Risk Of Mould Growth Research Articles

Assessing the “sufficient ventilation” requirement for Austrian buildings: development of a Monte Carlo based spreadsheet calculation to estimate airing intervals and mould risk in window ventilated buildings

In Austria the lack of guidelines or standards has caused many discussions and disputes on the question if “sufficient ventilation” can be ensured with window airing only, in particular in newly constructed, airtight residential buildings. This work presents the development of a calculation method aiming to provide a simple-to-use tool to estimate the risk of mould growth and the window airing interval required to ensure good indoor air quality assuming a range of different boundary conditions and occupant behaviours. The method implements a Monte Carlo approach calculating 1000 single zone mass balances for carbon dioxide (on a room level) and water vapor (on a housing level). Air infiltration through the building envelope is accounted using the so-called LBL-model. The time interval between window airing required to comply with CO2 limit value is estimated by calculating the time evolution of the CO2 concentration for 1000 different parameter combinations. The mould risk is estimated by a 1000-fold calculation of the daily averaged indoor air humidity and the resulting water activity on critical wall surfaces. The results are displayed as probability distributions providing information on the risk that the queried situation can or cannot ensure “sufficient ventilation”. Exemplary calculations for bedrooms of new multifamily buildings estimate that intervals between window airing events (to keep time-averaged CO2-concentration below 1000 ppm), will vary between 23 and 190 minutes (representing the 5th and the 95th percentile). This is clearly below an acceptable intervention interval for bedrooms. For living rooms, the assessment shows a strong sensitivity on the “accessible” air volume. The humidity assessment for this type of housing suggests that mould growth could occur in about 17% of the cases even though air exchange corresponding to two airing events per day were assumed. An additional outdoor air exchange of up to 40 m³/h would be required to reduce the mould risk fraction to <1%, suggesting the need for mechanical ventilation concepts in residential housing to enable healthy indoor environment independently of occupant behaviour.

Evaluation of Moisture Performance of Tall Wood Building Envelope under Climate Change in Different Canadian Climatic Regions

A study was realized to assess the effects of historical and projected future climates on the hygrothermal performance of cross-laminated timber wall assemblies in 12 Canadian cities belonging to several climate regions and zones and for two cladding and ventilation types. Water ingress in the wall assembly was supposed to be 1% wind-driven rain (WDR), and the airflow rate in the drainage cavity was calculated using local climate data. The hygrothermal simulation results showed that under the assumption of no deficiencies allowing wind-driven rain to enter into the wall (perfect wall), there is no risk of mold growth in the future for both claddings, either vented or ventilated. Under the assumption of high moisture loads (1% WDR), the mold growth risk could increase significantly in all climate regions and cities considered. However, in those cities located in the Cordillera and Prairie regions, the increase was not found to be problematic as the maximum mold growth remained under the acceptable level, whereas for cities located in coastal and southeastern regions, the increase in mold growth risk could be considerable. The impacts of cladding and ventilation types on the relative performance of the walls varied with city location.

Hygrothermal performance of a self-insulated exterior wall with various exterior/interior insulation thicknesses in two climate zones in China: A novel moisture-energy-environment-economic (M3E) method in insulation thickness optimization

