Conference Agenda

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Session Overview
Session 06: Moisture transfer in wooden structures
Wednesday, 25/Aug/2021:
2:00pm - 3:30pm

Session Chair: Dr. Berit Time, SINTEF
Location: Room 1 - Room 082, Building: 116

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2:00pm - 2:15pm

Pitched unventilated wood frame roof with smart vapour barrier – field measurements

Nora Schjøth Bunkholt1, Lars Gullbrekken1, Berit Time1, Tore Kvande2

1SINTEF, Norway; 2NTNU, Norway

Compact wooden roofs may provide reduced roof height and less material use compared to conventional compact roofs. However, compact roofs are normally built without organic materials between the vapour barrier and the roof membrane due to moisture safety risks. The aim of this study is to contribute to verification of simulations and laboratory measurements indicating that organic materials could be used in compact roofs provided a smart vapour barrier (SVB) is applied at the interior side of the construction. The study is carried out on the ZEB Laboratory (Norway), a full-scale office building with a 19 m long pitched compact wooden roof with a roof angle of 32°. The main purpose of the paper is to give a thorough presentation of the roof and present initial field measurements documenting the performance of this kind of roof construction. The roof is constructed with wooden beams and mineral wool insulation between the beams, vapour tight underlayer roof and ventilated BIPV roofing. Sensors are installed in different positions in the beams for measurement of temperature and moisture content (MC). It is of particular interest to investigate differences in MC close to the underlayer roof and close to the SVB. Hence, sensors are positioned in the top and bottom of the beam cross section. The results show that the MC close to the SVB was relatively low throughout the first year of measurements. Close to the underlayer roof, however, the MC increased during winter due to built-in moisture. MC-levels up to 25 weight-% was reached. This indicates that there might be a risk of mould growth in the roof if the temperature level is sufficiently high. The MC decreased rapidly to around 15 weight-% when summer arrived, which shows an expected redistribution of moisture inside the construction and indicates possible drying of the construction.

2:15pm - 2:30pm

Impact of the 3D Morphology on the Hygric Transfer of Hygroscopic Materials : Application to Spruce Wood

Hiam DAHANNI1,2, Aya RIMA1, Kamilia ABAHRI1, Chady EL HACHEM3, Hassan ASSOUM4

1ENS Paris Saclay, France; 2Lebanese University, Lebanon; 3Cnam Liban; 4Beirut Arab University, Lebanon

Spruce wood is a bio-based material that has been massively used in the construction sector because it plays a significant role in the reduction of environmental issues. It is one of the most species of wood that is widespread in the building industry because it meets the requirements of the thermal regulations and has good acoustic properties. Also, this material has a hygroscopic nature that tends to absorb or release moisture which respectively generates swelling or shrinkage. It is also a highly heterogeneous and complex structure that is composed of solid and air phases. The aim of this paper is to investigate the influence of the heterogeneous and hygroscopic nature of spruce wood on the mechanisms of heat and mass transfers. Furthermore, the novelty originated in this work is that a real 3D spruce wood structure is considered for modelling heat and humidity transfers within the material. A 3D X-ray tomography was investigated for the reconstruction of the material at a resolution of 3.35 µm/pixels. Moreover, an extremely fine meshing was performed on the material so that a generation of both phases: solid, air is obtained. Then, a numerical model was developed in order to simulate the coupling of heat and moisture transfer phenomena within the considered sample. The resulting 3D relative humidity profiles display the high heterogeneity and complexity the material is characterized by due to the variability of the hygrothermal behavior of the components of the material and the topology of the interfaces between solid and air in spruce wood. Finally, the numerical modelling that was done on a homogeneous model showed that the consideration of the real geometry led to a better comprehension of the coupling of heat and mass transfers phenomena and therefore the homogeneous model couldn’t provide us with sufficient information concerning the hygrothermal fluctuations.

2:30pm - 2:45pm

Water vapour permeability of wood – remarks on cup method procedure and measured data of spruce wood

Jan Richter, Kamil Staněk

UCEEB, CTU in Prague, Czech Republic

Although the water vapour permeability of wood has been determined many times, there can be found a wide spread of values even related to only one wood specie and its single anatomical direction. This spread can possibly be attributed to the measurement method itself, since the cup method often shows high inter-laboratory error. However, even if the measurement and evaluation processes are well performed and under control, there were found several factors that can still highly affect the resulting value – up to ca. 70 %. These factors are neither mentioned in standards, nor were found in other sources. This paper theoretically describes those factors and their possible impact to the final value of water vapour permeability. Paper also presents one particular measurement scheme and obtained data from four cup tests performed on spruce wood in transverse direction.

