The science behind the health benefits of clay plasters

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Posted Aug 9 2012 by Adam Weismann of Clayworks

Clay plasters are beautiful alright – see Clayworks’ efforts in a restaurant in London (above), and see here for some basic information about clay plasters. But for those of you who want more evidence for the benefits of clay plasters, Adam has compiled the information below, together with sources. This article is about the health benefits of clay plasters, and it will be followed by an article outlining the environmental benefits. We’d like to see a shift from gypsum plasters towards clay plasters, and therefore we feel that it’s important for people to understand that there is scientific data backing up the benefits of clay plaster.

Unfired clay is a material suited to high performance building products, where health, beauty and sustainability of the natural environment are drivers and considerations. Here we outline research and offer comment on what we have come across, directly related to unfired clay and indirectly related to clay plaster, a principal derivative product.

Clay plasters are made from a blend of unfired clay and sands. They are uniquely beautiful and inspiring to live with. But equally attractive are their  functional properties, principally, that of breathability and known abilities to regulate relative humidity (RH). Next to airtightness and embodied energy, breathability is possibly the most critical consideration in building design. If your building is airtight, what about all that moisture created by you, your kitchen, your bathroom – where is it going?

Breathability and associated health benefits

Healthy, durable, working buildings can only be brought about by designing with a full understanding of breathability (see footnote 1, May, N 2005). The current focus on airtightness in design needs to also consider how vapour inside a building will be treated. Clay plasters, made from unfired clays and sands, are considered breathable (with excellent vapour permeability) and hygroscopic. Unfired clay can absorb and desorb indoor humidity faster than any other building material (see footnote 2, Minke, G 2006). Clay plasters regulate relative interior humidity between 40% and 70%. By keeping RH  between 40% and 70% research has shown that the likelihood for airborne infectious bacteria and virus to survive is the lowest (see footnote 3, Arundel, A. V. et al. 1986). Keeping RH between 40 and 60% also prevents building materials from off gassing toxins, such as formaldehyde (Arundel, A. V. 1986).

Water controls the life or the demise of building fabric. Wall moulds and areas of damp are minimised by the hygroscopic properties of clay plaster. Experiments at the University of Kassel in Germany proved that a 1-sided 15mm sample of clay plaster could absorb 5x the moisture of a sample of gypsum plaster. As shown in the comparative graph below, the ability to absorb humidity varies significantly depending on clay content.


Perhaps the best description of the need for breathable walls is from Tim Padfield – a description on indoor air quality:
In the home and in the office, porous, absorbent walls are equally beneficial. The “Sick Building Syndrome” has become a cliché, used to berate designers for all manner of defects which cause psychological or physiological harm to the occupants. The extraordinary number of synthetic chemicals which outgas from modern interiors cannot be blamed on impermeability, but the mould growth that adds natural irritants such as spores to the air can certainly be reduced by permeable walls. Impermeable walls are much more prone to transient episodes of condensation caused by cooking and washing, or simply by the breathing of a large gathering. Insects also thrive where liquid water is available. Dust mites, whose excrement is a potent allergen, thrive only above about 50% relative humidity. A bedroom with windows closed against the night cold will rise considerably in RH during the night, from moisture from the breath and bodies of the sleepers. A porous wall will absorb this moisture and release it when the room is aired during the day, giving a lower average RH. This will reduce the operating time of a dehumidifier, or make it unnecessary (see footnote 4, Dr Tim Padfield, 1998).

According to research carried out by Padfield (see footnote 5) a 20mm clay plaster layer will substantially regulate daily fluxes in RH. The type of clay used has an impact on a plaster’s ability to absorb moisture. Through regulating RH the occurrence of mould can also be prevented. The graph below shows the relationship between mould occurrence and RH; Ucci, M, 2009 (see footnote 6). Terms like Sick Building Syndome (SBS) and Building Related Illness (BRI) are mentioned more often in the context of mould fungi.

Factors of influence are not only viruses, pollen, mites, nitrogen oxides, carbon monoxide, ozone, radon, emissions from building and facility materials and electromagnetic fields but also “Microbial Volatile Organic Compounds” (MVOC) and fungus spores (see footnote 7) Klaus Sedlbauer, undated.

In the study of using unfired clay materials in a test house in Scotland, Tom Morton, Principal Architect at Arc, Fife, UK, suggests in the bathroom, the clay plaster had such a strong ability to absorb peaks of air moisture after showers that it cleared the air without surface condensation. The effect of the extractor fan was of no statistical significance. (see footnote 8, Tom Morton, 2006).

Clayworks have come across related evidence for evidence of how clay plaster can treat pollutants and neutralise odours in rooms. It [clay] absorbs pollutants and helps maintain a healthy atmosphere (see footnote 9, Ruth Busbridge, 2009). Consider also that clay plasters are anti-static and can screen electromagnetic radiation. Tests conducted at [a] University [in] Munich, Germany in 1999 showed that solid timber and clay had by far better radiation shielding properties than for example concrete, bricks, concrete blocks or stud & plasterboard walls. From these tests they concluded that the superior performance of natural materials such as timber and clay is due to their unique cell structures made up of cavities, capillary tubes, cell walls, encased resins and various other materials and that man-made building materials can just not compete with nature (see footnote 10, E. Thoma, 2003).


(1) Breathability: The Key to Building Performance Neil May, Natural Building Technologies, April 2005
(2) Minke, G (2006) Building with earth: Design and technology of a sustainable architecture. Publishers for Architecture, Basel, Switzerland.‐eBook‐Building‐With‐Earth‐Design‐and‐Technology‐of‐a‐Sustainable‐Architecture‐Gernot‐Minke‐Birkhauser‐2006dg2005
(3) Arundel, A. V. (1986) Indirect Health Effects of Relative Humidity in Indoor Environments. Environmental Health Perspectives. Vol. 68 p. 651-661
(4) The Role of Absorbent Building Materials in Moderating Changes of Relative Humidity; Tim Padfield Ph.D. October 1998, The Technical University of Denmark, Department of Structural Engineering and Materials
(5) Padfield, T. (1999) Humidity buffering of the indoor climate by absorbent walls. The Technical University of Denmark, Department of Structural Engineering and Materials
(6) Ucci, M. (2009) Energy Efficiency, Health and Housing. Bartlett School of Graduate Studies, UCL
(7) Prediction of mould fungus formation on the surface of and inside building components. Klaus Sedlbauer, Fraunhofer Institute for Building Physics, undated.
(8) Materials World, Tom Morton, Jan 2006 to
(9) Ruth Busbridge MsC Architecture, Environment and Energy Studies Jan 2009, Centre for Alternative Technology, Powys,
(10) E. Thoma, 2003, [paper unknown] pp. 62-63; information sourced from Frank Thomas at www. The actual university in question is not stated either, but we have come independently come across a Professor Dipl.Ing. Pauli at the Microwave Laboratory of the University of the German Federal Armed Forces in Munich, who tests shielding attenuation in mineral paints.