The accumulation of moisture inside the envelope under high-humid climates can strongly affect the hygrothermal properties and can lead to moisture-related damage / moulds to the materials, especially in severe weather conditions. However, the optimal insulation thickness of the exterior envelope is typically determined by factors such as overall heat transfer performance, carbon dioxide production and economic cost. The effect of insulation thickness on the annual average moisture content (AAMC) of a self-insulated exterior envelope is often neglected and seldom considered in the optimization process. In this study, a coupled heat, air and moisture transfer (HAM) model was derived and validated. Theoretical indices for water vapour transfer were implemented, such as moisture storage coefficient and moisture inertia factor. A simplified model for liquid water penetration and evaporation was also implemented. A novel four-dimensional moisture-energy-environment-economic (M3E) method that incorporates moisture content analysis was proposed for insulation thickness optimization. A case study was conducted on a self-insulated aerated concrete wall in two climate zones: the hot summer and cold winter (HSCW) zone and the cold zone. The insulation thickness interval investigated were 0–50 mm for interior thermal insulation (ITI) and 0–60 mm for exterior thermal insulation (ETI). The results of the case study showed that the AAMC of the entire envelope varied up to 15.2% (HSCW zone) and 10.0% (cold zone) for different insulation thicknesses. In the HSCW zone, the lowest AAMC of the outer part of the envelope adjacent to the atmosphere was found at 30 mm (ITI) or 20 mm (ETI), indicating the presence of turning thicknesses; while in the cold zones, the lowest AAMC was found at 50 mm (ITI) or 10 mm (ETI) with no turning thickness. The proposed theoretical implementations accounted for changes of AAMC in different interior/exterior insulation thicknesses and were verified with the results of numerical calculation. The proposed theoretical indices revealed an opposite effect of water vapour transfer and liquid water transfer on the AAMC for different insulation thicknesses, which can explain the presence of turning thickness. Furthermore, the selected optimal insulation thicknesses from Pareto fronts were found around 25–29 mm for ITI and 36–38 mm for ETI in two climate zones. The results from the M3E method showed a reduction in moisture risks (up to 62.91%) and mould growth risks (up to 4.84%) in the case study, successfully meeting the acceptable range of moisture risk and mould growth risk. The findings of this paper can help generate a new understanding of moisture transfer, and the proposed M3E method can be applied in all cases to reduce the moisture/mould growth risks in the design stage.

Practical Use of Damage Functions for Environmental Preventive Conservation and Sustainability—Examples from Naturally Ventilated Buildings

This work explores the potential of using damage functions to assess cultural heritage environments. Changes caused by dimensional variation due to fluctuations in relative humidity are assessed using two accessible functions, and a third is discussed. The risk of mould growth is assessed from a time series of temperature and RH data. The results of previous studies comparing predictions from four functions to observed mould formation are reviewed, and the practical aspects of using the functions are described. Two situations related to metal and stone risk are described, comparing environments for display and assessing new or refitted buildings for storage. The use of functions to improve sustainability and their combination with performance models to predict carbon footprints are discussed.

A long-term monitoring study on the thermal comfort and durability of a straw bale Passivhaus cottage

The built environment sector accounts for 40% of the UK's total carbon footprint; bio-based construction materials can play an important role in reducing the whole-life carbon of a new build. Straw bale construction is one of the most promising bio-based methods of construction, due to its availability and material properties. Among the declared benefits of straw bale construction are the internal regulation of heat and moisture and the ability of the fabric to dry out. This paper presents a long-term monitoring study aimed at understanding the indoor temperature and moisture balance of a straw bale cottage built in the UK to a near Passivhaus level. The study lasted 6 years, monitoring the temperature and relative humidity of the indoor and outdoor environments, and in ten locations within the straw bale walls. The analysis has shown that the indoor environment achieved thermal comfort throughout the monitoring period, even when the building was used intermittently. Also, an analysis of surface temperatures and a mould growth risk analysis identified very limited mould growth risk within the building fabric. This paper shows the potential of straw bale low-energy construction in providing thermal comfort and a durable building fabric while minimising the whole-life carbon of buildings.

Prediction of mould growth rate within building envelopes: development and validation of an improved model

Mould growth is a common problem in building envelopes. This issue is usually caused by poor design and construction of walls and results from the difference between indoor and outdoor climatic conditions. Mould spores produced by mouldy walls may diffuse into the air, thereby affecting indoor air quality and threatening occupant health. Therefore, it is important to predict the risk of mould growth in building envelopes under various conditions. This study selected three buildings from a traditional community in Shanghai, China. First, the mould species in these building envelopes were identified. Based on the identification results, the growth rate of the corresponding genera was extracted from the literature to establish an isoline model that describes mould growth on the agar surface. In addition, the mould growth rate between and outside the isoline areas was predicted by modifying the Sautour model to relevant air temperature and humidity conditions. According to the results of the proposed model, the critical temperature and humidity that allow the growth of representative moulds from the buildings selected for this study can be expressed as φ=0.002633·cosh[0.10083·( θ-30)]+0.7153. The accuracy of the above model was verified experimentally, and the maximum relative error of the growth rate was within 25%.

