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Hygrothermal simulation and risk evaluation-The impact of discretization on numerical results and performance
Andreas Sarkany, Thomas Bednar
TU Wien, Austria
In the last decade the hygrothermal simulation of building parts has become a staple in the assessment and risk evaluation in building physics  although the level of model detail is almost never discussed, e.g. . The following paper should show that the level of modelling detail regarding the simulation model is a crucial factor for comparing and reproducing results of hygrothermal simulations. This is shown using the results of the VTT-Model  because its inherent Mould Index is a scale to grade the visual findings of mould growth on surfaces. Therefore, it is a necessity to choose a mesh fine enough to represent the hygrothermal behaviour on the surface. In addition, a different way to calculate hygrothermal behaviour on surfaces is shown. To reduce the computational performance issues caused by very fine finite element/volume meshes, the hygrothermal properties of the connecting surfaces of the finite elements/volumes can be calculated instead. This procedure is a key factor in the hygrothermal calculation model of HAM4D_VIE. Therefore, the VTT-model was implemented in HAM4D_VIE followed by a comparison between the levels of modelling detail as well as the calculation of finite element/volume connections. These comparisons are carried out in regard to numerical results and their impact on computational performance. This paper shows the necessary extensions of the hygrothermal calculation model of HAM4D_VIE to implement the VTT-model, the validation of the implemented model and a discussion of the comparative findings.
1:15pm - 1:30pm
Influences on the drying behavior of a concrete ceiling below a cold attic.
Thomas Lewis, Andreas Sarkany, Thomas Bednar, Ernst Heiduk, Manfred Grüner, Harald Hofbauer
TU Wien, Austria
The article describes the current state of a project examining the influences on the moisture distribution in cold attics above concrete ceilings of residential buildings. Considerable research has been done on moisture damages in cold attics, especially in Scandinavia and North America, focussing on spaces above wooden ceilings. The project (ongoing until Sept 2021) underlying the article deals with cold attics above concrete ceilings resting on masonry walls, a frequent variant in Austria. Research was triggered by a regional Austrian building industry association to shed light onto recent detrimental moisture accumulation in the wooden wall plate (= bearing for the rafters along the eaves) and in the two EPS insulation layers on top of the ceiling. Suspected reasons for the moisture problems and for the local moisture distribution are 1) a too small diffusion resistance of the vapour retarder covering the ceiling, 2) insufficient (natural) attic ventilation and 3) convection, e. g. in the creepings between the polystyrene blocks. In order to rank these potential causes by influence and also to find a practical solution a two stage experimental approach was chosen: First a handy small scale replica (order of dimension: 1m) of the situation was exposed to the according indoor and outdoor climate in a climate chamber. Different vapour retarders on top of the ceiling were chosen. Eventual boundary effects due to the small scale of the replica were monitored. In parallel, a hygrothermic model taking convection into account was established and simulations carried out. The project will deliver a contribution to the Austrian standard on moisture safety 8110-2 on how to judge the moisture safety of joints via simulation.
1:30pm - 1:45pm
Heat and Moisture Modelling of Exterior Insulated Wall Assemblies with Vacuum Insulation
Brock Conley, Cynthia A. Cruickshank, Christopher Baldwin
Mechanical and Aerospace Engineering, Carleton University, Ottawa, Canada
The need for thin, durable, high performance wall assemblies is apparent as new homes are built to higher energy standards while also maximizing occupant space and comfort. Vacuum insulation panels (VIPs) offer 8-10 times the insulation performance of fiberglass insulation, and would fit the need for a low conductivity exterior insulation. Drawbacks of VIPs are the non-continuous insulation creates a thermal bridge, an exterior vapour barrier and increased difficulty to install. The low conductivity properties show great potential to reduce the heat transfer through the envelope, but the vapour impermeable foil that maintains the vacuum slows the rate of vapour transmission through the assembly and could lead to mould growth at the sheathing over time.This paper outlines the numerical study of the hygrothermal performance of wall assemblies using VIPs as the exterior insulation in Canada. The objective was to find the size and thickness of VIP that would provide the optimal performance.
A simulation study was performed to find the optimal VIP solution that maximizes the effective thermal conductivity and minimizes the risk for moisture related issues. In total, 12 wall assemblies with VIPs used as the exterior insulation were simulated using WUFI and WUFI2D. A range of VIP area and thickness were simulated to determine the optimal heat and moisture performance of a composite insulation panel. The simulations showed that the humidity levels in board of the VIPs decreased when 200 mm by 300 mm VIPs were used, but they did not reach the thermal performance thresholds. As the VIP gap decreased to 25 mm for 560 mm by 864 mm VIPs, the humidity level behind the VIP approached cautious design levels. These simulation were compared to the experimental results from previously published in-situ and steady-state work and showed good agreement between the sets of findings.
1:45pm - 2:00pm
A mathematical model for predicting the indoor moisture variation by using moisture buffering theory
Kan Zu, Menghao Qin
Technical University of Denmark, Denmark
Indoor air humidity plays an important role in thermal comfort and building energy consumption. The utilization of hygroscopic building materials can passively regulate indoor humidity fluctuations and then reduce the energy consumption of the air conditioning system. Theoretically, the physical processes inside these hygroscopic materials require the determinations of hygrothermal properties, which normally signify extensive and reiterative experiments. In many building simulation toolboxes, moisture buffering behavior has been evaluated by either simple approximations or complicated heat and mass models. In this paper, we developed a mathematical model of moisture transport with fast solution time and acceptable accuracy by using the moisture buffer value (MBV) theory. Since the MBV originally represents the moisture buffering capacity of hygroscopic materials, we did some mathematical deductions about MBVs under different boundary conditions. Then the definition of time-average MBV has been used, and all the required parameters were obtained from the practical MBV test. By comparing the new moisture buffer value model (MBM) with the HAMT model, the results indicated that MBM could provide a reasonably accurate prediction for indoor moisture variation.