Approach to Assessing the Long-term Performance of Wall Assemblies – Durability of Low-rise Wood-frame walls

Given the importance of moisture to bring about deterioration in building materials over time, very generally, the long-term performance of building components depends on the hygrothermal response of the component when subjected to interior environmental and exterior climatic loads. In respect to the durability of components having a wood frame structure, this depends on whether the wood components remain dry and if not, the time over which they are exposed to conditions that generate temperatures and elevated levels of moisture content suitable for the onset of the formation of wood damaging fungi. Critical factors in estimating the longevity of wood frame structures include assessing conditions suitable for the onset, growth, and propagation of damage to occur, more specifically, the temperature range, wood moisture content and time of exposure. In ISO 13823 [ISO 2008] recommendations are made on the use of different methods for the design and verification of structures for durability. For example, verifying a component of a structure for durability can be done by applying service-life formats such as: (i) the factor method, or; (ii) the limit states format. When employing the factor method the design life is specified and the predicted service life is thereafter determined for the component or structure based on sets of factors that relate service life as determined from field studies to the expected life of a component for a similar use and subjected to a like environmental load. Whereas the limit-states format consists in checking the performance of a component or structure against a performance criterion, or a given performance limit state. Different approaches to assessing the vulnerability of wood frame structures to deterioration have been developed in recent years. Some of these approaches suggest applying a limit-states design approach to the performance assessment of the wood frame assembly. For example, a generalized limit states method was presented by Bomberg and Allen [1996] as a systematic framework for limit state design in respect to the durability of building envelopes. Isaksen et al. [2010] presented a performance based model employing a dose-response function to predict the onset of mould growth, which was used as a limit state. The moisture management within wood frame structures has been investigated by hygrothermal simulation where the temperature, relative humidity and moisture content of wood-based components over time are extracted from the results of simulation. As such, the limit states format can be applied to wood frame structures given that the threshold values for mould growth and wood decay, expressed in terms of relative humidity (moisture content), are known, and as well, the influence of temperature and exposure time can be extracted from simulation results. Thus results from hygrothermal simulations can be used as an evaluation of durability, based on limit states, using e.g., mould isopleths [Sedlbauer 2002] or mould growth and wood decay models [Hukka and Viitanen 1999]. This paper is intended to demonstrate a systematic method for the performance assessment of wood frame wall assemblies that follows a limit-states design (LSD) approach and additionally respects the requirements set out in ISO 13823 “General principles on the design of structures for durability” [ISO 2008]. The use of the method is highlighted by applying it to a project focused on a performance evaluation of proprietary drainage components and sheathing membranes used in wood frame wall assemblies when subjected to the climate of Canada [Saber & Lacasse, 2014]. The basis for evaluating the long-term performance of these wood frame wall assemblies incorporating drainage components was the method described in the Moisture Management of Exterior Wall Systems (MEWS) project [Beaulieu et al. 2002] and in which the concept referred to as the Relative Humidity Temperature-index (RHT-index) was used as a performance attribute of the wall assembly. This use of the RHT-index as a performance attribute for assessing the hygrothermal performance of wall assemblies, and where the approach for the assessment followed the broad precepts set out in ISO 13823[ISO, 2008], has previously been reported by Lacasse and Morelli [2016]. In the present paper, further advances to this approach are described for which an additional performance attribute is integrated to that of the RHT-index. The mould index, as was developed by Viitanen [Ojanen et al., 2010; Viitanen et al., 2010] is shown to be a highly useful performance criterion from which the hygrothermal performance of a wood frame wall may be estimated. The intent in this paper is to demonstrate that this revised approach, as was previously demonstrated by Lacasse and Morelli [2016], is likewise consistent with the guidelines described in ISO 13823 [ISO 2008]. The improvements to that which had previously been described as a standard and systematic method for ssessing the long-term performance (i.e. durability) of wood fame wall assemblies includes a more robust performance attribute in the mould risk index, this index being based on mould growth results obtain from experimental work in a laboratory setting. The Limit States Design (LSD) approach as provided in ISO 13823 [ISO 2008], is first briefly described and thereafter, an example of LSD approach for assessing the performance of a wood-frame wall assembly is described. References: - Beaulieu, P. et al. (2002), Final Report from Task 8 of MEWS Project (T8-03) - Hygrothermal Response of Exterior Wall Systems to Climate Loading: Methodology & Interpretation of Results for Stucco, EIFS, Masonry & Siding Clad Wood-Frame Wall; IRC-RR-118; NRC-Construction; Ottawa; 184 p - Bomberg, M. and Allen, D., 1996. Use of Generalized Limit States Method for Design of Building Envelopes for Durability. J. of Thermal Insulation & Building Envelopes, vol. 20, pp. 18-39. - Hukka, A. and Viitanen, H., 1999. A Mathematical Model of Mould Growth on Wooden Material. Wood Science and Technology, vol. 33(6); pp. 475-485. - Isaksson, T., Thelandersson, S., Ekstrand-Tobin, A. and Johansson, P., 2010. Critical Conditions for Onset of Mould Growth Under Varying Climate Conditions. Building & Environment, Vol. 45(7); pp. 1712-1721. - ISO 13823, 2008. General Principles on the Design of Structures for Durability. - Lacasse, M. A. & Morelli, M. (2016), “A Systematic Method of Assessing the Durability of Wood-Frame Wall Assemblies: Towards the Limit-States Design Approach”; Thermal Perf. of Exterior Envelopes of Whole Bldgs. XIII; Intl Conf.; Dec. 4–8, 2016; Clearwater (FL), ASRHAE, Atlanta (GA), USA, pp. 235-245 - Ojanen, T., Viitanen, H.A., Peuhkuri, R, Lähdesmäki, K., Vinha, J., and Salminen, K., "Mould Growth Modeling of Building Structures Using Sensitivity Classes of Materials", 11th Itnl. Conf. on Thermal Perf. of Exterior Envelopes of Whole Bldgs. XI (Clearwater, (FL), USA, December-05-10), 10 p., 2010. - Saber, H. H. and M. A. Lacasse (2015) Performance Evaluation of Proprietary Drainage Components and Sheathing Membranes when Subjected to Climate Loads – Task 6 – Hygrothermal Performance of NBCCompliant Reference Wall for Selected Canadian Locations; Research Report; National Research Council Canada; Ottawa, ON; 59 pgs. - Viitanen, H., Toratti, T., Makkonen, L., Peuhkuri, R.., Ojanen. T., Ruokolainen, L., Räisänen, J. (2010), Towards modelling of decay risk of wooden materials; Eur. J. Wood Prod. 68: 303–313