Clay plasters why use them?

Unfired clay is a material suited to high performance building products, where health, beauty and sustainability of the natural environment are drivers and considerations.

We outline the research we have come across that relates to unfired clay and clay plaster, a principle 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; (1: May, Neil April 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 (2: Minke, G. 2006). Clay plasters regulate relative interior humidity between 40% and 70%. (3: Arundel, A. V. 1986) By keeping RH between 40% and 70% research has shown that the likelihood for airborne infectious bacteria and virus to survive is the lowest. Keeping RH between 40 and 60% also prevents building materials from off gassing toxins, such as formaldehyde. (3: Arundel, A. V. 1986)

Source: www.greenspec.co.uk/unfired-clay-bricks.php

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.
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Perhaps the best available description of the need for breathable walls is from Tim Padfield:

“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.”
(4: Padfield, Tim Ph.D. October 1998)

According to research carried out by Padfield, (5: Padfield, Tim Ph.D. 1999) a 20mm clay plaster layer will substantially regulate daily fluxes in RH. And we note in separate papers that the type of clay used impacts the extent of moisture absorption.

Through regulating RH the occurrence of mould can also be prevented. The graph below shows the relationship between mould occurrence and RH (6: Ucci, M. 2009). 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. (7: Sedlbauer, Klaus)
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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.
(8: Morton, Tom Jan 2006)

Clayworks have come across related evidence for evidence how clay plaster can treat pollutants and odours neutralise odours in rooms. It [clay] absorbs pollutants and helps maintain a healthy atmosphere.
(9: Busbridge, Ruth. MsC Architecture, Jan 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. (10: E. Thoma, 2003)

Sustainable products, made in the UK

Claypod™.

Clay is one of the most abundant raw materials in the world. In 2010, Clayworks developed Claypod™, a concept which acts to localise production of unfired clay-based building materials. In terms of clay block masonry and base coat clay plasters, we have the capability to identify and convert available and suitable raw material, wherever it is found. Clayworks also developed a protocol to convert waste and secondary stream clay into valuable material.

Environmental fundamentals.

Providers of unfired clay products should endeavour to source as sustainably and as locally as they can. The following is achieveable:

Environmental specifics.

Embodied energy is a valid way to assess the environmental impact of a (manufactured) product. The most relevant surveys we have come across is the ‘Inventory of Carbon & Energy (ICE)’ (11: Prof. Geoff Hammond & Craig Jones, 2011). We have formed a graph based on some relevant comparison materials we found in the ICE. Against these, we have used the EE (Mj/kg) figures for the composite materials for clay blocks and base coat clay plasters found by Busbridge. (9: Busbridge, Ruth MsC Architecture). This is helpful to suggest the embodied energy of clay blocks and base coat clay plasters going into a building deisgn, where the raw materials can be sourced and converted, on site:
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www.greenspec.co.uk/embodied-energy.php carries a shorter, more digestible version.

Another useful paper to have come from Centre for Alternative Technology (CAT) on embodied energy is that of Carol Atkinson.
(12: Carol Atkinson, MSc Architecture)

Carol lists the measurements she took (kWh/m3) of all the constituent materials. The clay plasters were sourced/made in Yorkshire and included hemp.

The full paper is available for download here Energy assessment of a straw-bale building.

Commercial relevance of unfired clay products.

Critical aspects of the Code for Sustainable Homes (CfSH) are making their way into UK Building Regulations. An example of this, is Part L, with implications for the viability of core building units, which cannot be recycled. Products that are recyclable with low embodied and carbon energies and which reduce the need for energy, can only become more the case, rather than the exception. Considering the credentials of unfired clay products, we predict that they could be remarkably productive in contemporary building design.

Potential cost savings of unfired clay materials over conventional building materials

Given the implication of CfSH, the true ‘cost’ of a product to the consumer, must be measured beyond mere face value ‘unit’ price. In the same way that locally sourced, organic food more accurately reflects the true price of food, we can suggest that products and concepts in unfired clay are priced appropriately and allow for adoption within the mainstream market. Admittedly, producers of sustainable building materials cannot currently enjoy the economies of scale achieved by large and leveraged corporates.

Unit prices start at a moderate, but affordable premium to conventional, high carbon materials. But considering the wider context, and the related cost savings you are also ‘buying’, when choosing a building designed with unfired clay and clay plaster, for instance:

In the case where unfired clay is used in the form of bricks this material forms a good body of thermal mass, which helps to even out temperature swings. However, even a 15mm plaster coat has significant thermal mass to store heat.
(13: Morton, T et al, 2005)
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According to this same research by Tom Morton et al, the physical mass of clay plaster is 20.6 kg/m2, compared with that of gypsum plaster 8kg/m2, which is as much as 2.5 times of the latter. According to Borer and Harris (14: Borer, P. Harris, C. 1998) the denser building materials perform better in storing heat, with dense soil as the best performer in comparison with other building materials such as fired brick, concrete and stone (Borer and Harris, 1998). N.B. Clayworks Top Coat clay plaster has a density of 1900kg/m3 as opposed to the 1370kg/m3 of the clay plaster in these tests.

Specific comments we have come across:

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References:

(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.

(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

(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. www.strawtec.com.au 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.

(11) Prof. Geoff Hammond & Craig Jones, 2011. Sustainable Energy Research Team (SERT) of the University of Bath. The full detailed survey, complete with original data, methodology and notes, is available from www.bath.ac.uk/mech-eng/sert/embodied/.

(12) Carol Atkinson, MSc Architecture: Advanced Environmental and Energy Studies Energy Assessment of a Straw Bale Building, Jan 2008.

(13) Morton, T et al (2005) Low Cost Earth Brick Construction – 2 Kirk Park, Dalguise: Monitoring & Evaluation. Arc, Chartered Architects 69 Burnside, Auchtermuchty, Fife.

(14) Borer, P. Harris, C. (1998) The Whole House Book. The Centre for Alternative Technology. Machynlleth, Wales

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