What clay plaster can do and can not do

The quality of clay plasters is regulated in the same way as other building materials. This is in order to define minimum quality requirements, to make it possible to compare different offers and to protect consumers. The DIN 18947 standard was adopted in Germany first, but is valid throughout Europe and is promoted in Spain and Portugal by AENOR, the competent authority for the certification of building products.

The neutral test focuses in particular on workability, strength, resistance and moisture behaviour.

Clay plasters are intended for indoor use only. Once dried, clay becomes soluble again at any time with the addition of water.

As disadvantageous as this property would be for a pure clay plaster outdoors or on damp walls, the property of clay plaster on dry walls and ceilings for regulating air humidity indoors is equally advantageous.

Due to its nature, clay plaster is able to absorb up to 10 times more moisture from the air than gypsum plaster (see table page 7) , 5 times as much as cement plaster, and 4 times as much as lime plaster. The special properties of the clay with an impact on its moisture-regulating behavior and its adhesive strength are based on the unimaginably large “inner” surface of 1,000m² per gram of clay mineral (Montmorillonite).

The moisture content of the air indoors is primarily determined by two factors: on the one hand, usage behaviour. Wherever people spend time, moisture accumulates; each person produces up to four litres of moisture every day through breathing and sweating. Thus, the room with the highest dwelling time, the bedroom, is often also the room with the greatest humidity problems. On the other hand, the ventilation behaviour is important. If the humidity is high outdoors, it is more difficult to regulate the humidity indoors through ventilation.

In this context, one often hears about “breathable” walls. This does not mean that the migration of indoor air through the exterior walls can contribute to the indoor climate. It is solely about the absorption capacity of the surfaces surrounding us. It is especially the first 20mm of thickness of the surrounding walls and ceilings which are in contact with the air we breath that are the most important for the indoor air quality. The more surfaces of the interior are sealed, the more air quality and comfort suffer.

Let’s take a close look at which surfaces in our interiors can not contribute to moisture regulation and what is left: Tiled floors, impregnated natural stone and terracotta floors. Lacquered wooden floors and surfaces, glass surfaces, plaster painted with plastic paint, furniture… On the contrary, many of the above-mentioned substances contribute to the deterioration of air quality through their own emissions.

Clay plasters “breathe”. Clay plasters replace other conventional plasters and are usually applied in a thickness of around 15mm. They absorb moisture from the air and release it when the moisture supply decreases. Nowadays, fine colored 2mm clay finishes replace the final coat of paint. To ensure that the clay finish has a positive effect on the room climate, it is again important to make sure in the product data sheet that no additional acrylic or hydraulic additives are included or that additional surface fixation is necessary, which is again acrylic-based.

A misunderstanding that comes up again and again is the supposed insulating properties of clay plasters. Clay plaster has no more and no less insulating properties than any other plaster (does not) have. Each plaster -also clay plaster- is characterized by a relatively high density with a specific weight of around 1,800kg/m². Normal thermal insulation has a specific weight of 30kg/m³! Even the thermal insulation plasters advertised as such hardly weigh less than 600kg/m² and – also given their low thickness – contribute more to a noticeable improvement on paper than in practice. When talking about thermal quality when it comes to clay plasters, it is about the hygro-thermal quality. This refers to the subjective feeling of warmth, which improves because the clay plaster prevents excessive humidity in the room.

In conjunction with wall heating, clay plaster is the favorite because it does not put pressure on the pipes during drying and would promote cracks, as is the case with all other mineral plasters…., and only clay plaster combines two important factors of thermal insulation to the advantage of use, namely temperature and humidity.

Clay plasters and/or clay finishes can be applied to almost all surfaces. It is less important how breathable the surface is. It can be concrete, cement plaster, paint or plasterboard. Nevertheless, the clay layer on the surface -depending its thickness- will bring about a noticeable improvement in indoor air quality.

Two words about the ecological aspect of clay building materials: Clay is the only building material that can be reused indefinitely. In contrast to advertising-effective recycling, in which materials are shredded and used for a secondary purpose – for example as filling material in road construction. The CO₂ footprint is another important factor that is playing more and more of a role. The graphic, the blue line shows the CO₂ footprint of lime plasters taking into account the much advertised CO₂ compensation.

There is no space here for a visual representation of the attractiveness of clay surfaces. My Instagram offers some examples…
Clay has the unique ability to gain color and depth through light – regardless of whether it is artificial or natural light. Depending on the perspective and time of day, the appearance changes and spreads automatic well-being. Even without giving particular importance to its beneficial properties for the indoor climate, clay has become a constant in modern interior design.

Moral of the story: clay is a modern building material whose quality is defined by official standards and not only meets modern requirements for use but can also – if used correctly – make an irreplaceable contribution to our well-being. Countless examples show clay surfaces as an unobtrusive design highlight in both, modern and traditional architecture.

The best choice as an English-language reference book on the subject: Earth Building Practice by Ziegert/Roehlen

We prefer to work with according to DIN 18947-49 certified earth building products from Europe’s leading manufacturer CLAYTEC, founded in 1984, and thus benefit from 40 years of application and production experience. I am official representative and craftsman for CLAYTEC in Portugal and Spain.