Risk Of Mould Growth Research Articles

The impacts of COVID-19 pandemic on the hygrothermal environment of our homes

In 2020 the residential sector witnessed a complete transformation of the way people live and occupy the spaces. Indeed, different Countries introduced total lockdowns as a measure to contain and prevent the spread of COVID-19, forcing people to stay at home. These measures impact the indoor hygrothermal environment: higher internal thermal loads and moisture generation rate may create the perfect situation to support mould growth. This project aims to understand the impacts of increased work-from-home practices on the hygrothermal performance of residential buildings. The assessment uses a two-step methodology: firstly, whole building transient simulations (software trnsys) are used to generate the indoor temperature and humidity profiles, secondly hygrothermal transient simulations (software WUFI) are used to quantify the risk of mould growth. This research reveals the inadequacy of current design and construction practices to support flexible occupation patterns.

Development of moisture reference years for assessing long-term mould growth risk of wood-frame building envelopes

A moisture reference year (MRY) is generally used to assess the durability, or long-term performance of building envelopes within a long climatological time period, e.g. a 31 year timeframe. The intent of this paper is to develop a set of moisture reference years that can be used to assess risk to the formation of mould growth in wood-frame buildings over the long-term. The set of moisture reference years have been developed based on 15 realizations of 31-year climate data. Replicated Latin Hypercube Sampling is applied to select 15 sub-realizations with 7 representative years having different levels of moisture index (MI) from each realization. Thereafter, hygrothermal simulations are performed for a brick veneer clad wood-frame wall assembly using the 15 sub-realizations; that sub-realization which produces the highest value of maximum mould growth index over 7-year period is selected as the MRY. The selection process is then implemented for all 15 realizations of the 31-years of data sets, from which 15 sets of 7-year long MRYs are selected to represent the original 15 realizations. It is shown that the 15 sets of 7-year long MRYs can produce the same value of maximum mould growth index as well as the uncertainty as compared to the original 15 realizations having a 31-year climate data set.

Exterior Wood-Frame Walls—Wind–Vapour Barrier Ratio in Denmark

Wood-frame walls in cold climates are traditional constructed with a vapour barrier that also constitutes the air-tightness layer. Polyethylene foil as a vapour barrier is likely used; however, other building materials can be used to obtain correspondingly sufficient properties. 1D hygrothermal simulations were conducted for a wood-frame structure to investigate the wind–vapour barrier ratio, and if the vapour barrier of polyethylene foil could be omitted and replaced by other materials. The results were postprocessed using the VTT mould model. The results showed how wood-frame walls can be designed with respect to internal humidity class and diffusion resistance divided into three categories: no risk for mould growth, needs further investigation, and is not performing well as the risk for mould growth is present. For internal humidity classes 1–3, the ratio between wind and vapour barrier must be about 1:5, and 1:10 for classes 4 and 5 to be on the safe side. Simulations were performed for the climate of Lund, Sweden, which were used to simulate climate in Denmark too. Nevertheless, the results are related to climate data and, thus, the location.

Stochastic Simulation of Mould Growth Performance of Wood-Frame Building Envelopes under Climate Change: Risk Assessment and Error Estimation

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.

Hygrothermal Analysis of CLT-Based Retrofit Strategy of Existing Wall Assemblies According to EN 13788 Standard

In the framework of the ongoing EU-funded innovation project called e-SAFE (energy and Seismic Affordable rEnovation solutions), several solutions for the energy and seismic deep renovation of reinforced-concrete (RC) framed buildings in the EU countries are going to be developed and demonstrated. One of these solutions makes use of cross laminated timber (CLT) panels connected to the existing RC frame through specifically designed dampers to increase the seismic and energy performances of the existing envelope. This paper aims to preliminary assess the hygrothermal performance of such CLT panels when applied to various typical wall structures under different climate conditions in Italy through numerical simulations carried out according to the EN 13788 Standard and considering various indoor vapor production classes. Results show that the most problematic existing wall structures are uninsulated concrete walls, for which a risk of surface condensation and mold growth is predicted in all climate zones because of their low thermal resistance (U-value of 3.55 W·m-2·K-1), followed by uninsulated solid brick walls (U-value of 1.81 W·m-2·K-1). The application of CLT panels is found to not only significantly improve the thermal behaviour of the walls, but also to eliminate any surface and interstitial condensation issues in all climate zones.

