Moisture control design has to respond to all relevant hygrothermal loads

Moisture related damage is still a formidable cost factor in the building sector. Besides installation deficiencies, moisture control design failures are the most frequent reasons for moisture problems. Therefore, adequate moisture control analysis has become the key for sustainable buildings. However, by focusing on vapour diffusion only other important moisture loads such as driving rain, construction moisture or air infiltration are mostly neglected. Therefore, international moisture control standards often refer to simulation models for more realistic analysis, leaving many practitioners wondering how to handle these tools. To overcome this dilemma, the updated German moisture control standard has introduced a three-pathway approach for design evaluation: 1 st deemed to satisfy list, 2 nd restricted Glaser calculation and 3 rd fully fledged hygrothermal simulation. The third pathway includes the option to account for small leaks or imperfections in building envelope components. Guidelines in other countries are also embracing similar moisture control approaches which gives hope for more durable and sustainable building design. To reach this aim, moisture control should also become an integral part of the design process instead of a secondary chore.

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

Hygrothermal simulations can be used as a reliable tool in analysing moisture performance. For an efficient analysis, it is important to appropriately select the wall orientation in the simulations. ASHRAE 160 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 different wall assemblies under historical climate in different Canadian cities. Four cardinal orientations (North, East, South, and West) and orientation receiving the highest amount of annual WDR (Default) were tested in this study. The simulations were carried out assuming three scenarios of moisture loads for four different wood-frame (2×6 wood stud) wall systems that differ by their claddings: brick, fibreboard, stucco, and vinyl. With an assumption of no WDR, north facing wall always leads to the worst moisture performance. In the presence of WDR, with and without water source, default orientation leads to the worst moisture performance with few exceptions. As 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.

Hygrothermal simulation and risk evaluation - The impact of discretization on numerical results and performance

Regulations for modelling when deducting thermal simulations are represented in the standards [1]. However, the level of model detail regarding discretization in hygrothermal simulations and especially for evaluating the mould risk on surfaces of organic vapour barriers is almost never discussed. The presented approach shows that the chosen discretization of the simulation model is one of the most influencing factors for the risk analysis of surfaces of very fine layers, such as paper vapour barriers, in walls with interior insulation via hygrothermal simulations. To reduce the computational performance issues caused by very fine finite volume meshes [2], the hygrothermal properties of the connecting surfaces of the finite volumes can be calculated instead. For the risk analysis the VTT-Model was implemented in the hygrothermal simulation program HAM4D_VIE, followed by a comparison of the effect of discretization on the results of the surfaces of the vapour barrier. The results of the comparison are discussed with regard to numerical results and their qualitative impact on computational performance. The presented numerical model will be proposed as an alternative for risk analysis on surfaces of vapour barriers, where mould growth would either stay undetected or the necessary discretization with elements comes at the cost of computational performance.

Hygrothermal simulation on effective dehumidification methods in a museum storage room

The hygrothermal environment must be controlled in facilities like museums and galleries to suitably conserve the stored cultural artifacts. The present study proposes a humidity control technique for a museum storage room in Kyoto, Japan. This method requires limited energy and no large-scale equipment or major building renovation. The relative humidity of the room measured during the preliminary field survey exceeded the range for the conservation of metal artifacts (under 45%RH) throughout the year, and dehumidification was experimentally performed. The possible range of humidity control and the energy are quantitatively evaluated in the present study by simulating varied ways of operating a dehumidifier in combination with the improvement of the room’s property of being airtight. The results of the study indicated that simple building modifications and operational improvements could improve the storage environment. For instance, measures to ameliorate airtightness and sensing control along with the addition of small-scale equipment such as a home-use compressor-type dehumidifier can yield long-term low humidity suitable for the conservation of metal cultural artifacts. Such measures are also considered advantageous in terms of energy and labor consumption.

Interstitial condensation in Chinese residential buildings: cliché or challenge?

In this paper, we compare the predictions of interstitial condensation by the steady-state method and the transient method under different climate conditions in China. Simulations reveal significant differences between the two methods, and the wind-driven rain also plays an important role. As a result, the transient hygrothermal simulation considering wind-driven rain should be recommended instead of the steady-state method for predicting interstitial condensation under complicated climate conditions.

