Tuomo Ojanen

Moisture and Bio-Deterioration Risk of Building Materials and Structures

There are several biological processes causing aging and damage to buildings. This is partly due to natural aging of materials and excessive moisture. The demands on durability, energy balance, and health of houses are continually rising. For mold development, the minimum (critical) ambient humidity requirement is shown to be between RH 80% and 95% depending on other factors like ambient temperature, exposure time, and the type and surface conditions of building materials. For decay development, the critical humidity is above RH 95%. Mold typically affects the quality of the adjacent air space with volatile compounds and spores. The next stage of moisture-induced damage, the decay development, forms a serious risk for structural strength depending on moisture content, materials, temperature, and time. The worst decay damage cases in North Europe are found in the floors and lower parts of walls, where water accumulates due to different reasons. Modeling of mold growth and decay development based on humidity, temperature, exposure time, and material will give new tools for the evaluation of durability of different building materials and structures. The models make it possible to evaluate the risk and development of mold growth and to analyze the critical conditions needed for the start of biological growth. The model is also a tool to simulate the progress of mold and decay development under different conditions on the structure surfaces. This requires that the moisture capacity and moisture transport properties in the material and at the surface layer be taken into account in the simulations. In practice there are even more parameters affecting mold growth, e.g., thickness of the material layers combined with the local surface heat and mass transfer coefficients. Therefore, the outcome of the simulations and in situ observations of biological deterioration may not agree. In the present article, results on mold growth in different materials and wall assemblies will be shown and existing models on the risk of mold growth development will be evaluated. One of the results of a newly finished large Finnish research project ‘modeling of mold growth’ is an improved and extended mathematical model for mold growth. This model and more detailed research results will be published in other papers.

Towards modelling of decay risk of wooden materials

An empirical model for wood decay development which can be incorporated into a hygrothermal model of building physics is presented. The model is applied to the ERA-40 reanalysis data, based on six-hour weather observations in Europe, to estimate wood decay in different parts of Europe. These studies provide new tools for evaluating the durability and service life of wooden products and a preliminary European wood decay risk level map. The effects of the projected climate change on wood decay may also be considered by this methodology.ZusammenfassungVorgestellt wird ein empirisches Modell zur Holzfäuleentwicklung, welches sich in ein bauphysikalisches hygrothermisches Modell einbauen lässt. Zur Bestimmung der Holzfäule in verschiedenen Teilen Europas benutzt das Modell die aufbereiteten ERA-40 Daten, die auf sechsstündigen Wetterbeobachtungen in Europa basieren. Diese Untersuchungen liefern neue Möglichkeiten zur Bestimmung der Dauerhaftigkeit und der Nutzungsdauer von Holzprodukten sowie eine vorläufige Darstellung des Holzfäulerisikos in Europa. Die Einflüsse der erwarteten Klimaänderung auf die Holzfäule können mit diesem Verfahren ebenfalls untersucht werden.

Heat and Mass Transfer between Indoor Air and a Permeable and Hygroscopic Building Envelope: Part I – Field Measurements

In this paper, measurements are presented which quantify the mass transfer of tracer gases and water vapor between indoor air and a permeable and hygroscopic building envelope. The transfer of tracer gases through the envelope requires the entire envelope to be permeable, while the transfer of moisture requires sufficient hygroscopic mass to be in contact with the indoor air. The results show that mass transfer can improve the indoor air quality and climate. The diffusion of gases through the building envelope significantly increases the effective ventilation rate for poorly ventilated rooms, but only moderately increases the effective ventilation for well-ventilated rooms. Moisture transfer, on the other hand, has a significant influence on the indoor humidity for both poorly and well-ventilated rooms.

Moisture Buffer Value of Building Materials

Building materials and furnishing used in contact with indoor air may have a positive effect to moderate the variations of indoor humidity seen in occupied buildings. Thus, very low humidity can be alleviated in winter, as well as can high indoor humidity in summer and during high occupancy loads. This way, materials can be used as a passive means of establishing indoor climatic conditions, which are comfortable for human occupancy, or for safe storing of artefacts which are sensible to humidity variation. But so far there has been a lack of a standardized figure to characterize the moisture buffering ability of materials. It has been the objective of a recent (ongoing until mid-2005) Nordic project to come up with such a definition, and to declare it in the form of a NORDTEST method. Apart from the definition of the term Moisture Buffer Value, there will also be a declaration of a test protocol which expresses how materials should be tested. Finally as a part of the project, some Round Robin Tests will be carried out on various typical building materials. The paper gives an account on the definition of the Moisture Buffer Value, it will outline the content of the test protocol, and it will give some examples of results from the Round Robin Tests. (Less)

Heat and Mass Transfer between Indoor Air and a Permeable and Hygroscopic Building Envelope: Part II – Verification and Numerical Studies

As simultaneous heat and mass transfer between building envelopes and indoor air is complicated and expensive to measure in laboratory and field experiments, a numerical model is important in understanding and extrapolating experimental results. In this paper a numerical model that solves simultaneous heat and mass transfer between building envelopes and indoor air is verified using the field measurements presented in Part I of this paper. The verification results show that the model is able to predict the transfer of water vapor, CO2, and SF6 between the building envelope and air. The model is then applied to investigate the humidity, comfort, and air quality in a bedroom of a wooden building located in four European countries (Finland, Belgium, Germany, and Italy). The numerical results show that moisture transfer between indoor air and the hygroscopic structure significantly reduces the peak indoor humidity (up to 35% RH), percent dissatisfied with warm respiratory comfort (up to 10%) and the percent dissatisfied with indoor air quality (up to 25%).

Effect of Exfiltration on the Hygrothermal Behaviour of a Residential Wall Assembly

The hygrothermal behaviour of timber frame wall is analysed using a steady-state calculation method and a two-dimensional heat, air, and moisture transport computer model. The conditions associated with exfiltration of warm and humid indoor air are examined. The physical quantities investigated included the amount of moisture accumulated in the wall cavity during the heating season and the heat loss across the wall. Several interesting correlations between moisture accumula tion in a cavity and parameters such as leakage rate, vapour permeance characteristics of the exterior boundary, additional thermal resistance offered by an exterior sheath ing, and indoor humidity level emerge. These correlations show the advantage of us ing the analytical methods in deriving design guidelines for building components. The results from the analysis are used to identify and quantify various parameters that govern the performance of air barrier systems.

Moisture performance of an airtight, vapor-permeable building envelope in a cold climate

Vapor-permeable building envelopes have received renewed interest because they can moderate indoor humidity levels and improve the drying of the envelope during summer condensation conditions. In this paper, the moisture performance of a vapor-permeable building envelope is presented with field measurements and numerical simulations. The results show that the diffusion resistance of the internal surface should be greater than that of the external surface (typically recommended ratio of 3: 1 or 5: 1), but that the vapor resistance of the vapor retarder can be significantly below that provided by polyethylene and still result in a safe structure, even in a cold climate.