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Cavity walls are a widely used external wall type in north-western Europe with a good moisture tolerance in cool humid climates. In this work, a cavity wall configuration with a brick veneer outside leaf and a wood fibre board inside leaf is analysed with a newly developed coupled computational fluid dynamics–heat, air and moisture model. Drying of...
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Context 1
... this article, the coupled CFD-HAM model that is discussed in the previous section is applied to a ventilated cavity wall. Figure 4 shows an example of a cavity wall configuration. This configuration will be used for a more detailed study on heat and moisture transport. ...
Context 2
... as reference in equations (22) and (23), the temperature and mass fraction at a specified distance from the wall are used. This is indicated by the dashed line in Figure 4. So for each wall face, there is a different reference, the cell value being at a fixed normal distance from the face. ...
Context 3
... evaluate the performance of a simplified cavity model, a cavity wall configuration under specific boundary conditions was simulated for a period of 1 day with WUFI-2D ( Zirkelbach et al., 2007) and compared with simulations performed with the coupled CFD-HAM model. Figure 4 shows the cavity wall configuration that is used. The simulations were performed under summer and winter boundary conditions. ...
Citations
... In low-rise residential construction the exterior cladding is commonly considered the first plane of envelope protection against the exterior climate, with the waterresistant barrier (WRB) membrane or material considered the second plane of protection. In some jurisdictions, a rain-screen or ventilation cavity included interior of the cladding is also considered part of the second plane of protection and can increase the outward drying capacity of the assembly (Karagiozis and Kuenzel, 2009;Van Belleghem et al., 2015). Rainwater penetration past the first plane of protection in an exterior wall assembly may occur as a result of cladding material properties, installation methodology or incorrect installation practices. ...
Current exterior wall assembly designs for new low-rise residential buildings targeting low-energy demand in heating dominated countries include split-insulation wall and thick-wall assembly designs. Both have been shown to result in thermal efficiency gains compared to building-code minimum assemblies, however long-term hygrothermal performance can vary depending on boundary conditions and the presence of construction deficiencies. Future climate scenarios estimate many heating-dominated climates will experience a reduction in heating-degree day hours and an increase in annual rainfall. Using validated assembly performance data from a Passive House certified facility, a sensitivity analysis is performed to determine the impact of rainwater wetting, air exfiltration and insulation material properties on the hygrothermal response of a thick-wall assembly. Results show that rainwater leakage values of 0.50% and greater of the adhering rainfall on the exterior surface of the assembly results in the greatest risk for failure. The hygrothermal response of the assembly is then examined under a global temperature rise scenario of 3.5°C for five geographic locations across Canada. Results show that an increase in average annual total rainfall does not directly result in an increase in the failure rate of the assembly when a rainwater leak is present. Additional climatic factors, including outdoor air temperature, driving rain and solar radiation received will influence the hygrothermal response of the assembly and need to be considered when modelling the performance under future climate change scenarios.
... WDR is important factor in brick wall function due to the high porosity of the materials. As such, the ventilation of cavity walls has been increasingly common area of research [4][5][6][7][8][9][10][11]. More precise material properties are needed for quantifying the principles of cavity wall ventilation. ...
The material properties of a brick masonry are important when modelling or comparing different veneers. The effect of mortar joint, however, is somewhat an unknown variable. In this study, two different firing batches of a hollow burnt-clay brick product were tested for thermal conductivity, water vapor permeability, water absorption coefficient and capillary moisture content. Brick and mortar materials were tested both exclusively and as masonry specimens. The effect of brick-mortar interface was within margin of error for the first brick batch with slightly lower density, while batch two water vapor permeability, water absorption coefficient and capillary moisture content were noticeably affected. The effect of brick-mortar interface seems to be dependent on brick properties. The differences in hygric properties between brick batches were large, but below 100 %. Accounting for mortar using area weighted properties can improve modelling accuracy, but other factors such as workmanship, degradation and brick batch differences are likely more important.
