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Cavity walls consisting of an outer leaf, a cavity and an inner leaf are a widespread building enclosure configuration because of their good performance regarding rain tightness. To increase the drying potential, open head joints are typically provided in the brick outer leaf, creating cavity ventilation. Even though this cavity ventilation has a limited effect on the drying out of the brick veneer, it can significantly reduce the moisture levels inside the cavity. This might be crucial when the brick veneer is combined with, for example, a wooden load-bearing wall. A reliable prediction of the cavity moisture levels is hence essential. However, previous studies showed that the ventilation rate in the cavity is highly fluctuating in both magnitude and direction. That is why most numerical models simplify cavity ventilation by neglecting it, replacing it by an equivalent resistance, assuming a constant air change rate and so on. This article verifies common assumptions in numerical models to incorporate cavity ventilation behind a brick veneer cladding, by confronting the simulations with detailed field studies. The results showed that for walls exposed to driving rain and solar radiation, most simplified one-dimensional simulations do not suffice to predict the moisture content in the cavity in a reliable way. Only with two-dimensional simulations, incorporating the airflow in the cavity, a good agreement with the in situ measurements was obtained. Furthermore, the two-dimensional models showed to be able to capture the moisture gradient along with the height of the wall rather precisely.

© 2017 Elsevier Ltd A qualitative and durable built environment requires an effective moisture control strategy. Moisture related problems can have an impact on the health of inhabitants or even jeopardize the building's structural integrity. Knowledge of the hygrothermal response and the drying potential of the current building enclosures under environmental loads is therefore crucial. In the current study detailed field experiments have been conducted in Belgium to study cavity ventilation behind brick veneer cladding. A simplified version of a South-West oriented cavity wall finished with brick veneer cladding has been analysed regarding the airflow pattern inside the cavity and the cavity's hygrothermal conditions. In this way, the drying potential of cavity ventilation behind brick veneer cladding can be assessed. For the dataset considered in the present article, the study showed that buoyancy induced cavity ventilation leads to a higher effective air change rate compared to cavity ventilation due to wind pressure. Furthermore, it was found that cavity ventilation is not of great help to dry out the whole outer leaf. However, it proved to lower the moisture levels inside the cavity, which – if e.g. the brick veneer is combined with a wooden loadbearing wall – might be important to avoid mould growth problems. The study also showed that climatic conditions play an important role in the drying potential: sunny but windless days have larger drying potential than cloudy but windy days. ispartof: Building and Environment vol:123 pages:133-145 status: published

In this study, two typical timber frame walls with brick veneer cladding have been constructed at KU Leuven to investigate the hygrothermal response of these constructions in a moderate sea climate. Main topic of research is the contradictory criterion for the wind barrier when it comes to the risk on interstitial condensation for winter and summer conditions: in winter a vapour open wind barrier is appropriate, in summer a more vapour tight. Therefore, similar walls but with different types of wind barrier have been investigated. In one set-up a vapour open bituminous impregnated wood fibre board is used as wind barrier, whereas in the second set-up a more vapour tight wood fibre cement board is used. The study shows that a high relative humidity can be expected at the interface between insulation and wind barrier during winter conditions, leading to a high mould growth index. In contrast, the relative humidity at the interface between insulation and inner vapour retarder during summer is lower than expected. This can be caused by the buffering capacity of the hygroscopic materials in the wall.

Even small air paths around the insulation layer in cavity walls may result in a significant reduction of its thermal resistance. This topic of air flow patterns bypassing the insulation layer has been studied in detail from the ‘80s and ‘90s on and let to clear recommendations for building practice [1]. However, the question remains to which extend the proposed research results of that time are adopted in today’s building practice and if so, to which extent they are still valid for current building techniques. The overall aim of the present article is to gain insights into the current installation quality of the insulation layer in cavity walls in Belgium. Moreover, the present article targets to quantify at the same time the impact of workmanship inaccuracies on the thermal performance of these building elements. In a first step the current building practice is mapped in relation to the installation quality of rigid insulation panels in cavity walls in Belgium. A high number of building construction sites has been visited and the installation methods and accuracy are documented. The outcome of this work provides an overview of the configuration of typical air leakage paths in and around the rigid insulation boards in cavity walls for today’s building practice. In the second step of the research, the air flow resistances of the observed air flow paths (e.g. air gap between inner leaf and insulation board, tongue and groove connection of the insulation boards,…) have been measured in laboratory conditions. The characterization of these air flow resistances allows modelling the thermal behaviour of these elements in a following step. Numerical simulations, taking both heat and air transport into account, have been conducted to assess the performance of today’s levels of workmanship. Based on this numerical parameter study, recommendations are proposed to obtain optimal thermal performance in cavity walls.

