Mould growth in buildings is becoming an increasing concern for building owners because of health and aesthetic problems. Mould usually appears as black or greenish-brown patches on surfaces in humid environments and is common in houses. In fact, mould growth is caused by moisture problems. To avoid moisture problems, the most important consideration in ensuring the durability of wood-frame houses is to utilise design features, construction tools and practices that keep wood as dry as possible and promote drying if the wood gets wet. One critical factor in these designs is the accurate estimate of the effects of various temperature and moisture conditions on the rates of fungal attack. The absence of definitive data forces engineers to make extremely conservative estimates of mould growth that may not accurately reflect the risk. This report provides valuable data on mould growth on various wood and fibre products used for the construction of homes in North America.
Sapwood and heartwood of jack pine, white spruce and aspen and heartwood of white cedar, commercial aspen OSB bonded with PF resin, softwood plywood, low-density fibreboard, gypsum board, and fibreglass insulation materials were tested by a modified method of ASTM D 3273-94 standard for mould growth test. Test materials were cut into wafers of 5 cm x 12 cm x 1 cm sample size and were placed in specifically designed incubation containers for 8 weeks. Temperatures were targeted at 20°C and 30°C, and relative humidities (RH) were targeted between 65 and 100% by means of saturated salt solutions. Test samples were inspected weekly for mould growth for 8 weeks. A RCS Biotest air sampler monitored mould spore densities inside containers. Moisture contents (MC) of wood wafers were determined by oven-dry method (105°C) at the beginning and at the end of the test. Volatile organic compounds (VOC) were collected from clean and mouldy samples at the beginning and the end of the test and were analyzed according to the ASTM D5116-97 and EPA TO-17 methods. Thermal Desorption/Gas Chromatograph/Mass Spectrometer (TDU/GC/MS) analytical equipment was used to desorb, characterize and quantify the VOC collected on the sorbent tube. The compounds were identified with the NBS/NIH Mass Spectra database and quantified by extrapolating the chromatogram peak area of each compound with the calibration curve of alpha-Pinene, which was the selected chemical for VOC measurement. Another test was conducted on mould growth on various building materials under fluctuating high and low humidity conditions. In this test, the same materials as those listed above were used. The temperature was controlled in a stable 20°C and the relative humidities were kept one week at a high RH between 85 and 100% and another week at a low RH of 65%. The third test was conducted on the effect of drying/planing process on the mould resistance of solid wood. In this test, one group of samples were planed before drying and another group were planed after drying. The samples were placed in sample containers and inspected weekly for mould growth for 8 weeks.
Results showed that the targeted temperatures were measured as 20°C and 28°C and the targeted relative humidities were measured as 63% to 95%. At 20ºC and 95% RH, mould growth was found as the following: on the sapwood of jack pine and aspen and on fibreboard in the second week of incubation, on the sapwood of white spruce and OSB in the third week, on plywood and gypsum board in the fourth week, and on the heartwood of jack pine and aspen in the fifth week. No mould growth was detected on the heartwood of white spruce and cedar and on fibreglass insulation materials at the end of the in eight- week testing period. Slight mould growth was found on sapwood of white spruce, jack pine and aspen and on 4 types of composite boards after 3 to 8 weeks at 20ºC and 85% RH. Moulds were unable to attack wafer samples at 76% and 73% RH. At 28ºC, a similar mould growth pattern was observed as those tested at 20ºC. No mould growth was detected on any materials tested at 63% and 71% RH. Mould growth was detected on the sapwood of white spruce and aspen, and on plywood and fibreboard at 84% RH for this temperature. At 93% RH, moulds appeared on plywood and fibreboard in the second week of the test. Fungi at the end of the test did not affect the heartwood of white cedar and white spruce and the fibreglass insulation material. For all materials tested, low-density fibreboard was the most susceptible to mould growth, followed by OSB, plywood, gypsum board and the sapwood of all solid wood species. Heartwood of jack pine and aspen was less infested by moulds. Heartwood of white cedar and white spruce and fibreglass insulation material were resistant to mould growth. On most materials, rapid mould growth was found in 4 to 6 weeks of incubation in the favourable environmental conditions.
No mould growth was detected on any sample in containers maintained at a constant RH of 80% and at a fluctuating RH of 85/65%. Slight mould growth was found on sapwood of jack pine and aspen and on heartwood of aspen in container with a fluctuating RH of 95/65%. Heavy mould growth was found on samples of sapwood of aspen and jack pine, followed by the heartwood of aspen and fiberboard in a lesser degree in container with a fluctuating RH of 100/65%. Other samples were not affected by moulds in the 8-week test period. The results of this test showed that at 20ºC with fluctuating RH of 100/65% or 95/65%, sapwood of aspen and jack pine was the most susceptible to mould infection, followed by the heartwood of aspen and fiberboard.
The planing and drying process (planing wood surfaces before or after drying) largely affected severity of mould infections on sapwood but slightly on heartwood samples. The sapwood samples planed after drying were much less affected by moulds than those planed before drying.
Spore densities in incubation containers increased correspondingly with incubation time. More spores were collected from containers maintained above 88% RH than other containers. There was no significant difference of spore density among containers maintained at less than 85% RH. Majority of moulds present on sampling media were identified as Penicillium citrinum, P. vermiculatum and Aspergillus niger.
For all wood materials tested, low-density fibreboard was the most prone to absorb water from air, while cedar heartwood was the least. At 20ºC and 95% RH, fibreboard increased its MC from 4% MC at the beginning of the test to 33% MC at the end of the 8-week test, while cedar heartwood increased its MC from 11% at the beginning to 21.6% at the end. The other wood materials gained their MC from 17.7% to 22% in the same environmental condition.
The VOC profile analysis showed that several compounds were not detected from clean reference samples but only detected from the mouldy samples, which are called microbial VOC. Compounds detected from the mouldy OSB samples were different from those detected from the mouldy fibreboard samples. From the mouldy OSB samples, the predominant detected VOC were 2-, or 3-pentanone and 2-petanol. From the mouldy fibreboard, they were identified as borneol, camphor, 3-cyclohexen-1-ol (4-methyl), pyrazine-methyl and 1-octen-3-ol. These compounds have potential to be used as mould growth indicators on wall materials.