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The impact of the ventilation cavity design on the energy performance of the rainscreen wall assembly was numerically investigated in CFD. The ventilated cavity design parameters considered were the cladding material type (i.e. Fibre cement, brick, and metal cladding), the sheathing membrane emissivity coefficient (i.e. 0.1–0.9), the ventilated cavity height (i.e. 1H and 2H) and the air gap width (i.e. 13 mm–50 mm). The CFD model was validated with experimental data and a dynamic simulation for a typical hot day and cold day is performed for a timestep interval of 5 min. The heat flux through the wall assembly was compared amongst the design alternatives. For the range of parameters considered, the ventilated cavity construct with brick cladding, sheathing membrane emissivity of 0.1, one-storey ventilated cavity, and 13 mm air gap width minimized the heat loss for a typical cold day. However, ventilated cavity construct with brick cladding, sheathing membrane emissivity of 0.1, one-storey ventilated cavity, and 50 mm air gap width minimized the heat gain for a typical hot day.

© 2015 Elsevier Ltd. The article at hand presents the results of an extensive field study in which the air change rate behind rainscreen claddings has been measured. In total eight different full-scale test walls have been tested. The main parameter variations are: 1) the cladding system (brick veneer and sidings), 2) orientation (South-West and North-East) and 3) area of ventilation openings. To increase the reliablity of the results four measuring techniques to determine the air change rate have been applied. The accuray and applicability of these methods have been discussed in a previous paper. The current article focusses on the impact of the three abovementioned parameter variations on the overall air change rate. Moreover their effect on the hygrothermal conditions in the cavities will be outlined.The results show that the cavity ventilation for brick veneer (1-10 ACH) is two order of magnitude lower than behind sidings (100-1000 ACH). This difference also reflects on the drying potential of both systems. The vapour content behind the sidings follows closely the outer climate. In contrast the vapour pressure behind brick veneer can be significantly higher than the outer climate which is induced by its moisture buffer capacity and its low ventilation rate. publisher: Elsevier articletitle: Field study on the air change rate behind residential rainscreen cladding systems: A parameter analysis journaltitle: Building and Environment articlelink: http://dx.doi.org/10.1016/j.buildenv.2015.09.012 content_type: article copyright: Copyright © 2015 Elsevier Ltd. All rights reserved. ispartof: Building and Environment vol:95 issue:1 pages:1-12 status: published

© 2015 Elsevier Ltd. The present article elaborates on the comparison of four measuring techniques quantifying the ventilation rate behind typical cladding systems in wall elements. In total eight full-scale test walls have been investigated in a test building located in a maritime temperate climate (Belgium). Two different commonly used cladding systems are studied: (a) brick veneer and (b) fibre cement sidings. The test walls were installed on the Southwest and Northeast façades of the building corresponding to the orientations with most extreme exposure regarding solar radiation, wind directions and wind-driven rain. In total four different measurement techniques to quantify the ventilation rate in these systems have been applied: a) tracer gas techniques, b) indirect method based on pressure measurements, c) direct method based on anemometers and d) method based temperature and relative humidity registration. The results indicate that the ventilation rate behind vented brick veneer is two orders of magnitude lower than behind ventilated cladding systems with sidings. It was found that the most appropriate measuring technique of a wall system is depending on the ventilation rate, and hence on the type of cladding. For brick veneer claddings, the method deriving the air flow rate from the pressure gauges was found to be most suitable. In the wall systems with sidings, on the other hand, the ventilation rates were sufficiently high to be measured accurately with the anemometers. publisher: Elsevier articletitle: Experimental analysis of cavity ventilation behind rainscreen cladding systems: A comparison of four measuring techniques journaltitle: Building and Environment articlelink: http://dx.doi.org/10.1016/j.buildenv.2015.01.030 content_type: article copyright: Copyright © 2015 Elsevier Ltd. All rights reserved. ispartof: Building and Environment vol:87 issue:5 pages:177-192 status: published

Hygrothermal simulation programs are commonly used by design engineers to analyse moisture performance of building envelopes. For ventilated assemblies, programs typically require the user to enter cavity ventilation rates. If such input data are not prepared on physical grounds, it can impair the quality of the simulation results. In earlier papers: (1) estimations of ventilation rates in experimental walls based on cavity air velocity measurements; (2) comparative calculations based on monthly and annual tabular climate data, a simple driving force model and models of cavity airflow and heat balance, have been presented. In this study, the models and hourly climate field data were used to perform calculations of transient cavity ventilation rates for comparison with previous experimental results. For 13 different time periods extending from 24 to 91 h, the calculated average ventilation rates were within or very close to the experimentally estimated limits for the ventilation rate. Additionally, the calculations captured the temporal variability and the physical cause of ventilation airflow in the cavities reasonably well. The applied calculation methodology can be developed into a user friendly approach to estimate realistic ventilation rate input data for hygrothermal purposes. Limitations and possible improvements of the methodology are discussed. The influence of the accuracy and resolution of the ventilation rate input data in simulations is demonstrated in a case study.

