Hygrothermal behaviour of external thermal insulation composite systems (ETICS) to withstand biological colonisation

Abstract

ETICS are multilayer building solutions applied to the building external walls to provide an improved thermal performance to the building envelope. However, several questions have been raised concerning the durability of ETICS, namely related to biological colonisation phenomena. Considering the high susceptibility of ETICS to bio-colonisation, the following research questions arise: (i) what is the impact of surface temperature (ST) and surface relative humidity (SRH) fluctuation on mould growth in ETICS facades? (ii) is it possible to predict mould growth on ETICS under fluctuating conditions considering favourable and unfavourable growth conditions? This study aims to investigate the influence of the hygrothermal behaviour of five different ETICS (with thermal mortars and insulation boards) on mould growth. ETICS were exposed for one year at an urban site in Lisbon, Portugal, facing North, during which the ST and the SRH were monitored. Concurrently, numerical simulations were performed to evaluate the hygrothermal behaviour of the ETICS. Three theoretical indices were applied, using numerically and experimentally obtained values of ST and SRH as input to provide an indication of the risk of mould growth. The results were complemented and validated by assessing the bio-colonisation, water performance and aesthetic properties of the ETICS. Index 1 (percentage of time with SRH ≥80%) indicated similar potential of mould growth for all systems. Both index 2 (percentage of time with SRH = 100%) and index 3 (percentage of time with SRH ≥80% and 15 °C ≤ ST ≤ 30 °C) indicated a higher potential of mould growth for the lime-based ETICS and a lower potential for the acrylic-based ETICS with an EPS-based mortar. Moreover, the lime-based system obtained the highest rate of mould growth after one year of outdoor exposure. Therefore, results suggested that indices 2 and 3 are in agreement with field observations and thus can provide an indication on mould growth for the analysed ETICS. Results also showed that an increase of capillary water absorption after ageing to levels higher than 1 kg/m2 after 24 h in direct contact with water can favour mould growth. Thus, the combination of unfavourable microclimatic conditions (SRH <80%; ST < 15 °C or ST > 30 °C) and surface hydrophobicity are fundamental to avoid mould growth on ETICS, regardless of the incorporation of biocide in the finishing coat composition.