our relatively conservative, British market. However, the extremely detailed and reliable hygrothermal modelling software, WUFI, has gained a reputation as being the gold standard for building modelling and it shows very clearly how materials, such as clay plasters, can make a significant difference to the overall performance of insulation systems.

The structure of clay plasters allows humidity to be adsorbed and desorbed quickly and this process can repeat infinitely without any detriment to the plaster and through using WUFI we can see how incredibly useful they can be.

Using clay plasters as an adhesive coat/parge coat on masonry walls prior to applying wood fibre insulation seems to increase the zone over which interstitial condensation is buffered and helps keep the wood fibre insulation drier. It can make the difference between whether or not a facade impregnating cream is required externally or how thick the insulation can be that is applied internally without moisture risk.

Lime plasters are the norm when applying an adhesive/parge coat to walls prior to applying internal wall insulation and were our preference for many years. However, as WUFI shows, they do not buffer moisture particularly, meaning that any condensation that occurs does so on the surface of the wood fibre insulation.

This interstitial condensation results in higher moisture levels at the surface of the wood fibre board, where it meets the wall or plaster layer, and can result in the surface of the board being at relative humidity (RH) levels of 95% or more for long periods of time. Whilst in the short term this is not usually a problem, keeping the RH levels below 95% would make the system more durable.

Embodied energy

Clay plasters generally contain much less embodied energy than lime plasters since the clay is simply dried rather than fired at over 1000°C, as is the case with limes. This makes them a serious option for projects trying to reduce embodied carbon and the impact of buildings more generally. Clay plasters can be re-worked if they’ve dried out too, potentially reducing site waste almost completely.

Finally, clay finish plasters can look beautiful too. We supply brilliant white clay finishes which require no painting and other companies, such as ClayWorks in Cornwall, supply an amazing array of different colours and textures too. Once installed, these plasters, which are notoriously soft, can be hardened by spraying with a potassium silicate solution, such as Baumit Viton Festiger, which create surfaces just as hard as any lime plaster, ensuring their longevity.

See our full range of clay boards and plasters or visit our Specification Generator to see how they can be used.