The air exchange in rainscreen walls is expected to provide ventilation drying if excess moisture is absorbed in the wall construction. In an earlier paper, we presented a cavity airflow model and measurement based estimations of air change rates (ACH) in south-facing experimental walls. Here, focus is on the ventilation drying process and its practical implications. ACH in the experimental walls were calculated and converted into drying rates at different stages in the drying process. Furthermore, changes in the drying rates due to changes in the cavity design and in the outdoor climate were investigated. The significance of the drying rates was demonstrated in a case study. Findings showed that the cavity design is of major importance for the drying rate if the material adjacent to the cavity is wet over its entire extension. For such extreme cases, a light facade colour, vented horizontal battens and, in particular, a small cavity depth, are adverse factors for the drying rate. As the drying process proceeds and the moisture transport from the material to the cavity air becomes dependent on vapour diffusion, the drying rates for different cavity designs tend to be evened out. If the resistance to vapour flow in the material reaches high levels, a favourable outdoor climate is significantly more important than the cavity design to promote drying. Also for the case where the material adjacent to the cavity initially is only locally wet, findings showed that the cavity design is of minor importance for the drying time.

To analyse the moisture performance of wall systems with a ventilated rainscreen cladding, the air change rate per hour (ACH) is required. However, the average ACH and its variation depend on many factors. This study focuses on performing field measurements of air velocities and temperatures in south oriented wall cavities characterised by either vertical wooden battens or horizontal vented metal battens. A physical cavity airflow model together with laboratory test of loss factors were used to analyse the data and interpret the results. With vertical battens, findings estimated the average ACH during a measurement period to be 230–310 ACH. In the cavities with horizontal battens, the ACH was 60–70% lower. The daily variations were considerable and hours with solar radiation and clear skies resulted in ACH that exceeded the average values 2–3 times. In contrast to airflow induced by thermal buoyancy, wind-induced airflow was irregular with frequent changes in both velocity and direction. This pattern was observed independent of the angle between the wind and the cladding. The frequent changes in flow direction significantly reduced the efficiency of wind-driven airflow to create air exchange. The wind-induced airflow in wall cavities with a pronounced non-linear relationship between the driving force and the air velocity is suppressed in the presence of buoyancy. For rainscreen claddings exposed to many hours of solar radiation, this effect increases the possibility of accurate estimations of ACH.

This paper reviews the state-of-the-art information concerning pressure equalization of rainscreen walls with a view to disseminate the existing knowledge for architects and building engineers. During this review, emphasis is given to (1) the recent (post 1980) experimental and theoretical investigations concerning pressure equalization of rainscreen walls and (2) the design guidelines and codification for Pressure Equalized Rainscreen (PER) walls. Based on the knowledge gathered from previous studies, the needs and new directions for further research have been identified and reported.

A process is developed to forecast cladding joint air pressure variations which affect rainscreen joint weathertightness in buildings, without recourse to experimental testing. A numerical model, PERAM, is presented to solve equations governing the pressure response of air within the cavity of a rainscreened joint in a building cladding when subjected to an external sinusoidal air pressure variation. A measure of the pressure equalisation performance of the joint as a percentage (the PEP) can then be calculated from the model waveforms derived, as a determination of the degree of weathertightness of the joint system. The PEPs calculated by PERAM are shown to fit the experimental data by incorporating two physically well-defined floating parameters into PERAM. This suggests that the model can be applied at the design stage of detailing weathertight pressure equalised cladding systems.

Advancements in Computational Fluid Dynamics (CFD) and numerical modeling techniques provide new approaches to quantify air flow around buildings and wind induced pressures on the building envelope. This paper describes the progress made on developing a numerical model to predict the performance of Pressure Equalized Rainscreen (PER) wall assemblies. The model is three dimensional and time dependent. The Navier Stokes Equations (NSE) are discretized using a control volume method. External wall boundaries are treated with cyclic pressure differentials, the onlyfluid property available as a boundary condition from the experiment. This paper presents the steps involved in the numerical model development and the effect of static and dynamic parameters on the model predictions. Two types of wall assemblies, one representing an airtight air barrier system and another with a leaky air barrier system, are modeled. Predicted performances of both wall types are analyzed with respect to time and space. Preliminary model validation is performed by comparing computed results with limited experimental data. The paper also includes some of these comparisons for various geometrical configurations of the PER wall assembly.