The Influence of Paint on the Thermal and Moisture performance of Traditional Solid Walls

It is now widely recognized that traditional buildings are typically constructed with vapour permeable materials, necessitating the use of compatible materials in renovation or thermal upgrading projects. Today, significant effort is dedicated to preserving the historic plaster in such buildings, or when necessary, replacing it with similar vapour-permeable materials. However, the selection of paint is often overlooked, and the use of inappropriate products can compromise the moisture-regulating benefits offered by lime plaster. This research focuses on the impact of 200 years of paint layers on the vapour permeability of lime plaster in a Georgian house, which significantly increases its vapour resistance. The study also assesses the effect of four paints-mineral paint, matt emulsion, soft sheen emulsion, and acrylic-based paint-according to EN ISO 12572, with average diffusion resistance factors (Sd) of the paint layers measuring 0.32m, 0.25m, 0.95m, and 0.53m, respectively. Hygrothermal modelling, using WUFI, is conducted using experimentally measured hygrothermal properties of Irish building materials and the aforementioned paints on traditional brick walls with varying moisture conditions in the wall. The modelling reveals that in instances of moderate moisture content in the wall, paint selection may not significantly affect moisture performance. However, in cases nearing wall saturation, paints with high Sd values can have a serious detrimental impact, contributing to high moisture levels on the interior side of the wall. This research forms part of the FabTrads-TradFabs series of projects funded by the SEAI, aimed at measuring the hygrothermal properties of traditional Irish building materials.