2:45pm - 3:00pm

Sensitivity analysis of hygrothermal performance of wood framed wall assembly under different climatic conditions: the impact of cladding properties

Chetan Aggarwal1, Lin Wang2, Maurice Defo2, Hua Ge1, Max Junginger1, Michael A. Lacasse2

1Concordia University, Montreal, Quebec, Canada; 2Construction Research Centre, NRCC, Ottawa, Ontario, Canada

One of the parameters that influences the moisture performance of the wood framed wall assembly is the material properties of exterior cladding. The uncertainties of its properties, would result in a range of wall performance. The objective of this study was to investigate the impact of uncertainties in cladding material properties on moisture performance of wood framed wall assembly under different climatic conditions. A wood framed (2x6 wood stud) wall with exterior brick cladding was simulated assuming 1% rain leakage deposited on the exterior side of sheathing membrane. A parametric study was carried out to analyze the impact of the cladding properties on the moisture response of OSB. The simulations were conducted in five different cities located at different climate zones across Canada. The aim was to identify the most influential cladding property on the moisture response of OSB, i.e., mould growth index and moisture content, to the varying cladding properties under different climatic conditions i.e., different cities under historical and future conditions. In general, it was found that liquid diffusivity is the parameter that has the most influence on moisture response of OSB in all the five cities. Also, the significance of this influence varies depending on the climatic conditions.

3:00pm - 3:15pm

Identifying the critical orientation of wood-frame walls in assessing moisture risks using hygrothermal simulation

Chetan Aggarwal1, Maurice Defo2, Hua Ge1, Michael A. Lacasse2

1Concordia University, Montreal, Quebec, Canada; 2Construction Research Centre, NRCC, Ottawa, Ontario, Canada

Hygrothermal simulations can be used as a reliable tool in analysing moisture performance for the design of durable building envelope systems. For an efficient analysis, it is important to specify the appropriate climatic loads, including selecting the wall orientation in the simulations. ASHRAE 160 standard recommends to using orientation with highest amount of annual wind-driven rain (WDR) and the orientation with the least annual solar radiation. The objective of this work was to identify the orientation which leads to the worst moisture performance of the different wall assemblies under historical climate loads in different Canadian cities. Four cardinal orientations (North, East, South, and West) and the orientation receiving the highest amount of annual WDR (Default) were tested in this study. The simulations were carried out for four different wood-frame (2x6 wood stud) wall systems that differ by their claddings assembly: brick, fibreboard, stucco, and vinyl. Eleven Canadian cities belonging to different climate zones were selected for simulations. Four different performance indicators were evaluated: Maximum Moisture Content (MC) and Average MC in the OSB, Maximum Mould Index (MoI) and Average MoI value in the outer layer of OSB. Three scenarios of moisture loads were investigated: (i) No WDR (ii) WDR but no water source (iii) WDR and water source as 1% of WDR deposited on the exterior side of sheathing membrane. When there is no WDR, the north facing wall always leads to the worst moisture performance. In the presence of WDR, with and without water source, the default orientation leads to the worst moisture performance with few exceptions. As the default orientation was based on total sum of WDR, it sometimes may not lead to worst performance and hence hourly distribution of WDR should be taken into consideration.

3:15pm - 3:30pm

Moisture-induced deformations of wood and shape memory

Chi Zhang1, Dominique Derome2, Jan Carmeliet1

1ETH Zurich, Switzerland; 2Universite de Sherbrooke, Qc, Canada

Wood is known to swell substantially during adsorption and shrink during desorption. These deformations may lead to damage in wood in the form of cracking and disjoining of wooden components in e.g. floor or windows. This swelling and shrinkage of wood is reversible and can be used as a natural actuation mechanism, which is at the origin of the design of functional materials with moisture-induced stimuli responsiveness, a rapidly developing field in materials science.

Another intriguing moisture-induced deformation mechanism of wood is its shape memory effect. Here the material is deformed in the wet state and afterwards dried under maintained deformation, in order that wood retains its deformed shape even after the removal of the mechanical loading, called fixation. When wood is wetted again it can partially regain its original shape, called recovery. Wood fixation is used for example in furniture design to obtain permanent curved shapes. Densified wood is an example of wood fixation and an undesired effect of this type of wood is its hygrolock-springback upon being exposed to higher moisture contents.

In this presentation, we explain these two mechanisms from a microscopic point of view. Reversible hysteretic sorption and swelling is explained by the opening and closing of sorption sites in ad- and desorption. The moisture-induced shape memory is explained the moisture-induced activation of the interfaces between the reinforcing crystalline cellulose fibers and its matrix. When wetted, the interfaces are weakened and the material can be easily deformed. Upon drying, the interfaces become again stiff and the deformation of the composite can be fixed. When the interfaces are wetted again and weaken, the frozen deformation cannot be sustained and recovery happens.

The presentation finishes with highlighting the importance of understanding the reversible deformation and moisture-induced shape memory in engineering and in building physics applications of wood.

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