Impact of built-in moisture on the design of hygrothermally safe cross-laminated timber external walls: A stochastic approach

In terms of hygrothermal performance, solid wood panels composed of cross-laminated timber (CLT) are sensitive to moisture. However, to the best of our knowledge, no clear indications of critical initial moisture conditions for CLT envelopes regarding the dry-out of built-in moisture have been provided. Therefore, our main objective was to set critical limit values for CLT external wall design in terms of their initial moisture content (built-in moisture) and dry-out capacity (vapour resistance of wall layers) using a stochastic approach. The focus is on five types of CLT external walls that differ in their dry-out capacity. The key factors in the hygrothermally safe design of the CLT external envelope are sufficient dry-out capacity and control of the CLT initial moisture content level during the construction phase. The results of our stochastic analysis confirmed that a high initial moisture content in CLT causes a high mould growth risk when CLT is covered with vapour tight layers. Therefore, the limit values for critical initial moisture content of CLT and the water vapour resistance of the wall layers that ensure dry-out capacity of different external walls to prevent the risk of mould growth on the CLT surface in a cold and humid climate were determined. These limit values take into consideration the weather conditions during the construction phase and classes of material sensitivity to mould growth.

Mould Growth Risks for a Clay Masonry Veneer External Wall System in a Temperate Climate

To reduce greenhouse gas emissions, nations have introduced energy efficiency regulations for new and existing buildings. This has been considered advantageous as more efficient building envelopes would reduce energy consumed to heat and cool home interiors to within accepted thermal comfort bandwidths. However, as these methods have been adopted, many nations have identified an unintended visible presence of surface and interstitial condensation and mould in new code-compliant buildings. In Australia, it has been estimated that up to 50% of Australian houses constructed in the last decade (2006–2016) have a presence of condensation and mould. Australia introduced its first condensation and mould-related building regulations for new homes in 2019. This paper reports on the hygrothermal and mould growth analysis of the most common low-rise residential external wall system, a timber-framed clay masonry veneer wall. A key component of this paper discusses the application of innovative methods in the Australian context. The external wall’s moisture accumulation and mould growth were simulated for a period of ten years using the transient hygrothermal simulation tool, WUFI® Pro, and the mould growth model, WUFI® VTT. This study identified significant risks for this typical external wall system when constructed in a temperate climate.

Microclimate and Mould Growth Potential of Air Cavities in Ventilated Wooden Façade and Roof Systems—Case Studies from Norway

Harsh climatic conditions in the Nordic countries are being worsened by climate change, which increases the moisture load on building façades. New types of defects are being observed in air cavities in well-designed and well-built wooden façades and roofs. More knowledge is required on the microclimatic conditions in air cavities and roofs, and their implications for organic growth and biological deterioration. The present study collects and presents sensor data from three buildings in Norway. Collected air temperature, air humidity, and wood moisture data are compared to mould growth criteria found in scientific literature, building physics software, and national and international standards. The data shows great differences in air cavity microclimates between the case buildings and between different sensor positions within the instrumented air cavities. Air cavity temperatures are found to be lower than exterior temperatures for a substantial portion of the time. For tall buildings, the vertical positioning of a sensor influences the data more than the orientation of the façade. All three buildings feature monitoring positions with both acceptable and critically high levels of moisture to indicate mould risk. There is great variation in the estimated risk of mould growth according to the different criteria. The study indicates that the coastal climate in the south-west of Norway presents a challenge in terms of resilient building design to avoid mould growth in the ventilated air cavity.

Determining the Vapour Resistance of Breather Membrane Adhesive Joints.

