Decay of wood and wood-based products above ground in buildings, Mechanism of Brown-Rot Decay: Paradigm or Paradox
Carll, C. G. and Highley, T. L.
1999 Journal of Testing and Evaluation, JTEVA, Vol. 27, No. 2, pp. 150-158
wood decay, Basidiomycete fungi, fiber saturation, moisture conditions, temperature conditions, incipient decay, anabiosis,spore germination, field surveys, field studies, sheathing
Carll, C. G. and Highley, T. L., (1999), "Decay of wood and wood-based products above ground in buildings, Mechanism of Brown-Rot Decay: Paradigm or Paradox", Journal of Testing and Evaluation, JTEVA, Vol. 27, No. 2, pp. 150-158.
Abstract:
This paper is an overview of what we know about occurrence of wood decay above ground within buildings. It presents information concerning under what conditions decay may become established. In laboratory tests involving optimum moisture and temperature conditions for decay fungi, and direct contact with large quantities of specific well-developed decay fungi, substantial decay in small specimens of untreated wood of nondurable species can occur in a few weeks. The simultaneous occurrence of optimum conditions for decay and high degree of inoculation with mature decay fungi is probably very rare in buildings. However, spore germination also proceeds rapidly at optimum moisture and temperature conditions. For most decay fungi, optimum moisture conditions mean moisture contents above fiber saturation (usually around 25 to 30% mc,) but well below the waterlogged condition. Optimal temperatures for most decay fungi are in the range of 21 to 32ˇăC. Untreated wood and wood-based products will not decay if intermittently wetted for short periods to moisture contents above fiber saturation or if wetted to such levels for periods of a few months when temperature is low. However, little is known in quantitative terms about decay development under fluctuating conditions. Moisture and temperature conditions are not expected to fluctuate appreciably behind external insulation and finish system (EIFS) claddings. Given this, we can find nothing in the research literature that would contradict the 20% wood moisture content rule for this application.
Survival of mold:
Carll & Highley (1999) described the fungal survival in dry wood. They observed that the decay fungi could turn to dormant state "if moisture conditions fall much below the range of fiber saturation, and survive in a dormant vegetative state, and reviving once moisture conditions again reach levels around fiber saturation". This phenomenon is term as anabiosis in mycology. The anabiosis ability of wood decay fungi varies with species, can last as long as a decade, and only occurs for well-developed fungi. This is the reason why kiln dried wood is more resistant to fungi occurrence, since the non kiln dried wood might still have dormant and previously grown fungi colonies. They do not have a qualitative model for the survival rate as a function the moisture conditions and duration.
This publication in whole or part may be found online at: here.
