**2. Critical environmental conditions for bio-deterioration**

There are several biological processes causing aging and damage to buildings and building components. This is due to natural ageing of materials but also caused by excessive moisture and damage of materials. For mould 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 (Table 1) For decay development, the critical humidity is above RH 95 %. Mould typically affects the quality of the surfaces and 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.

Moisture and Bio-Deterioration Risk of Building Materials and Structures 581

activity on germination and growth of selected mould fungi. They developed isopleths for the growth conditions of mould fungi on agar media. An isopleths is a boundary that defines all combinations of temperature and relative humidity that permit a particular mould growth rate. Grant et al. (1989) analysed and modelled the moisture requirements of some mould fungi isolated from dwellings. A certain succession, depending on the moisture requirements of different fungal species: primary, secondary and tertiary colonizers, was

Research with building materials will be better fitted to the moisture problems in buildings. Adan (1994) used a non-linear regression technique to model sigmoid curves describing vegetative fungal growth of *Penicillium chrysogenum* on gypsum board material. He used the time-of-wetness (TOW) as an overall measure of water availability for fungal growth under fluctuating humidity conditions. The TOW is defined by the ratio of the cyclic wet period (RH ≥ 80 %) and the cyclic dry period. The mould growth is a function of the effect of lowest humidity, time of wetness and high relative humidity frequency, and finally of periods of wet and dry conditions. He used Low Temperature Scanning Electronic Microscope LTSEM to analyse the growth and studied the effect of coatings and surface quality on the mould growth. He also evaluated the effect of distribution of growth density on test results. Clarke et al. (1998) developed a simulation model and tool for mould growth prediction in buildings based on an analysis of published data using growth limit curves for six generic mould categories. These limits have been incorporated within the ESP-r (building Energy Software) system for use in conjunction within combined heat and moisture flow

Modelling of mould growth and decay development based on humidity, temperature, exposure time and material will give tools for the evaluation of durability of different building materials and structures. The models make it possible to evaluate the risk and development of mould growth and to analyse the critical conditions needed for the start of growth of microbes and fungi. The model is also a tool to simulate the progress of mould 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 have been taken into account in the simulations. In practice there are even more parameters affecting mould 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. One of the results of a newly finished large Finnish research project "Modelling of mould growth" is an improved and extended mathematical model for mould growth based on development of

mould index in different materials under different exposure conditions (table 2).

the large laboratory studies on Scots pine and Norway spruce sapwood.

Hukka and Viitanen (1999) and Viitanen et al. (2000) presented a model of mould growth which is based on duration of suitable exposure conditions required before microbial growth will start or the damage will reach a certain degree. Particular emphasis is focused on this time period, the so-called response time or response duration, in different humidity and temperature conditions for the start of mould growth (Figure 1). The model is based on

found.

simulation.

**3. Modeling of development of mould growth** 

**3.1 Mould growth on building materials** 


Table 1. Organisms involving damages and defects of building components (Viitanen and Salonvaara 2001, Viitanen et al. 2003)

In Northern Europe, the roofs, floors and lower parts of walls are most often exposed to high humidity and potential attack by biodeterioration processes (Paajanen and Viitanen 1989, Viitanen 2001a, Kääriäinen et al. 1998) when also decay will develop. For the decay development, the humidity and moisture conditions will be higher than that for mould growth, and modelling of decay risk is a separate task. Mould growth is often typical in materials in exterior conditions. In damage conditions, however, different decay types can be found: brown rot, soft rot, and white rot. In buildings suffering from excessive moisture loading, brown rot is the most common decay type (Paajanen and Viitanen 1989, Viitanen 2001a).

The other a-biotic factors like UV radiation and quality of substrate (nutrients, pH, hygroscopicity, water permeability) are also significant for the growth of organisms. Different organisms, e.g. bacteria, fungi and insects, can grow and live in the building materials; microbiologically clean buildings probably do not exist, as some contamination begins as early as during the construction phase. The humidity / moisture conditions connected with temperature and exposure time are the most important factor for development of biological problems and damage in buildings.

