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.


Introduction
During the service life of buildings, natural aging and eventual damage of materials due to different chemical, physical, and biological processes can take place. Ageing of the materials is one aspect of the environmental processes and involve different chemical, mechanical and biological reactions of the materials. Bio-deterioration, e.g. mould, decay and insect damage in buildings, is caused when moisture exceeds the tolerance of structures which may be a critical factor for durability and usage of different building materials. Modelling of the development of mould growth and decay development is a tool for evaluate the eventual risk of ambient humidity or moisture conditions of materials for biodeterioration of materials. The modelling can be used in combination of hygro-thermal analyses of building and building components. Moisture availability is the primary factor controlling mould growth and decay development, but the characteristics of the substrate and environmental conditions determine the dynamics of the growth. However, moist materials may also dry and become wet again thus, resulting in fluctuating moisture conditions. Mould and decay problems in buildings are most often caused by moisture damage: water leakage, convection of damp air and moisture condensation, rising damp from the ground and moisture accumulation in the structure. Repeated or prolonged moisture penetration into the structure is needed for damage to develop.

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.  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 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 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.

Type of organism
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.

Mould growth on building materials
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). 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 the large laboratory studies on Scots pine and Norway spruce sapwood. , time (weeks) and temperature needed to start mould growth on pine sapwood (Viitanen 1996) The growth of mould in this model was evaluated using the "mould index" scale shown in Table 2. The model can be used to evaluate the mould growth in different exposure conditions, and it can be introduced to building physic modeling to evaluate the performance of different structure. The model is not suitable for evaluate the development of decay, for which different models exist (Viitanen 1996, Viitanen et al. 2000. The model describes also the dynamic nature of mould growth under varying temperature and humidity conditions as it gives the predicted mould index as a function of time. Simulation results with the model show that under fluctuating humidity, the mould index will decrease during low humidity or temperature periods, depending on the time periods ( Figure 2). This kind of behaviour can also be found in the "Modelling of mould growth" study (Viitanen et al 2010  The original index is based on wood materials (Viitanen and Ritschkoff 1991a). New determinations for index levels 3 and 4 for other materials are presented using bold fonts and has been presented by Viitanen et al (2011a).

Index
Description of the growth rate 0 No growth 1 Small amounts of mould on surface (microscope), initial stages of local growth 2 Several local mould growth colonies on surface (microscope) 3 Visual findings of mould on surface, < 10 % coverage, or, < 50 % coverage of mould (microscope)

4
Visual findings of mould on surface, 10 -50 % coverage, or, >50 % coverage of mould (microscope) 5 Plenty of growth on surface, > 50 % coverage (visual) 6 Heavy and tight growth, coverage about 100 %  (2001) studied different models to evaluate spore germination and growth of different mould species on different types of materials. He found, that the isopleths developed by growth of mould on an artificial medium can be used to evaluate the growth rate of different fungi. He used a hygrothermal model based on the relative humidity, temperature and exposure time needed for the spore germination of mould fungi based on the osmotic potential of spores. He analysed the effect of different climatic conditions on the spore moisture content and germination. He also evaluated the spore moisture content and germination time based on calculated time courses of temperature and relative humidity in various positions of the exterior plaster of an external wall using WUFI program (Sedbauer and Krus 2003). In Figure 3, a comparison of the critical conditions for mould growth assumed by some of mould growth models is shown. These curves represent lower limiting isopleths (humidity levels) for mould growth.  (2001) with data from results of building materials after Viitanen et al. (2000), Clarke et al. (1998) and Hens (1999) The first version of the mould growth model was based on large laboratory studies with pine sapwood (Viitanen and Ritschkoff 1989). The mould growth intensities were determined at the constant conditions. In the later stages, studies in varied and fluctuated humidity conditions were performed and based on these studies, mould growth model (equation 1) was presented by Hukka and Viitanen 1999. 12 1 7 exp( 0.68ln 13.9 ln 0.14 0.33 66.02) where the factor k 1 represents the intensity of growth (Equation 3), W is the timber species (0 = pine and 1 = spruce) and SQ is the term for surface quality (SQ = 0 for sawn surface, SQ = 1 for kiln dried quality) based on Hukka and Viitanen (1999). For other materials than wood the value SQ = 0 is used, which omits this factor. Numerical simulation is typically carried out using one hour time steps (climate data intervals) and hours are used in the equations instead of days.
In the equation, the factor t M=1 is the time needed to start of the growth (M = 1, Table 2), and t M=3 the time needed to reach the level M =3. The factor k 2 (Equation 3) represents the moderation of the growth intensity when the mould index (M) level approaches the maximum peak value in the range of 4 < M < 6.
( ) In Equation 4 RH crit is the limit RH level to start the mould growth (Viitanen et al 2011a). For other materials than wood, the model has to be modified. The new mould growth intensity factors are presented as relative values compared to those of the reference material pine by using Equations (5) and (6).
In this equation the coefficients A, B and C can have values that depend on the material class. The new M max has an effect on the factor k 2 (Equation 3) and it contributes to the simulation results. Table 4 presents the maximum levels of mould index values for different materials under different conditions. These results were classified to material sensitivity groups, presented both for growth intensities and maximum mould index levels. Table 3 gives the values for the growth intensity parameter k 1 classes and for the coefficients of the maximum mould index factors M max and k 2 . The factor RH min represents the minimum humidity level for starting mould growth for each material group. The factors presented in Table 3 form the new basis for numerical simulation of mould growth on different material surfaces. These values will be applied in the following studies where the model performance will be evaluated. Table 4 represents the tested materials, whose resulting mould indexes were used for the determination of k 1 for the respective classes. The k 1 classes were determined by using expert estimation for most suitable values.

