Zhe Xiao

Moisture ingress is a critical concern in buildings, as it may profoundly affect structural integrity, the energy efficiency of a building, and as well the quality of the indoor environment that, in turn, could influence the health and safety of building occupants. Moisture ingress can occur during any phase in the lifecycle of a building component, where environmental loads, such as precipitation, wind, snow, and elevated relative humidity, play a fundamental role in affecting the building structure. Climate change exacerbates the issue of moisture ingress by intensifying these loads. In this review paper, the statistical perspective on publications related to moisture ingress in building envelope materials (BEMs) was first assessed through a scientometric study. All relevant publications were gathered and manually filtered, and the selected papers were categorized based on the topics discussed. The results of the scientometric study, as presented in this paper, include a bar chart in which the number of publications in each category is illustrated; a science journal mapping diagram showing the interdisciplinary connections of the research; a cluster map depicting the network between topics; and an R&D momentum analysis reflecting the rate of growth and publication count in this field. Given the strong focus on material properties, this review also examines experimental methods for characterizing moisture transport properties in building materials used in BEMs. Additionally, the differences between various codes and standards centered on this topic are reviewed and discussed. This combined strategy is intended to comprehensively evaluate available information and approaches to permit identifying the knowledge gaps that need to be addressed.

The primary goal of this review is to explore both the fundamental dynamics of moisture ingress and practical strategies for its mitigation. Moisture ingress remains a critical issue due to its impact on the structural integrity of buildings and the health and safety of occupants. This work adopts a systematic approach, focusing on key mechanisms of water transport—capillary action, vapour diffusion, and condensation—and how different parameters influence the process of moisture transport. Moisture ingress, whether through direct leakage, capillary action, air infiltration, or vapour diffusion, poses significant risks to the premature degradation of building envelope materials. In this study, emphasis has been placed on describing the methods for controlling liquid water movement, preventing condensation, and using moisture-resistant materials. Additionally in this study, the advanced design and hygrothermal performance simulation tools are examined; the use of such tools is considered essential for predicting and managing moisture-related issues in building envelopes. Finally, the significance of complying with moisture control standards and guidelines is highlighted, ensuring a comprehensive framework for effective moisture management in building design and maintenance. Beyond this review, key knowledge gaps in moisture control strategies have been identified, particularly in respect to material performance, the accuracy of predictive modeling, and the standardization of mitigation techniques. Addressing these gaps is essential for advancing building design, maintenance practices, and regulatory frameworks that together combine to enhance moisture resilience.

Resilience has been used as a building performance metric that measures the building’s capability of absorption, response, and recovery from one or a series of disruptive events, e.g., extreme weather events or power outage events. With respect to resilience, in relation to the moisture performance of the building envelope (moisture resilience), this aspect has not yet been thoroughly explored nor defined. Given the expected increase in annual precipitation in certain regions of Canada as induced by climate change effects occurring both currently and in the future, the moisture resilience of building envelops will require immediate attention given that wall assemblies of buildings are predicted to be subjected to excessive moisture loads in the coming years. In this study, the moisture resilience of wood frame wall assemblies to mould growth was described from three aspects: (i) absorption—the ability of the wall to maintain a low level of relative humidity on the OSB; (ii) response—the fluctuation of the relative humidity on the OSB; and (iii) recovery—the rate at which the relative humidity recovers to an acceptable level. The metrics used to demonstrate the relative impact of these factors on moisture performance were also developed. The results have revealed a robust correlation between moisture performance and the relative influence of various newly defined aspects of moisture resilience.

Hygrothermal simulation is commonly used to understand the effects of climate loads on wall assemblies. This helps determine the risk of deterioration of wall components due to factors such as mould, wood rot, corrosion, and other modes of degradation. Owing to the lengthy computing time and high cost of undertaking simulations over extended periods of time in excess of 30 years, common practice is to select representative years, generally referred to as moisture reference years (MRYs), amongst the climate data series with the assumption that they would provide results similar to that obtained using the entire climate data series. In this study, simulations were performed for a wood-frame wall having brick veneer and fibreboard as cladding and located in 4 Canadian cities using both a 31-year climate data series and MRYs. The mould growth index on the outer layer of the sheathing panel was used as the performance indicator to permit comparing the moisture performance obtained using the long-term climate series to that obtained using MRYs. It was determined that different strategies should be implemented to analyse the MRY simulation results for different types of wall assemblies to obtain a similar conclusion as that of 31-year simulations.

