A decay risk model for wood frame walls using the 4Ds with special reference to New Zealand

Abstract

The substantial economic burden due to the failure of wall cladding systems with respect to leakage, supports the need for a locally relevant and easily understood method to predict the performance of wall systems. This article describes a methodology for evaluating moisture balance, specifically in a wood frame wall system, and determining the risk of decay due to moisture accumulation in the wood framing. Two common house forms are considered: one with a flat roof and no overhangs, and one with a peaked roof and overhangs. Two cladding system types are considered: direct fixed fibre-cement sheet cladding and fibre-cement sheet cladding over a drained cavity. A wall's ability to manage the moisture load is quantified in terms of the 4Ds: Deflection, Drainage, Drying, and Durability. The moisture balance methodology uses the rainfall and water penetration loads as defined by local climate data and the ASHRAE 160 standard, with special reference to Wellington, New Zealand. Data from laboratory testing and outdoor test hut measurements of sample walls is used to quantify drainage and drying. Data from decay testing of wood samples is used to quantify durability. The model is simple to use and can serve as an easily accessible performance assessment tool for building material manufacturers, architects, builders, and codes officials to evaluate the durability of conventional direct fixed wall cladding systems to determine if they are suitable for the climate and exposure type of their intended site and building form, thereby minimize cladding failure due to water leakage.