ECO-SEE clay plaster conference paper abstract

Direct measurement of effective moisture buffering penetration depths in clay plasters

Dr Daniel Maskell, Dr Andrew Thomson, Prof Pete Walker and Manfred Lemke

Abstract

The beneficial effects of clay-based building materials on indoor environment quality are acknowledged by architects, engineers and building scientists. Clay has unique physiochemical properties that provide excellent moisture buffering properties. The optimum levels of Relative Humidity for building occupants are between 40% and 60%, with levels outside of this optimum range associated with unintended discomfort, health risks and degradation of the building fabric. The option to passively regulate internal Relative Humidity can significantly reduce energy use as well as improve indoor air quality. Typical earth based interior wall construction consists of a wall substrate finished with base and top coats of plaster. Different substrates can be plastered with clay coatings, including traditional surfaces such as adobe or cob, or contemporary surfaces such as brickwork, lime stone or plasterboard. These wall surfaces offer large areas for exposed hygric-active material.

The NORD test provides a method of calculation, based on measurements of material properties, to establish the depth of moisture buffering material required to be effective. The empirical formulae require measurements of water vapour permeability and moisture storage function. This paper proposes a direct method of quantifying the effect of thickness on moisture buffering of materials.

Two types of commercially available clay plaster are considered: a base coat and top coat. Methods of quantifying moisture buffering properties are typically quoted with respect to exposed surface area, but do not directly rationalise performance with respect to the depth. The effect that depth has on the moisture buffering capacity was investigated by testing specimens of both plasters with thicknesses of 2mm, 4mm, 10mm, 20mm and 40mm. Moisture buffering properties were determined in accordance with ISO 24353:2008. The results indicate that under the above exposure conditions there is a limiting thickness of plaster beyond which there is no statistically significant improvement in moisture buffering.

Understanding the minimum depth to achieve the maximum humidity buffering potential can have significant impacts on resource specification, wall construction with clay and its use for passive air quality improvement.