In typical New Zealand timber framed house construction, the timber framing has a vapour permeable building wrap applied to the exterior and a cladding applied over 20 mm battens. An interior lining usually of gypsum plasterboard is then fixed once the timber framing has a moisture content of < 20%. Timber framing within the building envelope is not expected to be wet in-service or exposed to a leaching risk if the building is compliant with the NZ Building Code. However, there remains a potential risk that timber framing may still be exposed to inadvertent wetting during the lifetime of the building due to construction errors, lack of maintenance or extreme weather events. In New Zealand, it is usual to preservative treat timber framing to prevent or contain any fungal decay that might develop under these conditions until the moisture source can be identified and rectified. This paper describes a method developed by BRANZ for a study to gain a better understanding of the redistribution and possible leaching of boron, an approved preservative for timber framing that might occur over time in wall framing being wetted regularly and in a non-drying environment. The water dosage rate of 100g per linear metre that was chosen exceeds natural drying rates in a wall cavity in summer and winter, and has been shown to increase the timber moisture content above the fibre saturation value. Additional studies of the distribution of the boron preservative concentration along radiata pine boards showed variation and this is a challenge when trying to determine a benchmark reference value to measure redistribution effects or possible losses against over time. The boron concentrations (expressed as Boric Acid Equivalent) in timber samples were determined at intervals over a 18 month period. At the time of treatment some boards showed an uneven boron distribution in the cross-section. Over time the boron redistributed within the cross-section. Some boards also showed a downward trend in boron concentration and with depletion from the timber edge where water was being directly applied over the surface every 4 days. The overall trend was for a reduction in average % m/m BAE of approximately 30% over the first 6-8 months of the testing then the BAE levels levelled off (or increased).

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