... Since measuring air change rates (ACH) in air gaps is often difficult, the literature mainly provides ACH from computational fluid dynamic simulations. For air gap widths from 20 mm to 50 mm values range from 30 to 875 h -1 (Falk and Sandin, 2013;Hauswirth and Kehl, 2010;Paskert et al., 2022;Sánchez et al., 2013;Sanjuan et al., 2011;van Belleghem et al., 2015). To simulate the ventilation in the air gap, EnergyPlus provides two numerical models: Wind and Stack open area (WS), and the Design Flow Rate (DFR). ...
... According to further test simulations with different design ACH for a 50 mm wide air gap, 288 h -1 was chosen for the following investigations according tovan Belleghem et al. (2015). The constants A to D were defined by the ...
Green facades can be applied to reduce building energy demands. However, their effect has hardly been reported in the literature. Since Green Wall Models (GWM) are not yet integrated into Building Energy Modeling (BEM) tools, this study modified the Green Roof Model (GRM) of EnergyPlus for the application on a vertical surface and, subsequently, validated it as a ventilated Green Wall (GW). Afterwards, a sensitivity analysis was conducted to investigate the energy-saving potential for different GW orientations and coverage percentages for a residential building equipped with central heating and cooling systems. Finally, the achieved energy savings of the GW were compared with those of a conventional façade renovation. The results showed that greening the south-east façade was the most efficient case, which reduced the cooling and heating energy demand by 10.6% and 0.2%, respectively. Additionally, the results showed that the ventilated GW had a significantly better cooling effect compared to the conventional façade renovation measure.
... According to the authors, the major difference between the two models is the convective effect, which may lead to convective drying as soon as the temperature of the condensation plane rises above the dew point of the indoor air. A ventilated cavity (which means convective effect), dries out much faster than an unventilated one and this effect is discussed elsewhere (Kalamees & Kurnitski, 2010;Karagiozis & Kuenzel, 2009;Salonvara et al., 2007;Van Belleghem et al., 2015). Wang & Ge (2017) analyzed the relative position of the air layer inside the cavity by comparing the simulation results from both 1D models with the results from laboratory experiments. ...
Air leakage is a crucial factor when assessing the hygrothermal performance of wood-frame walls since it can lead to moisture accumulation during the cold season. The seriousness of this prob-lem may change in a warmer climate in the future and hygrothermal simulations are widely used as a tool to predict this effect. However, since 2D models are required for detailed air leakage as-sessment and the high number of input variables leads to having to conduct thousands of simula-tions for a single type of building cladding, downsizing the simulation grid to the lowest number of cells is a crucial task to help ensure reduced computational time. Using a hygrothermal simulation tool, the steps needed to build the smallest 2D grid were explained; as well, convergence and ac-curacy of the results were evaluated and the functional relations between air leakage rate and air permeability of the insulation were clarified. Hygrothermal simulations were performed for wood-frame walls having brick veneer and stucco cladding for three Canadian cities: Whitehorse, Van-couver, and Ottawa. While the air leakage rate has a significant impact on the inner surface of OSB, wind driven rain is the key factor on the outer surface. The performance of stucco cladding is worse than brick in all cases and the future climate may reduce the risk of mould growth on the in-ner surface of OSB in all cities. The results also show that the simulation time can be reduced by 90%, with negligible loss of accuracy, when comparing fine to optimized meshes.
... Theoretically assessing the microclimate is a complex challenge, although the governing equations are known [9]. Simplified models used in commercially available building physics software tend to underestimate the moisture conditions by overestimating drying rates [10]. In Nordic climates in particular, this tendency could result in hazardous conditions going unnoticed. ...
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.
... Van Belleghem et al. [63] modelled the air, heat, and moisture transport in ventilated cavity walls in Belgium, comparing a simplified model in WUFI to a more detailed model. The WUFI model was found to overestimate the ventilation effect on drying and subsequently indicate lower moisture levels than was the case. ...
... The deposition of liquid water in general remains challenging to simulate in numerical models. It was reported by Van Belleghem et al. [63] that commonly used simulation software tends to underestimate moisture loads, highlighting a challenge in using computer models to investigate highmoisture environments such as air cavities subject to wind-driven rain. ...