Mould Growth Models and Risk Assessment for Emerging Timber Envelopes in Australia: A Comparative Study

Timber envelopes provide multiple benefits in reducing both operational and embodied energy environmental impacts in construction. However, when poorly designed, they may incur in high risk of mould growth, affecting both building performance and occupant’s wellbeing. This research investigates the risk of mould growth associated with emerging timber envelopes in Australia, particularly looking at mass-timber and timber-framed wall typologies. The study compares the use of two mould growth assessment models: the VTT and the IBP biohygrothermal. Results provide relevant insights on both current design approaches and performance assessment methodologies. Whilst the study is based on Australian practice, conclusions have international relevance and applicability.

Hygrothermal performance of cool roofs with reflective coating material subjected to hot, humid and dusty climate

The measurements for the short-wave solar reflectivity of a Reflective Coating Material (RCM) with various cleaning operations that were obtained in a previous study were used in this study to conduct numerical simulations in order to assess the moisture and energy performance of cool and black roofs when they were subjected to the weather conditions of Saudi Eastern Province and Kuwait City. The results of the numerical simulations showed that black roofs always work with less moisture than cool roofs. Because the highest relative humidity in the different components of the black and cool roofs was well below 80%, there was no risk of condensation and mold growth in these roofs. For both weather conditions of Saudi Eastern Province and Kuwait City, the results showed that installing cool roofs have resulted in increasing the heating energy loads in relation to black roofs. Conversely, the results showed that the decrease in the cooling energy loads due to installing cool roofs were typically much greater than the increase in the heating energy loads. As such, cool roofs have resulted in net energy savings in relation to black roofs. Replacing black roof by cool roof in Saudi climate, the results showed that the annual energy savings in the total energy load was 25% and 34% as a result of installing cool roof with RCM at no cleaning and weekly homemade cleaning, respectively. Additionally, replacing black roof by cool roof in Kuwaiti climate, the results showed that the annual energy savings in the total energy load was 23% and 31% a result of installing cool roof with RCM at no cleaning and weekly homemade cleaning, respectively.

CLT construction without weather protection requires extensive moisture control

This study examines how cross-laminated timber (CLT) constructions, including joints, connections and attachment points, are affected by precipitation during construction. The case studies are based on moisture content measurements and material sampling as well as microbiological analysis during the structure’s construction stage. The study does not include remediation control. The field measurements show microbiological growth in all buildings and almost all floor structures that were investigated. Of a total of 200 measuring points analysed, half had mould growth and around a third had moderate or extensive growth. The moisture content measurements for one of the locations with the largest percentage of elevated or high moisture content was at the top of the floor structure in the bottom gap between timbers in the CLT top layer. This is one example of several materials or construction components where there is limited possibility of dry out. Based on the outcome, it would appear difficult, or impossible, to avoid the appearance of microbial growth during construction with CLT without weather protection. Previous studies indicate that microbiological analysis of CLT is extremely rare in both laboratory and field studies, which implies that there are obvious shortcomings in the scientific work. The fact that mould growth is often invisible needs to be disseminated, especially in practical studies. However, there seems to be a good level of awareness in the literature that theoretical studies often conduct mould growth risk evaluations. There do not appear to be any moisture safety assembly methods or solutions for CLT construction that do not have weather protection or a declaration of the critical moisture conditions for CLT products.

Climate Resilience of Internally-Insulated Historic Masonry Assemblies: Comparison of Moisture Risk under Current and Future Climate Scenarios

The conservation of cultural heritage built of historical brick masonry alongside meeting targets in energy reduction will most likely require widespread installation of internal wall insulation (IWI). In London, traditional buildings (pre-1919) make up 40% of the existing stock and insulating from the interior is a likely retrofit solution for solid brick walls. Adding insulation may introduce a higher risk to moisture accumulation and consequences such as mould growth and material decay. To investigate resilience to future moisture loads, three interior insulation assemblies (conforming to two U-value guidelines) were simulated in DELPHIN under reference, near-future (2040), and far-future climate (2080) scenarios. Calcium silicate, phenolic foam, and wood fibre assemblies were simulated. The reference year climate file was compiled from observed data and future files developed using the UK Climate Projections 2018 (UKCP18). Assemblies were evaluated for moisture accumulation, mould growth risk, and freeze-thaw (FT) risk. Results show low-to-medium risks in 2040 and high risks in 2080, assemblies of higher absorptivity and thinner insulation comparatively performing best. The calcium silicate assembly fared best for moisture performance; however, all assemblies will be subject to high moisture risk levels in the far future and responsible retrofits must take this and alternative design solutions into account.