Due to increasingly stringent requirements, tapes and adhesive joints are a commonly used method to ensure tightness and energy efficiency in modern building envelopes. Previous studies have researched and tested properties such as the strength and tightness of adhesive joints. So far, water vapour resistance has been neglected. This article aims to determine the vapour resistance and shed light on possible consequences of vapour-tight adhesive joints in breather membranes used in roof assemblies. Laboratory measurements of vapour resistance were conducted according to NS-EN ISO 12572:2016, known as the cup method. Eleven products of breather membranes were tested. Results from the laboratory measurements were used to evaluate the impact of vapour-resistant adhesive joints related to the drying of built-in moisture. The simulation programs WUFI 2D and WUFI Mould Index VTT were used to model scenarios for moisture transport and risk for mould growth. Laboratory results show that the vapour resistance of breather membrane adhesive joints varies from 1.1 to 32 m in sd-value. Three of the tested products have a vapour resistance larger than 10 m, while four products have an sd-value less than 2.0 m. The sd-values of the membranes themselves range between 0.027 and 0.20 m. All tested adhesive joints are considerably more vapour tight than the Norwegian recommended value for breather membranes (<0.5 m). However, the mould growth analysis shows that the risk of mould growth is low in most practical cases, except when using adhesive joints with the highest vapour resistance in roofs assembled during autumn.

Investigation of material characteristics and hygrothermal performances of different bio-based insulation composites

• Four bio-based insulation composites are investigated for their hygrothermal performances and mould growth potential. • The cork composite has the best hygrothermal performance without mould growth. • The mycelium composite is highly susceptible to mould growth risk. • Medium mould growth risk is observed on both hemp and grass composites. The thermal and hygric characteristics of four bio-based insulation composites (mycelium, hemp, grass and cork) are investigated, and their hygrothermal performances including mould growth potential are simulated and analysed under typical construction details and various climates profiles. The cork composite is found to have the best hygrothermal performance and is suitable under all investigated climates without mould growth, due to its hydrophobic nature and low thermal conductivity. The mycelium composite is highly susceptible to mould growth risk; whereas medium risk is observed on both hemp and grass composites. The findings underline the suitability of using different bio-based composites according to the assembly design and the local climate conditions.

Hygrothermal performance assessment of wood frame walls under historical and future climates using partial least squares regression

The objective of this study was to develop a prediction model for predicting the mould growth risk in wood-frame walls. A machine learning algorithm, the Partial Least Squares (PLS) regression was used for developing the model. Hygrothermal simulations were performed for a wood-frame wall with brick cladding using hourly historical and projected future climate data for cities located in different climate zones across Canada. The mould index calculated at the exterior layer of OSB was used as the response variable in the PLS model and the most relevant climate parameters were used as inputs including: temperature, relative humidity, wind speed, wind-driven rain, and solar radiation normal to the façade. The model was trained using a training set comprising two climate periods and different wall orientations for three cities. It was then used to predict the mould index for other years and wall orientations without performing the simulations. The coefficient of determination (R2), the Root Mean Square Error (RMSE) and Mean Absolute Error (MAE) was used to evaluate the model's accuracy. Also, ranking and mould risk category analysis were carried out to assess the model's ability to rank the years according to their moisture severity. Results showed that with proper selection of training dataset, the model can be effectively used to predict the hygrothermal performance of brick-clad wood-frame wall assemblies in the selected cities and over both historical and future climates.

Hygrothermal Modelling of the Differences between Single versus Variable Relative Humidity Vapour Diffusion Resistivity Properties of Pliable Membranes

The study investigates through hygrothermal modelling the effect of different boundary conditions and varying measured vapour diffusion resistivity values on the hygrothermal performance of five pliable membranes. Previously, this research quantified the variable water vapour diffusion resistivity properties of five different pliable building membranes. The membranes were assessed under varying humidity conditions using the gravimetric wet and dry cup test method. The varying humidity conditions better represent the boundary conditions experienced by materials in the building envelope. The pliable membranes include two permeable, two impermeable, and one variable products, which are commonly used to provide air and vapour control layers in the construction of framed external wall systems. This article focusses on the transient hygrothermal modelling of each of these membranes as a component of a typical timber-framed, clay brick veneer external wall system. The simulations were completed for three different climate types, namely, hot and humid, temperate, and cool-temperate with snow, and with a northern and western orientation. The results from hygrothermal and bio-hygrothermal simulations highlighted different responses subject to climate type and orientation. These results show that there are significant differences in simulated moisture and mould growth risk between the results of pliable membranes with single vapour resistance factor value and pliable membranes with multipoint vapour resistance factor values.