Decline of the mould growth and mould index caused by frost or dry condition
As living organisms mould fungi need water and suitable temperature to grow. When conditions are unfavourable for fungi, activity of mould fungi will be inactivated depending on the extent of the frost or dryness and the time periods of unfavourable conditions. In Figure 4, the humidity and temperature conditions of microclimate for the favourable and unfavourable conditions for mould growth is shown. The rate of humidity and temperature and the time periods in favourable and unfavourable conditions will affect on the growth rate of mould. Especially the longer periods in low humidity or temperature will cause decline of the mould growth and development and even the decline of mould index.
where M is the mould index and t is the time (h) from the moment t 1 when the conditions on the critical surface changed from growth to outside growth conditions (Hukka and Viitanen 1999). Under long period seasonal variations of humidity conditions the decline of mould index may differ from that presented in Equation 8. Also the material may have a significant effect where (dM/dt) mat is the mould decline intensity for each material, (dM/dt) 0 is that for pine in the original model (Equation 9), and C mat is the relative coefficient for mould index decline used in the simulation model. The original decline model for wood could be applied using these additional factors (Ojanen et al 2010, Viitanen et al 2011a. The relative decline of mould for different materials was determined using laboratory experiments with walls (Ojanen et al 2010). The temperature and relative humidity conditions on the critical boundary layer between two different materials were monitored continuously. The mould index level of the material surfaces was determined with suitable intervals by opening the structure from three different parts. The experimental target conditions at the interface of the two materials are presented in Table 5. These experimental walls had mould growth after the first warm and humid period ('Summer/autumn'). The mould decline was determined by the change of the mould index during the second period, a four month long 'Winter' period causing freezing temperatures at the critical boundary. The mould index values were determined for both material surfaces on each critical interface. Figure 11 presents the relative mould decline values (C mat ) solved from the observations in the experiments. The results include the detected mean, minimum and maximum mould index values. The decline of mould intensity on different materials under unfavourable mould growth conditions could be presented as decline classes (Table 6). This classification is based on few measurements with relatively large scattering and it should be considered as the first approximation of these classes. It was found, that the decline was larger within wood