Effect of forced convection on the hygrothermal performance of a wood frame wall with wood fibre insulation

Wood-based thermal insulation materials have interesting hygroscopic characteristics because of their high moisture capacity. The present paper investigates the moisture conditions in wood-frame walls with wood fibre thermal insulation. The effect of forced convection of moist air into the construction is studied in particular. Laboratory measurements were carried out on a full-scale wall model divided in five sections with various configurations. The wall sections had different combinations of exterior air barrier, thermal insulation and interior vapour retarder. Each section was prepared with an air leakage to simulate supply of moist air to the wall construction through forced convection. The measurement results provided insight in the moisture conditions in the wall and internal distribution of moisture in the wall sections. The results show that walls with wood fibre insulation may have the same risk of high moisture levels as walls with mineral wool insulation. However, the results indicate that wood fibre insulation absorbs condensation and melting water, while the mineral wool does not. Hence, the results imply that the wood fibre insulation has the benefit of distributing the moisture over a larger volume than the mineral wool insulation. Furthermore, the investigations indicate that using an exterior air barrier with high thermal resistance results in a generally lower moisture level in the wall construction, which can be considered favourable regarding risk of mould growth.

How well do mould models predict mould growth in buildings, considering the end-user perspective?

Mould growth results from a complex interaction between environmental factors, material properties, and mould fungi characteristics. These interactions must be considered during the design, construction and maintenance of a building to prevent growth. Mould prediction models aim to predict whether mould will grow on a specific material in a part of building with a known, or simulated, relative humidity and temperature. They are often used in the design phase. Several models are available. There is limited research on the performance of the models in real buildings. This study aimed to evaluate six different models, using data from five building parts. The predictions on whether mould growth was expected or not were compared to actual mould growth observations on five building materials. The study was performed as a round-robin. Most models underestimated the possibility for mould when humidity and temperature varied a lot by time. The outcome also depended on the end-user, who needs to make assumptions and parameter values choices on, for example, material susceptibility for mould growth. Therefore, using the same climate data, mould growth prediction may differ depending on who makes the prediction. One model, MOGLI model, where input data comes from laboratory tests and no such assumptions must be made, predicted correct in most cases. One conclusion of the study is that when predictions are made in practice, the results must be used cautiously. More knowledge is needed to understand, and more accurately model, the relationships between the moisture and temperature variations in buildings and the risk for mould growth.

Effect of vapor diffusion port on the hygrothermal performance of wood-frame walls

Vapor diffusion ports (VDPs) that are drilled in the exterior sheathing of wood-frame walls are commonly used in wood-frame buildings in the coastal region of British Columbia with the intention to improve the drying capacity of wood-frame exterior walls. This practice originated following the systematic building envelope failures due to rain penetration that occurred in this region around 1985–1995. A previous laboratory study found that VDPs provided substantial improvement in the drying rates of OSB sheathed walls, but not for plywood sheathed walls. A more recent laboratory test using wood-frame walls with higher insulation levels in compliance with the current energy code found that VDPs did not significantly improve the drying rates. To provide a more comprehensive evaluation of the effect of VDPs, hygrothermal simulations using WUFI 2D are carried out based on the recent tests. The WUFI 2D model is firstly validated with measurements from the recent tests and the validated model is then used for a parametric study to evaluate the effect of VDPs under three wetting scenarios using yearly weather data for Vancouver. It is found that VDPs have the ability to improve the rate of drying, which is directly related to the moisture content level of the wall assemblies, although the improvement is moderate even for high moisture levels. The contribution of VDPs to reducing mold growth risk is insignificant under a moderate moisture load and becomes considerable only under a sustained high level of moisture load, i.e., east orientation with an ongoing rain infiltration. Therefore, efforts should be made towards properly designing and constructing building envelope details instead of relying on VDPs as a mitigation measure to improve durability performance.