Accurate values for the climatic conditions in an air cavity, hereby called the microclimate, are crucial when calculating and simulating the performance of a ventilated roof and façade system. The climatic stress of its components and their mould and rot potential influence the long-term durability of the roof or façade. A scoping study is conducted to gain an overview on research and the scientific literature on the microclimate of air cavities in ventilated roofing and claddings in Nordic climates. From the body of the research literature, 21 scientific works were of particular interest, and their findings are summarized. The review shows that only a limited number of studies discuss the microclimate of air cavities. Roofs are discussed to a greater and more varied degree compared to façades and air cavities behind solar panels. However, the results cannot be compared and validated against each other to generally describe the microclimate of air cavities, as the surveyed papers approach the subject differently. This knowledge gap indicates that calculations and simulations can be performed without knowing whether the results represent reality. If the structure of ventilated roof and façade systems are only designed based on experience, it can be difficult to be proactive and adapt to future climate changes. Further studies are needed to determine the relation between the exterior climate and the air cavity microclimate, so that future climate predictions can be used to simulate the long-term performance of ventilated roof and façade systems.
... This risk can be minimised by reducing the vapour pressure in J Mater Sci the living space by, for example, ensuring good indoors ventilation or by using a barrier with high vapour resistance and/or high thermal resistance near the colder side of the wall [34,92]. The exact details of the behaviour of ventilated cavities are complex and difficult to model [104]. ...
Timber cladding has been used since historical times as a locally available, affordable weather protection option. Nowadays, interest in timber cladding is again increasing because of ecological reasons as well as naturalistic viewpoints. This review presents a comprehensive report on timber cladding in a European context, beginning with a brief overview of the history before considering contemporary use of timber cladding for building envelopes. The basic principles of good design are considered, paying attention to timber orientation, fixings and environmental risk factors. The relationship of timber with moisture is discussed with respect to sorption behaviour, dimensional instability and design methods to minimise the negative consequences associated with wetting. The behaviour of timber cladding in fires, the effects of environmental stresses and weathering, as well as the cladding properties and the variation thereof with different types of wood and anatomical factors (including exposure of different timber faces), are examined. The review then moves on to considering different methods for protecting timber, such as the use of coatings, preservatives, fire retardants and wood modification. A brief discussion of various environmental considerations is also included, including life cycle assessment, embodied carbon and sequestered atmospheric carbon. The review finishes by making concluding remarks, providing a basis for the selection of appropriate cladding types for different environments.
... The thermal performance of STB and WCHB models was assessed using the inside air temperature measurements. Three transport modes for heat can be identified as radiation, convection, and conduction [30,31] as shown in Figures 8 (a, b). Figures 9 (a, b, c), 10 (a, b, c), and 11 (a, b, c) show variation in the temperature distribution of certain nodes in the STB model (T2, T5, and T8) and the corresponding nodes in the WCHB model (T13, T16, and T19) on three middays of June, July, and August, respectively. ...
Recently, many studies have focused on studying indoor air quality, especially with the outbreak of COVID-19, which is one of the reasons for the increased need to improve indoor air quality. Many ideas and applications are available to incorporate nature into buildings to improve indoor air quality (IAQ) and thus secure a higher percentage of natural ventilation and pollution reduction. One of these ideas is to use “breathing
walls” (BWs), which are envelope components based on porous materials. They decrease energy consumed for heating, ventilation, and air-conditioning of buildings. The study discusses the effect of using the BW approach on thermal comfort in buildings. Moreover, the improvement in the IAQ when using two models - one using BWs (applying natural and industrial materials together on the BWs, which are composed of wooden concrete hollow
bricks (WCHBs)), and the other model built with solid traditional bricks (STB) - was studied through an experiment to compare air temperature, carbon dioxide (CO2) concentration, indoor relative humidity, and thermal behavior. The experiments were conducted on the two models for five months in the summer of 2019, and the results of both models were compared. From the results, it may be concluded that the model with BWs exhibits
improved thermal behavior than the model with traditional bricks by recording on average three to five degrees lower than the outside temperature. Moreover, the relative humidity is lower in the WCHB model than in the STB model by ~41.66% in the same conditions; however, the CO2 concentration (ppm) in the WCHB model was lower than that in the STB model by ~28.5% in the same conditions.