Modeling of development of decay 4.1 Causes for decay damages in buildings
The excessive water into the building structure and materials is the basic cause to different bio-deterioration problems like decay. For instance, in washrooms water often penetrates through inside surfaces or pipe leakage into the structures causing long lasting high humidity conditions. In old wooden buildings, the floor has often been built above a cold ventilated basement or crawl space, where high humidity conditions may exist. If water is penetrated in the crawl space, the ventilation may not keep the floor dry and mould growth is obvious. If ventilation caps are closed, severe decay problems have been found, e.g. dry rot damage Viitanen 1989, Kääriäinen et al. 1998).
In connection of the decay, also microbial contamination on the surfaces in the crawl space is typically much higher than inside the building. The level of fungal spores in the crawl space is about ten times as high as indoors. In crawl spaces, spore concentrations in a range of 10 3 -10 5 colony-forming units per gram (cfu/g) of material are common. The levels have usually been highest on wood-based boards and on timber (Hyvärinen et al. 2002). In cases of heavy fungal colonisation, airborne spore concentrations of up to 10 3 -10 4 cfu/m 3 have been detected. The slab-on-ground structure without thermal insulation below the concrete slab has been used in old buildings. This type a floor is very sensitive for water damage and microbial growth. Especially in detached houses built between 1960 and 1980, wooden beams are often supported on concrete slabs on grade. Partial decay or insect damage is often found in the lower sill plate of exterior walls due to water penetration from the basement (Kääriäinen et al. 1998). Decay is the more severe result of high moisture exposure of wooden structures when the materials are wet for long periods. According to laboratory studies, the growth of decay fungi and decay development can start when the ambient humidity level in the microclimate remains for several weeks above RH 95 -100 % and moisture content of pine sapwood above 25 -30 % Ritschkoff 1991b, Morris et al 2006). According to experience, decay will develop when moisture content of wood exceeds the fibre saturation point (RH above 99.9 % or wood moisture content 30 %, but also the variation of conditions and temperature has an important effect.

Modeling the decay development 4.2.1 A model for decay development in pine sapwood
There is always a vide variation within the growth condition of different fungus species, and we need overall evaluation on the growth activity and decay development of a "typical" example fungi like typical decay fungi (e.g. Coniophora puteana or Gloeophyllum sepiarium). VTT has done comprehensive research in mould and decay growth and their numerical modelling on timber (Viitanen 1996, Viitanen et al 2003 presented a model of decay development in pine and spruce sapwood. Later a new model was developed from the work presented in references Viitanen and Ritschkoff (1991b), Viitanen (1996), andViitanen (1997). In these references, the decay growth of brown rot in spruce and pine sapwood is studied experimentally in different constant relative humidity and temperature conditions. In the present model, only the data of pine sapwood is considered. Based on the experimental findings presented in references, a model for variable conditions is proposed (Toratti et al 2009)). This model is a time stepping scheme. The development of decay is modelled with two consecutive processes:

a) Activation process:
This is termed as α parameter, which is initially 0 and gradually grows depending on the air conditions to a limit value of 1. This process is able to recover in favourable conditions (dry air) at a given rate (although no experimental evidence of recovery is available).

b) Mass loss process:
This occurs when the activation process has fully developed (α=1) otherwise it does not occur. This process is naturally irrecoverable. These processes only occur when the temperature is 0..30 °C and the relative humidity is 95% or above. Outside these condition bounds, the activation process may recover, but the mass loss process is simply stopped. The activation process is as given in Equation 2. The recovery time (i.e. α recovers from a value of 1 back to 0) is assumed to be 17520 hours (2 years). Recovery takes place when the conditions are outside the bounds of the decay growth. The model can be used for evaluation the exposure condition for the eventual risk of decay to develop. For example, recorded temperatures and relative humidity are given for the Helsinki area. This climate is shown in the figure 1 for a one year period. According to the model, this climate seems to induce a low mass loss of 1.1 % in 4 years (Figures 4 and 5).
During the first year, no decay development will occur in untreated pine sapwood. After 3 and 4 years exposure, decay is expected to occur only to a very limited extent in the surface of unprotected pine sapwood. Under normal use conditions, the cladding is protected by paints or other coatings. The direct influence of water on the wood surface is very small, and decay development will be significantly retarded or even negligible.
The mass loss process proceeds the activation process, when α has reached 1(Eq. 11).
For advanced decay to develop, a significantly longer period is needed, and after a 10 years period, severe decay in unprotected and uncovered pine sapwood can be expected in the Helsinki area. The design of details has a strongly marked effect on the durability and www.intechopen.com service life of wood structure. If there is a detail collecting the water, the moisture conditions are suitable for long time for decay to develop. If the structure and details are well planned so that there is no water sink and the structure can be dried after occasionally wetting, the conditions for decay development will not be reached, and there are actually no limits for the service life of wood.