Effects of Climate Change on the Moisture Performance of Tallwood Building Envelope

The objective of this study was to assess the potential effects of climate change on the moisture performance and durability of massive timber walls on the basis of results derived from hygrothermal simulations. One-dimensional simulations were run using DELPHIN 5.9.4 for 31 consecutive years of the 15 realizations of the modeled historical (1986–2016) and future (2062–2092) climates of five cities located across Canada. For all cities, water penetration in the wall assembly was assumed to be 1% wind-driven rain, and the air changes per hour in the drainage cavity was assumed to be 10. The mold growth index on the outer layer of the cross-laminated timber panel was used to compare the moisture performance for the historical and future periods. The simulation results showed that the risk of mold growth would increase in all the cities considered. However, the relative change varied from city to city. In the cities of Ottawa, Calgary and Winnipeg, the relative change in the mold growth index was higher than in the cities of Vancouver and St. John’s. For Vancouver and St. John’s, and under the assumptions used for these simulations, the risk was already higher under the historical period. This means that the mass timber walls in these two cities could not withstand a water penetration rate of 1% wind-driven rain, as used in the simulations, with a drainage cavity of 19 mm and an air changes per hour value of 10. Additional wall designs will be explored in respect to the moisture performance, and the results of these studies will be reported in a future publication.

Analysis of hygrothermal performance of low-energy house in Nordic climate

Analyses of hygrothermal conditions in low-energy houses is important because of their likely sensitivity for excessive moisture. The presented work deals with real-time measurement of temperature and relative humidity at multiple locations inside a low-energy house envelope. The measured data allows diagnosing approaches towards building design and understanding and evaluating the house performance. Suitability and accuracy of numerical computation was analysed. The Finnish mould growth model was used to monitor risk and extent of mould growth under measured and computed conditions. The measured conditions represent more favourable environment to avoid mould growth than the design values recommended by national and international guidelines. There was no mould growth indicated at any monitored points of the envelope. Monitoring the hygrothermal conditions provides valuable information about the performance of structural elements, building material and the house envelope and it helps to predict moisture related risks during the building’s service life.

Study of a new capillary active bio-insulation material by hygrothermal simulation of multilayer wall

This paper studies the influence of a new bio-based capillary active insulation (CAI) materials (corn stalk based vegetal derived concrete namely CS-1 and CS-2) and two traditional insulation materials (aerated autoclaved concrete (AAC) and aerated insulation brick (IB)) on the hygrothermal properties of multilayer wall. The reference wall consists of solid brick masonry plastered on the internal and external sides. Walls based on CAI materials consist of an extra layer of insulation and weather-resistive barrier (WRB) on the façade surface. Simulations were performed under the natural climatic condition of hot-humid and cold regions. Optimum thickness for the bio-based CAI materials to be used as internal insulation was 100 mm beyond which higher insulation thickness triggered the risk of mould growth. All walls based on CAI insulation materials (CS-1, CS-2, AAC, and IB) showed the mould index (MI) within the acceptable range. Bio-based CAI materials showed considerable improvement in the temperature and relative humidity at the interior surface. CAI materials reduced the heat losses through the wall by 58–69% as compared to the reference wall. Drying behavior of the wall was significantly improved with the combination of CAI and WRB and no risk of condensation was observed in the walls.

Assessing the hygrothermal performance of typical lightweight steel-framed wall assemblies in hot-humid climate regions by monitoring and numerical analysis

Lightweight steel-framed (LSF) construction has attracted interest and gained popularity worldwide in recent years; however, due to a lack of knowledge, LSF construction suffers from premature failure related to moisture problems in hot-humid regions. To assess the risk of durability failure in developing LSF assemblies, four typical LSF wall assemblies fabricated with two kinds of construction methods were monitored for 10 months under real environmental conditions in the typical hot-humid city of Guangzhou. Wall assembly can become wet when the weather is wet or the indoor environment is air conditioned. Wall assemblies with a ventilatable cladding system (Wall C) are the least susceptible to wetness, followed by assemblies with a ventilatable cladding system and external extruded polystyrene (XPS) board (Wall A), assemblies with stucco cladding (Wall D) and assemblies with stucco cladding and external XPS board (Wall B). Wall A performs the best in terms of mold growth and corrosion risk, while Wall B and Wall D have the highest risks with regard to mold growth and corrosion, respectively. However, the LSF components could reduce the time of wetness (TOW) on the surface of the external sheeting (insulation) board. The external XPS board could significantly reduce the TOW in the structural cavity, while the stucco cladding system leads to a higher moisture risk. Special attention should be given to the sealing quality of the edges, especially at the bottom, and at the interface between the stucco cladding and external insulation, as these locations have the greatest risk of moisture problems.

