Humidity regulation

Studies have shown the benefits on the indoor air quality when the relative humidity is kept between 40 and 60% [4]. These benefits are extremely relevant for the inhabitant’s health: reduction of the presence in the air of microorganisms (e.g. dust mites, bacteria and viruses), reduction of mould growth and therefore less emitted spores, and the occurrence of asthma and allergic rhinitis, which is the irritation and inflammation of the mucous membrane inside the nose. England is particularly concerned by this reality, as 15% of inhabitants suffer from asthma [5].

The use of permeable and porous coatings on interior walls help to regulate indoor air humidity. It also helps to avoid the problem of water condensation on walls. That being said, the building industry does rarely take this dynamic into account, nor do building physics calculations.

Academic research

The first research on moisture buffering was released by The Fraunhofer Institute for Building Physics (Germany), by Kunzel [6] in the 1960’s. The evaluation of dynamic vapour adsorption by interior surfaces, or how quickly a material adsorbs vapour, was carried out through a dynamic test, the “step-response” method. In 2006, the Nordtest project (Scandinavia) defined a method called “Moisture Buffering Value” (MBV) and published results of moisture buffering measurements realised on different conventional building materials. This method allows comparing the buffering capacity of each building material through one single value [7]. However, it was only very recently, in 2008, that we saw the publication of an ISO International Standard (Switzerland) describing a measurement method.

Academic publications in Germany and Denmark have demonstrated the superior ability of clay materials to regulate indoor humidity, through its ability to store humidity and release it when the air is drier [8,9]. Our research at the University of Bath (United Kingdom) focuses on this moisture buffering capacity of clay masonry (bricks and plasters) [10,11]. How the properties of materials influence moisture buffering is still little understood, especially when it comes to natural materials, such as clay, which has very variable consistency. We have therefore focused our research on defining the influence of these materials properties on the moisture buffering capacity. Systematic measurements have been carried out, using the dynamic MBV method on over 100 clay samples. Our measurements all included the water vapour permeability (the rate of vapour transmission) and the moisture storage capacity. Those measurements were combined with a precise determination of the materials properties: such as its density or its mineral composition.

“Good to excellent”

Based on our results, MBV of clay can vary from 1.13 (g/m2.%RH) to 3.73 (g/m2.%RH), which classifies – according to the Nordtest project – as a good to excellent buffering material, most materials are below 1 (g/m2.%RH). In order to have a high buffering performance, there has to be a good combination of very fine pores (acting as storage) and larger pores (allowing a fast vapour transmission).

Usually clay is used as it is with some minor additions (sand or fibres). Our research shows how clay materials can be specifically engineered to provide a higher buffering performance than what has been observed so far.

Clay is a very valuable, low impact, natural building material. This has already been recognised for a long time in Germany. Further academic research shall allow clay to reach a wider audience and be recognised for its benefits such as improving indoor air quality.

Fionn McGregor
University of Bath, March 2014

Fionn McGregor, MSc Clay science, PhD Candidate, Earth building materials, BRE Centre for Innovative Construction Materials. University of Bath, United Kingdom.


“Adsorption”: the adhesion of atoms, ions or molecules from a gas, liquid, or dissolved solid to a surface.

“Relative humidity”: a common term used in building physics that expresses the water vapour pressure in the air for a given temperature.

4. Arundel, A.V., et al., Indirect health effects of relative humidity in indoor environments. Environmental Health Perspectives, 1986. 65: p. 351.
5. Braman, S.S., The global burden of asthma. CHEST Journal, 2006. 130(1_suppl): p. 4S-12S.
6. Kunzel, H., Die Feuchtigkeitsabsorption von Innenoberflächen und Inneneinrichtungen. Report of Building Research, 1965. 42: p. 102–116.
7. Rode, C., et al. Moisture buffer value of materials in buildings. 2005.
8. Padfield, T., The role of absorbent building materials in moderating changes of relative humidity. Department of Structural Engineering and Materials, Lyngby, Technical University of Denmark, 1998. 150.
9. Lustig-Rössler, U., Untersuchungen zum feuchterverhalten von Lehm als Baustoff, 1992, Gesamthochschule Kassel. Universität: Kassel.
10. McGregor, F., et al., Conditions affecting the moisture buffering measurement performed on compressed earth blocks. Building and Environment, 2014. 75(0): p. 11-18.
11. McGregor, F., et al. The effect of stabilisation on humidity buffering of earth walls. in Proceedings of LEHM 2012: 6th International Conference on Building with Earth. 2012. Weimar, Germany: Dachverband Lehm e.V.