Evaluation of decay risk in different part of Europe using decay model
The empirical wood decay model was run using the ERA-40 data for air temperature, humidity and precipitation at 6 hour intervals (Viitanen et al 2010b). ERA-40 is a massive data archive produced by the European Centre of Medium-Range Weather Forecasts (ECMWF). The reanalysis involves a comprehensive use of a wide range of observational systems including, of course, the basic synoptic surface weather measurements. The ERA-40 domain covers all of Europe and has a grid spacing of approximately 270 km. The nature of the data and the reanalysis methods of ERA-40 are described in detail in Uppala et al. [2005]. The evaluation of decay development in the model is based on the mass loss caused by the decay fungus. Within specified limitations, the mass loss is an applicable variable for evaluating the decay development in wood. The decay development model will give a general assumption of the effect of humidity, temperature and exposure time on the start and progress of the decay. The resulting modelled mass loss in 1961-1970 at the calculation points of the ERA-40 grid were analyzed by a chart production software producing a maps of wood decay in Europe (Figure 7). First a map on decay risk protected from rain and then a map on decay risk of pine sapwood exposed to rain. A modification of the weather data was made so that the humidity of air was set to 100% during precipitation (at non-freezing temperatures) as this is thought to result in a full saturation in the wood surface.   Fig. 6. No activation of growth or decay development during the first and second years, an activation of decay process after 4 years exposure may be expected (Viitanen et al 2010b) Fig. 7. a) Modelled mass loss (in %) of small pieces of pine wood that are protected from the rain or b) exposed to rain in 10 years in Europe (from [Viitanen et al. 2010b[Viitanen et al. , 2011b The risk of decay activity in different part of Europe can be evaluated on the map. If we evaluate the decay activity rate in Helsinki to be 1, then the decay risk in north-western part of Portugal and in West Ireland is 2 times and in Atlantic part of France and Belgium it will www.intechopen.com be between 2 and 2.5 times higher than that in Helsinki. In North Scandinavia it would be between 0.5 and 0.25, which will point out, the effect if climate on risk of decay development in outdoor structure varied vide within Europe. These coefficients can be used as one step to evaluate the effect of macroclimate conditions on service life of cladding and decking. Another way to evaluate the macroclimate conditions is presented by Thelandersson et al (2011) using Meteonorm climate data. By calculating the daily dose and accumulating the dose for one year a measure of the risk of decay is obtained. This is made for several sites, and the result in terms of dosedays can be compared between the different sites. To be able to compare different sites, the dose was transferred to a relative dose by dividing it by the dose for the "base-station" Helsinki. Due to the variation of climate across Europe, relative doses between 0.6 (northern Scandinavia) and 2.1 (Atlantic coast in Southern Europe) were obtained.

Conclusion
There are several factors involved with the bio-deterioration of materials and buildings, and mathematic modelling that may help us to understand the complicated interaction of many factors. The presented numerical mould growth and decay development models are based on experimental results from several research projects. It is suitable for post-processing temperature and humidity data from any numerical simulation of hygrothermal conditions i n b u i l d i n g c o n s t r u c t i o n s . H o w e v e r , i t m u s t b e k e p t i n m i n d w h e n p e r f o r m i n g t h e assessment that there is a great uncertainty coupled to this kind of analysis: the variation of the material sensitivities is high, estimation of a product sensitivity class is difficult without testing, the surface treatments may enhance or reduce growth potential, different mould species have different requirements for growth and the evaluation of the actual conditions in the critical material layers may include uncertainties. The best way to use the predicted mould growth and decay development as an assessment tool is to compare different solutions with each others: The solution with the lowest risk for the mould growth or decay development would most probably also have least other moisture related problems

Acknowledgment
This chapter is based on several research performed at VTT and other research institutes and universities.