• CHRISTOPHER BLACK
    LDR Engineering
  • JOHN STRAUBE
    RDH Building Science

This paper reports on a full-scale experimental investigation of the mould growth on different types of wood products used for sheathing and framing wall assemblies. Objectives included examination of the difference in mould growth in full-scale wall systems under different temperature and humidity conditions for treated and untreated wood products.

Importantly this study used full scale wall assemblies; to date mould growth studies have only been performed within a laboratory on small samples of materials. Moreover, this study recreates the conditions which evidently cause mould growth on full scale wall assemblies. Tests were performed within a climate chamber on three wall assemblies. The scope of the study includes both the sheathing and framing components, but this paper focuses mainly on the sheathing.

Results indicate that the relative humidity conditions needed for mould growth to occur on wood in a reasonable time (less than several months) are higher than a surface relative humidity of 80%RH. During the first eight weeks of one test, the conditions at the surface of the sheathing was held constant at 26 ºC and 95%RH and little mould growth was observed on the untreated sheathing and little or no mould growth was seen on the borate-treated sheathing. The other tests demonstrated that the presence of liquid water greatly reduced the time to germination, the amount of mould growth, and the rate of mould growth. All three tests clearly showed that borate treatment reduced the amount of mould growth; however, the concentration of borate treatment, and the types of materials treated, does influence the resistance of mould growth. Furthermore, there was some evidence to suggest that borate treatments of the plywood increased the time to germination significantly, from a few weeks to 16 weeks in this study, but once mould growth was initiated, the rate of mould growth was similar to that of the untreated plywood. Recommendations include improvements to the test method and for future work.

This paper was presented at the 2007 Canadian Conference on Building Science and Technology.

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