© 2016 Published by Elsevier B.V. This paper reviews previous research work on the effect of air movement on the heat transmission losses in light weight building components in general and pitched roofs in particular. Air movement in pitched roofs is typically a mixture of three phenomena: (1) in/exfiltration, (2) natural convection and (3) wind-washing. While the importance of air barrier systems to guarantee the thermal and hygric performance by the exclusion of exfiltration is widely acknowledged today, less emphasis is put on the necessity of a continuous wind barrier to prevent wind-washing of the insulation layer. Also the effects of small air leakages around the insulation layer which can trigger natural convection is currently underestimated in practice. Based on findings in the literature, guidelines are put forward on how to construct a pitched roof with a robust high thermal performance. publisher: Elsevier articletitle: Highly insulated pitched roofs resilient to air flow patterns: Guidelines based on a literature review journaltitle: Energy and Buildings articlelink: http://dx.doi.org/10.1016/j.enbuild.2016.03.071 content_type: article copyright: © 2016 Elsevier B.V. All rights reserved. ispartof: Energy and Buildings vol:120 pages:10-18 status: published

© 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

© 2016 Informa UK Limited, trading as Taylor & Francis Group. In timber frame construction in Europe, air barrier systems are typically realised at the interior side of the building envelope. Yet, in some applications such as renovation projects, it can be easier to provide the air barrier layer at the exterior. An exterior air barrier system, typically board materials in which the joints are sealed with tape, needs to withstand severe outdoor weather conditions and maintain pre-defined air permeability values. The aim of the present article is to investigate the impact of severe climatic conditions on the airtightness of typical taped joints. The airtightness of 32 wood fibre cement board samples has been investigated. Each specimen has a 2 mm wide joint. Two different kinds of commercially available tapes were used to seal these joints. Airtightness of all specimens has been tested before and after accelerated ageing procedures that mimic real exterior climate conditions. Three different ageing protocols were selected: (1) temperature cycles, (2) temperature, rain and frost cycles and (3) UV exposure alternate with high humid conditions. The present paper discusses the durability of the two tapes by comparing the air permeability of the specimens before and after accelerated ageing exposure. The results indicate that for both tapes tested the impact of the three artificial ageing methods on the air permeability is limited (increment less than 2 × 10−5 m3/m/h/Pa). peerreview_statement: The publishing and review policy for this title is described in its Aims & Scope. aims_and_scope_url: http://www.tandfonline.com/action/journalInformation?show=aimsScope&journalCode=tjov20 ispartof: International Journal of Ventilation vol:16 issue:1 pages:30-41 status: published

For many years, the coupled heat, air, and moisture (HAM) transport through multilayered building components has been an important research topic. As a result, a great number of numerical simulation tools have been developed to asses and predict the hygrothermal behaviour of building enclosures. But, while the literature shows that air transport has a significant impact on the heat and moisture response of light weight constructions, only few existing heat and moisture transport models include air flow as an active mass component. One of the reasons is the different time scale of air transport compared to the other transported quantities and the resulting difficulties in achieving acceptable simulation performance.The current article presents a quasi-steady state airflow model implementation in an existing transient HAM-model. The air transport is described as Darcy flow in porous media including a body force term to capture natural convection. One particular aim is the prediction of significant effects such as overlaying buoyant and externally driven air flow on the hygrothermal behaviour of building envelope components with sound simulation performances. Furthermore, the article includes an evaluation of the model for which three validation cases from the literature are used to show the applicability and limitations of the presented approach. © 2012 Elsevier Ltd. ispartof: Building and Environment vol:58 issue:12 pages:208-218 status: published

This paper investigates the airtightness of a wood-frame passive house during the consecutive construction stages. Aim is to investigate whether an acceptable airtightness can be achieved by the external wind barrier only. The wind barrier consists of a promising new type of asphalt impregnated fibreboards. To quantify the different leakage paths, the study applies the reductive sealing technique. In total 12 pressurisation tests have been conducted during the construction process. In addition to these field tests on the entire building, the paper discusses the results from laboratory measurements on specimens of the wind barrier including typical joints, in order to verify the influence of different possible types of local air leakage paths. The results reveal that the external wind barrier has a significant airtightness. Moreover, by sealing only the most critical joints of the outer shell, the overall airtightness of the studied case fulfils the requirements of the passive house standard (less than 0.6 air changes per hour (ACH) at 50 Pa). The results presented in this paper, thus, indicate that the proposed solution may have potential to reduce the labour costs required to reach a sufficient level of airtightness.