Energy performance of a prefabricated timber-based retrofit solution applied to a pilot building in Southern Europe

This paper advances the current knowledge on the use of prefabricated timber-based panels in building renovation by analyzing in detail the thermal performance achieved by two different renovation solutions developed in the framework of the ongoing e-SAFE H2020 project. In particular, these solutions apply to the external walls of a pilot building located in Catania (Italy) as a double-skin façade that increases also the seismic performance of the building. The dynamic energy simulations reveal that the proposed solutions allow reducing the energy need for space heating and space cooling by 66% and 25%, respectively. One further finding is that, although the proposed timber-based renovation solutions are not affected by mould growth and surface condensation risk, the impact of thermal bridges cannot be neglected after renovation. Indeed, despite the strong reduction in the magnitude of heat losses due to thermal bridges (from 667 W·K−1 down to 213.1 W·K−1), they still account for about 21% of total heat losses after the renovation. This suggests that more complex and expensive technological solutions should be introduced to further reduce heat losses in some thermal bridges, but a cost-benefit analysis should justify their adoption. Finally, overlooking these thermal bridges in dynamic energy simulations can lead to an average underestimation of the heating and cooling energy demand after the renovation, by about 16% and 5% respectively. In this regard, the paper proposes a simplified yet reliable approach to include heat transfer through thermal bridges in the post-processing stage of dynamic energy simulations under thermostatic control.

Multilayer assembly of phase change material and bio-based concrete: A passive envelope to improve the energy and hygrothermal performance of buildings

Phase change materials (PCMs) can improve indoor thermal comfort and reduce energy consumption, while bio-based concrete is an environment-friendly material that enables indoor humidity regulation and heat insulation. However, only a few studies have explored the integrated application of the two materials and comprehensively analyzed the energy and hygrothermal performance. In this study, a passive envelope solution that integrates PCM and hemp concrete is proposed to improve buildings’ energy, thermal, and hygric performances simultaneously. Four integrated scenarios were considered and compared with a baseline scenario (hemp concrete only). The performance of the integrated envelope was studied numerically based on the impact of the PCM’s properties and its location in the envelope. The results highlight the indispensable role moisture transfer plays in determining the indoor hygric environment and heat load, as well as the valuable effect of the integrated envelope on improving both energy and hygrothermal performance. Scenario 4/5 (with PCM closest to the interior) in the summer showed the greatest performance improvement compared to the baseline scenario, with reductions of 8.2%, 46.3%, and 43.7% for heat load, temperature fluctuation, and partial water vapor pressure fluctuation, respectively. The impact of the PCM properties in scenario 4/5 illustrate that the optimization of the integrated envelope can be achieved by increasing the thickness and latent heat of the PCM and identifying its appropriate phase transition range. From a year-round perspective, scenario 4/5 is also notable, as it shows great potential for saving energy and adapting to climate humidity variation while guaranteeing moisture equilibrium within the hemp concrete. The three-year assessment confirmed a lack of condensation and no risk of mold growth for such an integrated envelope, as the relative humidity in key locations remains below 75%.

Combined in situ and in silico validation of a material model for hempcrete

Hempcrete is a very promising bio-based insulating material with a minor or even negative CO2 footprint and low thermal conductivity. To utilize it efficiently, it must be possible to make predictions regarding hempcrete hygrothermal performance and whether it is up to the modern standards—this requires a verified material model for use in numerical simulations. The results of ∼8 months of in situ temperature and relative humidity monitoring for a hempcrete wall are presented along with a documented manufacturing process, and the experiment is reproduced in silico, obtaining good agreement and observations in line with previous studies. Given the scarcity of in situ studies, the presented data could be used as a reference for simulations of moisture and heat transfer in building envelopes that contain hempcrete. Validation for hempcrete as a good insulating material for Latvian or similar climate conditions is also provided. Its high mold growth resistance even at high humidity is experimentally demonstrated. Combined with good water and vapor conductivity, this leads to consistently low mold growth risk as shown experimentally and in simulations.

