Decay of Wood and Wood-Based Products Above Ground in Buildings

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 32°C. 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.