The building sector is responsible for nearly 40% of the total global direct and indirect CO₂ emissions. Urban green infrastructure, which includes features such as urban trees, vegetation, green roofs, and green facades, are examples of nature-based solutions often employed as municipal climate mitigation and adaptation strategies. This approach offers a range of cost-effective strategies for reducing municipal CO₂ emissions and presents compelling public policy co-benefits such as improved urban livability and enhanced environmental conditions. For municipalities to confidently deploy these solutions at a scale necessary to achieve climate benefits, acquiring knowledge of quantifiable and demonstrated outcomes is an essential requirement. The objectives of this paper are to (1) provide a comprehensive analysis of the advantages and limitations of nature-based solutions (NBS) to address the challenge of reducing CO₂ emissions; (2) evaluate existing design guidelines and policies as may be available across Canada, and that that support the of implementation of NBS in urban agglomerations; (3) identify knowledge gaps and research needs to address challenges to the implementation of NBS. In this review, suggestions and requirements as presented in these documents are examined while giving due consideration to the scientific evidence available in research papers. It was found that the adoption of NBS can contribute to carbon neutral communities through reduced building energy consumption and carbon sequestration. Supportive guidelines and policies have been developed, or are in development, to promote the implementation of NBS at the city scale, despite challenges in assessing, quantitatively, their impact due to uncertainties in data, methods, and scale. Nonetheless, existing research provides sufficient evidence to qualify the measures and suggestions outlined in the guidelines and policies described in this paper.

Previous studies have shown that the effects of climate change on building structures will increase the mould growth risk of the wood-frame building envelope in many circumstances. This risk can be controlled by wind-driven rain deflection, improving water tightness of the exterior facade, and improving cladding ventilation. However, the effectiveness of these risk mitigation strategies are subject to various uncertainties, such as the uncertainties of wall component properties and micro-climatic conditions. The objective of this paper is to apply stochastic hygrothermal simulation to evaluate the mould growth risk of a brick veneer-clad wood-frame wall with a drainage cavity under historical and future climatic conditions of Ottawa, a Canadian city located in a cold climate zone. An extensive literature review was conducted to quantify the range of stochastic variables including rain deposition factor, rain leakage moisture source, cladding ventilation rate and material properties of brick. The randomised Sobol sequence-based sampling method, one of the Randomized Quasi-Monte Carlo (RQMC) methods, was applied for risk assessment and error estimation. It was found that, under the climatic condition of Ottawa, limiting the amount of wind-driven rain to which walls are subjected is a more robust mitigation measure than improving cladding ventilation in controlling mould growth risk, the improving of water tightness of exterior façade is not as robust as wind-driven rain deflection and cladding ventilation, however, the reduction of rainwater penetration can reduce the mould growth risk at different levels of rain deposition factor and cladding ventilation rate.

The moisture load in wall assemblies is typically considered as 1% of the Wind Driving Rain (WDR) load that is deposited on the surface of wall assemblies as specified in the ASHRAE-160 standard whereas this ratio has been shown to be inaccurate as compared to results derived from several watertightness tests. Accurate assessment of moisture loads arising from WDR can be obtained through the watertightness test during which different levels of WDR intensities and Driving Rain Wind Pressures (DRWPs) are applied to a test specimen and water that penetrates wall assembly can thus be quantified. Although many previous studies have included watertightness tests, only a few of these have attempted to correlate the moisture loads to WDR conditions as may occur in specific locations within a country. To improve the assessment of moisture loads for a vinyl-clad wall assembly, a wall test specimen was tested following a test protocol based on local climate data using National Research Council of Canada’s Dynamic Wind and Wall Testing Facility (DWTF). The use of this test protocol permitted quantifying the moisture load in the vinyl wall assembly when subjected to several different simulated WDR conditions. The moisture load was formulated as a function of the WDR intensity and DRWP which thereafter allowed evaluating the moisture load based on a given climate’s hourly rainfall intensity and wind velocity. Such work is particularly relevant considering that the intensity, duration and frequency of WDR events across Canada will in some regions increase due to the effects of climate change.