The research and modelling of mould growth is most often performed under constant conditions when the ambient humidity conditions and microclimate will prevail for longer periods. Ayerst (1969) and Smith and Hill (1982) studied the effect of temperature and water

120 % RH > 95 %

**(RH or MC %)** 

wet materials RH > 97 %

Ambient RH > 75 %, depends on duration,

Ambient RH > 95 %,

species and materials

Ambient RH > 65 % depends on duration, temperature, species and

wet materials also nitrogen and low pH are needed

environment

Table 1. Organisms involving damages and defects of building components (Viitanen and

In Northern Europe, the roofs, floors and lower parts of walls are most often exposed to high humidity and potential attack by biodeterioration processes (Paajanen and Viitanen 1989, Viitanen 2001a, Kääriäinen et al. 1998) when also decay will develop. For the decay development, the humidity and moisture conditions will be higher than that for mould growth, and modelling of decay risk is a separate task. Mould growth is often typical in materials in exterior conditions. In damage conditions, however, different decay types can be found: brown rot, soft rot, and white rot. In buildings suffering from excessive moisture loading, brown rot is the most common decay type (Paajanen and Viitanen 1989, Viitanen

The other a-biotic factors like UV radiation and quality of substrate (nutrients, pH, hygroscopicity, water permeability) are also significant for the growth of organisms. Different organisms, e.g. bacteria, fungi and insects, can grow and live in the building materials; microbiologically clean buildings probably do not exist, as some contamination begins as early as during the construction phase. The humidity / moisture conditions connected with temperature and exposure time are the most important factor for

The research and modelling of mould growth is most often performed under constant conditions when the ambient humidity conditions and microclimate will prevail for longer periods. Ayerst (1969) and Smith and Hill (1982) studied the effect of temperature and water

temperature and mould species

Wood moisture content > 25 -

MC > 25 - 120 %, depends on duration, temperature, fungus **Temperature range (**°**C)** 

ca. -5 to +60

ca. 0 to +50

ca. -5 to +45

ca. 0 to +45

ca. 0 to +45

ca. 5 to +50

**Damage / problem type Humidity or moisture range** 

**Type of organism** 

blue-stain fungi

algae and lichen

2001a).

bacteria bio corrosion of many

mould fungi surface growth on

different materials, smell,

wood (soft rot, brown rot or white rot), also many other materials can be

Strength loss of materials.

Surface growth of different materials on outside or weathered material.

development of biological problems and damage in buildings.

health problems

different materials, smell and health problems

blue-stain of wood permeability change of wood

decay fungi different types of decay in

deteriorated,

insects Different type of damage

Salonvaara 2001, Viitanen et al. 2003)

in organic materials, surface failures or strength loss.

activity on germination and growth of selected mould fungi. They developed isopleths for the growth conditions of mould fungi on agar media. An isopleths is a boundary that defines all combinations of temperature and relative humidity that permit a particular mould growth rate. Grant et al. (1989) analysed and modelled the moisture requirements of some mould fungi isolated from dwellings. A certain succession, depending on the moisture requirements of different fungal species: primary, secondary and tertiary colonizers, was found.

Research with building materials will be better fitted to the moisture problems in buildings. Adan (1994) used a non-linear regression technique to model sigmoid curves describing vegetative fungal growth of *Penicillium chrysogenum* on gypsum board material. He used the time-of-wetness (TOW) as an overall measure of water availability for fungal growth under fluctuating humidity conditions. The TOW is defined by the ratio of the cyclic wet period (RH ≥ 80 %) and the cyclic dry period. The mould growth is a function of the effect of lowest humidity, time of wetness and high relative humidity frequency, and finally of periods of wet and dry conditions. He used Low Temperature Scanning Electronic Microscope LTSEM to analyse the growth and studied the effect of coatings and surface quality on the mould growth. He also evaluated the effect of distribution of growth density on test results. Clarke et al. (1998) developed a simulation model and tool for mould growth prediction in buildings based on an analysis of published data using growth limit curves for six generic mould categories. These limits have been incorporated within the ESP-r (building Energy Software) system for use in conjunction within combined heat and moisture flow simulation.