© 2015 The Authors. Throughout the years, different numerical HAM-simulation tools have been developed to assess and predict the heat, air and moisture response of building components. But, though commercially available and commonly applied in building practice, still, several simplifications and shortcomings exist in the common models. Probably the most important one, is the fact that most tools neglect or strongly simplify air transport, focusing only on heat and moisture transport. Especially for the analysis of wood frame constructions, these simplified models may cause a large discrepancy between simulation results and real performance. This study aims at a comparison of the outcomes of numerical HAM-simulations for wood frame constructions with experimental data of real test cases. In particular, the focus of this paper is on cavity ventilation behind brick veneer. Therefore, a simplified version of a wood frame wall with brick veneer cladding is studied in this paper. Different common modelling assumptions are compared. Furthermore, a detailed measuring campaign has been conducted at the VLIET test building of the KU Leuven to validate the different modelling approaches. By verifying the results of the numerical simulations by the data of real test cases, the reliability of the modelling assumptions can be analysed. The results of this study clearly show that simplified assumptions on cavity ventilation in HAM-models might cause large discrepancies between simulation results and in-situ measurements. publisher: Elsevier articletitle: Modelling Cavity Ventilation Behind Brick Veneer Cladding: How Reliable are the Common Assumptions? journaltitle: Energy Procedia articlelink: http://dx.doi.org/10.1016/j.egypro.2015.11.172 content_type: article copyright: Copyright © 2015 The Authors. Published by Elsevier Ltd. ispartof: pages:1467-1477 ispartof: Energy Procedia vol:78 pages:1467-1477 ispartof: IBPC 2015 location:Turijn date:14 Jun - 17 Jun 2015 status: published

© 2014, International Building Performance Simulation Association (IBPSA). Moisture-induced damage is one of the major causes of degradations and reduced thermal performance in wood frame buildings. It is therefore crucial to incorporate the hygrothermal assessment of new timber frame building envelopes systems from the early development phase onwards. The article at hand presents the simulation results studying the hygrothermal performance of various timber frame wall configurations with exterior air barrier systems. A parameter analysis explores the impact of different European climates, insulation materials, exterior air barrier materials and verifies in addition to the impact of bad workmanship in the installation of the insulation layer. This study reveals that the application of mineral wool (MW) insulated timber frame walls in combination with exterior air barriers results in increased moisture loads. Moreover, small air gap channels between the MW and the adjacent exterior air barrier significantly increase natural convection and add up to harmful moisture levels. Yet the simulations indicate that the use of blown-in cellulose insulation can avoid these issues. The study further indicates that the technique of exterior air barrier is more suitable for continental climates rather than for moderate sea climates in Europe. peerreview_statement: The publishing and review policy for this title is described in its Aims & Scope. aims_and_scope_url: http://www.tandfonline.com/action/journalInformation?show=aimsScope&journalCode=tbps20 ispartof: Journal of Building Performance Simulation vol:8 issue:3 pages:1-22 status: published

Laboratory tests and numerical simulations to study the hygrothermal consequences of using an exterior air barrier in light-weight building envelopes are discussed. Full-scale test walls are placed between a vertical hot and coldbox, operating at controlled temperatures, humidity levels and total air pressures. The test walls are exposed to different boundary conditions during five consecutive measuring stages. Each test wall is provided with 18 thermocouples, 15 relative humidity, 3 heat flux and 1 pressure gauge sensor, covering the most important positions of the wall. In this way, the obtained results can be used as detailed validation data for heat, air and moisture (HAM)-models with a strong focus on natural and forced air convection. The main emphasis of this article is the evaluation of a numerical model with the current dataset. The model used is a modified version of DELPHIN 5 including a quasi-steady state airflow model with the ability to capture natural convection.

The current paper presents the results of a detailed laboratory experiment to study the hygrothermal behaviour of vertical light weight walls with an exterior air barrier. Four independent test walls (each 2.3 m by 0.5 m) are placed between a newly developed hot and cold box, operating at controlled temperatures, humidities and air pressures. All four walls are insulated with 30 cm of standard mineral wool to which OSB is applied as interior vapour retarder. The test walls differ from each other by the physical properties of applied exterior air barrier; airtightness, moisture buffer capacity, vapour permeability and thermal resistance. An additional objective of the current investigation is to generate validation data for numerical HAM models. As a consequence, great care was given to create precise two dimensional conditions and to minimize air leakages. The results reveal that using an exterior air barrier instead of traditional interior one, may increase the moisture load as a result of buoyancy driven convection. This leads to an increased risk for mold growth and interstitial condensation against the upper position of the exterior sheathing in winter conditions. © 2012 Elsevier Ltd. ispartof: Building and Environment vol:56 issue:10 pages:192-202 status: published

The article presents detailed experimental results on the hygrothermal performance of various ventilation cavities. In total 8 full-scale test walls have been installed in a test house to study the impact of different cladding materials (fibre cement sidings/brick veneer cladding) and the size of ventilation openings on the conditions in the cavity. Two measuring methods, hot bead anemometry and air pressure registration, are used to investigate the ventilation rate within the cavities configurations. In addition, equipment was installed to monitor the effect of the ventilation rate on the temperature and humidity conditions within the walls. The results indicate that the air change rate behind brick veneer (1-10ACH) is typically two orders of magnitude smaller than the ventilation behind wall systems with sidings (>100-ACH).