Wood sheathing durability from moisture sorption isotherm variability due to age and temperature

Hygrothermal modelling can be superior to experimental work in exploring durability of building components and assemblies, however, these models require accurate material property inputs. The results of hygrothermal models can be used to calculate mould growth risk, an indicator of building durability. Mould growth is particularly important in organic building materials such as wood. Wood framed buildings are extremely common in North American low-rise residential construction, and part of these systems is sheathing board. OSB and plywood are the most commonly used types of sheathing board. Material properties of OSB and plywood sheathing include moisture storage, represented by the sorption isotherm – however, some common hygrothermal modelling programs do not account for varying sorption isotherms. This study examines the variability of sorption isotherms in OSB and plywood sheathing after accelerated ageing and under different temperature conditions. The measured data was then input into hygrothermal models, and the results were post-processed to determine mould growth risk via the mould index, a scale for mould growth. Accounting for age and temperature resulted in differences of up to 60% in mould index, with the adjusted models generally displaying lower mould index values. Therefore, the use of a single static isotherm instead of one that accounts for age and temperature can result in the over-prediction of mould growth risk of up to 60% in low-rise residential wood framed walls using OSB or plywood sheathing board.

Hygrothermal Analysis of Masonry Wall with Reed Boards as Interior Insulation System

When the masonry walls of buildings under heritage protection need to be restored and thermally improved, the only option is to use an interior insulation system. This is also the riskiest method of insulating walls in cold climates. Capillary active interior insulation systems have been proven to be the most reliable, minimizing the risk of mold growth and decay caused by condensation. They have also been proven to be less risky in wind-driven rain. The building studied is situated in a heritage-conservation area in downtown Tartu, Estonia, and therefore cannot be insulated from the exterior. This paper compares the hygrothermal performance of four different interior insulation systems with and without a heating cable and vapor barrier. In the first case, Isover Vario KM Duplex UV was placed between reed panels. In the second case, reed panels were used without the vapor barrier. Data loggers were applied between the reed panels and the original wall and inside the room to measure temperature and relative humidity in one-hour intervals. Exterior temperature and relative humidity values were taken from the Estonian University of Life Sciences Institute of Technology weather service station. In addition to the measurements taken in the case study building, calculations were made using heat-air-moisture (HAM) Delphin software to simulate the situation. The use of a smart vapor retarder (Isover Vario KM Duplex UV) with reed panels in the interior insulation system reduced the relative humidity level inside the wall. The vapor retarder improved the drying-potential compared to the interior insulation system without the vapor barrier.

Indoor Air Design Parameters of Air Conditioners for Mold-Prevention and Antibacterial in Island Residential Buildings.

The climate characteristics of the islands in the Nansha Islands of China are a typical marine climate including high temperature, high relative humidity, high salt content, strong solar radiation, and long sunshine. These can provide suitable conditions for mold reproduction on the surface of the wall in a building. Therefore, mildew pollution on the wall for a long time can easily damage the building’s structure. It does not only directly affect the appearance of the building, but also indirectly affects the indoor environment and human health. In this paper, dominant fungi in the residential buildings on thee Nansha Islands of China are Aspergillus, Penicillium, and Cladosporium. Critical lines of temperature and relative humidity for mould growth on the interior surfaces of island residential building envelopes have been given and discussed. The results show that the risk of mould growth on the wall with different materials, from low to high, is reinforced concrete, aerated concrete block, coral aggregate, brick, and wood. Furthermore, in order to prevent the room regulated by air conditioner from being contaminated by mould, indoor air temperature should be set variable and controlled between 26 °C and 28 °C, the relative humidity should be changed between 50% and 80%.

Hygrothermal behaviour of emerging timber-based envelope technologies in Australia: A preliminary investigation on condensation and mould growth risk

This study uses dynamic hygrothermal simulations to assess the risk of failure of both timber-framed and mass-timber emerging external wall assemblies across Australian major cities, where most of the population lives. The transient analysis also investigates the sensitivity of the simulation outcomes with respect to the indoor climate input data. Results suggest that the mass-timber timber envelope performs overall better across the different scenarios of analysis compared to the timber-framed solution. However, the hygrothermal behaviour of the massive envelope solution consistently changes with the indoor climate model variation. This article highlights the importance of addressing condensation risk and moisture safety in timber envelopes from the early design stage. Input data and simulation scenarios must be carefully defined according to the specific case as they can highly impact the results’ outcome.