Factors influencing surface mould growth on wooden claddings exposed outdoors

Abstract

Development in surface mould growth on painted/unpainted wooden claddings and acting climatic factors were investigated over a period of 3 years. Eight wood substrates, including modified, preservative-treated and untreated wood, were tested in combination with three types of paint: (1) water-borne alkyd modified acrylic paint without fungicide; (2) solvent-borne alkyd paint without fungicide; and (3) ICP (internal comparison product). One set of samples was exposed unpainted. The samples were tested according to a modified version of EN 927-3. A logistic regression model was fitted to the data. The degree of mould growth varied with exposure time, coating typology, wood substrate, temperature and relative humidity. Exposure time and coating typology contributed most to the model. After 3 years of outdoor exposure unpainted panels and panels coated with solvent-borne paint without fungicide had more mould growth than panels coated with ICP and water-borne paint without fungicide. Unpainted oil/copper–organic preservative-treated claddings had higher resistance to mould growth than other unpainted wood substrates. Coated untreated pine and coated acetylated pine were more susceptible to mould growth than other coated wooden substrates.

Keywords:

• Coating
• field test
• mould
• relative humidity
• temperature
• wood substrate

Acknowledgements

The field work was performed in co-operation with Mycoteam AS and Norwegian Institute of Wood Technology as part of project “Svertesopp på trefasader” (Mould growth on wooden facades), project no. NFR-168554/I10. Cathrine Marie Whist, Mycoteam AS, is greatly acknowledged for skilful help with the evaluation. We would also like to thank Jotun AS for providing the paints, and Norwegian Institute for Air Research and Norwegian Institute of Wood Technology for providing climate data. The study was funded by Norwegian Research Council, project no. NFR-169272/I10.