... Generally, the thermo-hydrodynamic performance of the ventilated air gap is a function of multiple parameters. Different hygrothermal aspects of the ventilated cavity incorporated in the exterior surface of the traditional wall assemblies have been investigated in the literature [25,43,42,39,74,14,46,71,11,20,23,66,48,52,19,26,51]. The air change rate in the air-space has been shown to have a considerable impact on the amount of heat flux passing through the wall structure [54]. ...
Well-designed ventilated air-spaces behind external claddings in the building envelope can potentially reduce thermal energy flow in the entire wall structure. In this study, the impact of the dynamic thermo-hydraulic behavior of the flow in the naturally ventilated cavity on the performance of lightweight and heavyweight wall assemblies with traditional passive and active (i.e., BIPV) facades is investigated. A numerical transient 2-D model validated against experimental measurements is employed to perform the analysis using actual weather data and building practices in Europe, particularly in Switzerland. The results reveal that the change in the external cladding type from the passive fiber cement to the active PV façade can increase the time lag of the wall structure up to 2 hours in summer and decrease it up to 1 hour in winter. The maximum difference between the amplitudes of the heat flux through the interior surface for a wall assembly with the lightweight wall core is 1.5 times higher compared to a heavyweight wall core. The results show that enlarging the cavity thickness behind external claddings from 45 mm to 110 mm can increase the heat flow through the cavity up to 1.5 times. It is also shown that reflective insulation on the cavity surface adjacent to the wall core could increase the cladding surface temperature by more than 30% compared to the case without reflective insulation. This research shows that replacing the passive cladding with an active façade could affect the performance of the entire wall assembly, which highlights the importance of analyzing the active ventilated wall structures to transition towards modern building skins.
... The use of large ventilated cavities has already been required by some building codes (NBCC 2005). Several research projects have analyzed different hygrothermal aspects of the ventilated cavity incorporated in the exterior surface of the wall assemblies (Finch and Straube 2007;Balocco 2009;Manuel et al. 2013;Langmans and Roels 2015;Van Belleghem et al. 2015;Gagliano, Nocera, and Aneli 2016;Buratti et al. 2018, Meyer et al. 2019Tariku and Iffa 2019). A very comprehensive literature review on the factors affecting the airflow rate in the ventilated cavity behind different types of traditional external cladding systems is recently performed by Rahiminejad and Khovalyg (2021). ...
The presence of an air-space within a building envelope is known to have a varying contribution to the overall thermal performance of the wall assembly due to the combined effect of convection and radiation in the air cavity. In particular, the thermal resistance of a ventilated air-space can vary significantly depending on multiple environmental and thermo-physical parameters. Although the thermal resistance of enclosed air-spaces in the building structures has been thoroughly investigated in the literature, it has not been defined for a ventilated cavity. This paper aims to introduce the plausible definitions of the thermal resistance of a vertical ventilated air-space behind external cladding systems. Both theoretical and applied formulations are provided and compared. The energy balance method is used to model the steady-state heat transfer through two types of traditional external wall systems (i.e., brick and vinyl siding) in summer and winter conditions. A range of air exchange rates in the cavity is examined, and the effect of the presence of reflective insulation in the air-space on the thermal resistance of the air gap is analyzed. The results show that the presence of a ventilated cavity in the wall assembly can influence the thermal performance of the building envelope. In particular, the effective thermal resistance of a ventilated air-space behind a brick cladding wall could be between 0.17 and 1.85 times the thermal resistance of the cladding in the range of air change rate in the cavity from 0 to 100 1/h. The effective thermal resistance of the ventilated air gap behind vinyl siding could reach up to 9 times the thermal resistance of the cladding.