Hygrothermal Characteristics of Cold Roof Cavities in New Zealand

The New Zealand Building Code contains minimum durability requirements for components. For roof structures the requirement is 50 years if the component is structural or 15 years if it is not. Metal roof claddings are very common in New Zealand, and roof spaces are typically not deliberately ventilated. Recently, a number of roofs are failing to meet their durability requirement, and the lack of deliberate ventilation is a contributory factor in some cases. In this paper, we consider roof failures and analyse them using the hygrothermal simulation software WUFI® 2D (version 4.1). Using the National Research Council of Canada’s Guideline on Design for Durability of Building Envelopes, we evaluate to what extent the guideline can be used for such more complex models. Experimental data from a residential dwelling where excessive roof moisture issues were discovered shortly after occupancy are presented. A novel remedial solution using daytime-only ventilation to the roof cavity was trialled, and the data were used to benchmark a two-dimensional numerical simulation of the roof space using WUFI® 2D. A larger hygrothermal data set for 71 dwellings is presented together with relevant climatic conditions. The study works towards evidence-based building code changes for roof ventilation and is an example of using the guideline document for more complicated building envelope assemblies.

Hygrothermal simulation of building performance: data for Scottish masonry materials

Retrofitting thermal insulation to solid masonry walls alters their hygrothermal behaviour, which can be modelled by hygrothermal simulation software. However, such software needs values of key material properties to ensure satisfactory results and until now data has not been available for Scottish masonry buildings. This work aims to contribute to a Scotland-specific dataset of material properties for use by designers working on such buildings. Thermal conductivity, water vapour permeability, sorptivity, water absorption coefficient, hygroscopic sorption, density and porosity were all determined experimentally for selected historic and contemporary masonry materials. Within the range of materials tested three groups of materials properties emerge. Natural hydraulic lime mortars, hot-mixed quicklime mortar and earth mortar all show comparatively low density, high porosity, low thermal conductivity, high water vapour permeability and variable but generally high hygroscopic sorption. Craigleith, Hailes and Giffnock sandstones, no longer available but obtained from conservation works on historic buildings, and Locharbriggs and Hazeldean sandstones, obtained from current production, all show intermediate values of these properties. Crathes granodiorite and Scottish whinstone (from current production) show high density, low porosity, high thermal conductivity, low water vapour permeability and low hygroscopic sorption. It is shown that these materials are all relevant to Scottish buildings constructed in traditional masonry and this paper presents the first comprehensive set of hygrothermal property data for them.

A hygrothermal comparative analysis of split-­Insulated, high-RSI wall assemblies in three Canadian climates

As consciousness grows regarding the negative impacts most buildings have on the Earth's environment, techniques to mitigate this impact must emerge in mainstream design practices. A calibrated hygrothermal simulation was conducted using WUFI® Pro to assess predicted hygrothermal performance of a variety of wall assemblies that are likely to enter into mainstream design practices. The results of these simulations reveal the importance of designing assemblies that are resilient to field conditions that introduce more severe hygrothermal loads than standard vapour diffusion. It is clear that in order for a wall assembly to perform adequately under moisture ingress conditions, it must be able to dry freely to at least one side of the building enclosure. High-RSI assemblies with exterior XPS exhibited far diminished resiliency to driving rain penetration as compared to those without exterior insulation and those with exterior mineral wool.

A hygrothermal comparative analysis of split-­Insulated, high-RSI wall assemblies in three Canadian climates

As consciousness grows regarding the negative impacts most buildings have on the Earth's environment, techniques to mitigate this impact must emerge in mainstream design practices. A calibrated hygrothermal simulation was conducted using WUFI® Pro to assess predicted hygrothermal performance of a variety of wall assemblies that are likely to enter into mainstream design practices. The results of these simulations reveal the importance of designing assemblies that are resilient to field conditions that introduce more severe hygrothermal loads than standard vapour diffusion. It is clear that in order for a wall assembly to perform adequately under moisture ingress conditions, it must be able to dry freely to at least one side of the building enclosure. High-RSI assemblies with exterior XPS exhibited far diminished resiliency to driving rain penetration as compared to those without exterior insulation and those with exterior mineral wool.