Designing highly insulated cross-laminated timber external walls in terms of hygrothermal performance: Field measurements and simulations

Keeping wood dry both under construction and service is important to prevent possible moisture damages in wooden buildings. However, practice has shown that without weather protection, wooden structures may get soaked during construction. Therefore, our aim was to evaluate the impact of vapour permeable and low permeable highly insulated external wall assemblies, façade orientation, and initial moisture content on the hygrothermal performance of cross-laminated timber (CLT) external walls. We carried out field measurements in the cold and humid Estonian climate, for which five types of test walls were designed and manufactured. Simulation models for future research were also created and validated. Field measurements showed that the CLT external walls were hygrothermally safe in the observed cold and humid climate and the CLT panel provided sufficient control of the water vapour diffusion. The use of low vapour permeable external insulation can be beneficial against vapour permeable insulation if low moisture content of the CLT panel (factory dry) during construction is maintained and insulation is installed airtightly. Cellulose insulation can be a safe choice when selecting water vapour permeable insulation for the external wall in case the construction is planned without weather protection. Mould growth risk calculations based on measured hygrothermal conditions showed that the high moisture content in the thin surface layer of the CLT panel may not be critical when dry-out time is short. The probability of failure must be determined by stochastic analysis using a variety of influencing parameters.

The moisture distribution in wall-to-floor thermal bridges and its influence on mould growth

Moisture in building envelopes increases the energy consumption of buildings and induces mould growth, which may be amplified within the area of thermal bridges due to their different hygrothermal properties and complex structures. In this study, we aimed to (1) reveal the moisture distribution in the typical thermal bridge (i.e., wall-to-floor thermal bridge, WFTB) and its surrounding area and (2) investigate the mould growth in a building envelope that includes both a WFTB and the main part of a wall, in a humid and hot summer/cold winter region of China (Hangzhou City). The transient numerical simulations which lasted for 5 years were performed to model the moisture distribution. Simulated results indicate that the moisture distribution presents significant seasonal and spatial differences due to the WFTB. The areas where moisture accumulates have a higher risk of mould growth. The thermal insulation layer laid on the exterior surface of a WFTB can reduce the overall humidity while uneven moisture distribution may promote mould growth and water vapour condensation.

Assessment of effect of climate change on hygrothermal performance of cross-laminated timber building envelope with modular construction

Cross-laminated timber (CLT) modular construction possesses the advantages of wood, such as excellent carbon storage and thermal insulation, and of modular construction, such as considerably reduced construction period and cost as well as high productivity. This study evaluates the hygrothermal performance of CLT walls considering modular construction in future climatic conditions. Firstly, CLT walls with plywood applied to a core layer were manufactured. A mock-up of a CLT building was produced and its construction process was analyzed. Hygrothermal behavior of the CLT walls was simulated using WUFI simulation program, and the predicted results were verified against measurements obtained from the mock-up experiment. Finally, the hygrothermal performance of the CLT wall was evaluated for four types of insulation and future climate in eight cities of USA. The coefficient of variation—root mean square error (CV(RMSE))—of the temperature and relative humidity inside the ply-lam CLT wall from mock-up experiments and simulation evaluation were 6.43% and 7.02%, respectively, which met the validation criteria. Based on the hygrothermal performance, the ply-lam CLT wall with extruded polystyrene insulation was evaluated as safe from moisture problems in all the eight cities considered in this study. However, the risk of mold growth in all regions and insulation types increased under climate change with a rise of average annual temperature.

Hygrothermal performance of the building envelope with low environmental impact: case of a hemp concrete envelope

This document is a case study of hemp-based materials integrated into the building envelope for African and North American’s applications. The objective is to evaluate the energy performance of hemp concrete for construction in Montreal, Canada, where heating predominates and in Dori, Burkina Faso, where air conditioning predominates. The effect of thermal and hygrothermal comfort of hemp concrete, glass wool, cement block and compressed earth brick walls were simulated to quantify the benefits on overheating during the hottest months for the city of Dori and the risk of mould growth in the walls of the building in winter for the city of Montreal.