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 aboveRH95%. 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.