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Cost Action E37 Final Conference in Bordeaux 2008 Socio-economic perspectives of treated wood for the common European market

Service life of wooden materials – mathematical modelling as a tool for evaluating the development of mould and decay Hannu Viitanen1, Ruut Peuhkuri2, Tuomo Ojanen3, Tomi Toratti4, Lasse Makkonen5 1

VTT Technical Research Centre of Finland, PB 1000, FIN-02044 VTT, Finland e-mail: [email protected] 2

e-mail: [email protected]

3

e-mail: [email protected] 4

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e-mail: [email protected]

e-mail: [email protected]

Keywords: Wood durability, service life modelling, climatic loading, wood decay, mould

ABSTRACT Durability is defined "The ability of a product to maintain its required performance over a given time, under the influence of foreseeable actions, subject to normal maintenance". During their functional life, building components are exposed to moisture and other environment hazards in numerous ways, a fact that must be taken into account during manufacture and product development. For mould and decay development, different mathematical modelling exists based on laboratory and field studies. These can be used also for evaluating the different material properties for durability and service life of wooden products. In the future, the life time expectations and analyses of different building products will need more data on the durability of products, service life and resistance against mould and decay, not only data on wood material itself. The mould and decay models can be incorporated with climatic and building physic models to evaluate the effect of different exposure conditions on the durability and service life of wooden products. This paper presents state-of-art on modelling mould and decay of wooden structures exposed to climatic conditions. INTRODUCTION Service life means the period of time after installation during which a building or its parts meets or exceeds the performance requirements (ISO 15686-1, 2007). The long term durability of building structures depend typically on several factors, but the effect of excess moisture loads that, in combination with temperature conditions, may cause deterioration of materials and changes in their performance properties. Time of wetness is a useful factor when evaluating e.g. risks for corrosion of steel structures or mould growth on materials, but it alone does not give adequate information about the durability risks for organic, wooden materials. Long period, high moisture levels may start biological growth on timber surfaces, first mould or stain fungi and finally decay. When durability comprises only the decrease in structural strength, the decay of timber is the main factor to be considered in wooden structures. A very simple method to evaluate the damage or failure risk in materials and structures is to measure and calculate the moisture or humidity level of the materials for a longer period. The 85

Cost Action E37 Final Conference in Bordeaux 2008 Socio-economic perspectives of treated wood for the common European market

time of wetness in the exterior climate, however, does not necessary correspond with the time of wetness in different parts of the building envelope. The microclimatic conditions are the acting force for biological deterioration of materials. Microclimate is a result of several simultaneous factors: macroclimate (rainfall, temperature, humidity, air pressure conditions etc.), and mesoclimate (location of the building, structural details and the materials used). There are so many factors that mathematical models are needed to handle the complicated relations. The ISO factor method includes several general factors in order to evaluate the service life of building components. Most often the factors should be based on more detailed information to give a realistic view on the durability and service life. The life time expectations and analyses of different building products will need more data on the durability of products, service life and resistance against mould and decay, not only data on wood material itself. Moisture stress is partly due to the environment, weather and service conditions, and partly to moisture damage. Factors affecting the durability of wood also include the composition and structural properties of the material and other exposure conditions. The complicated interaction of different factors may be analysed using different mathematical models. PARAMETERS NEEDED FOR BIOLOGICAL DURABILITY AND SERVICE LIFE Exposure conditions and performance criteria Service life means the period of time after installation during which a building or its parts meets or exceeds the performance requirements (ISO 15686). The type of data which is needed for durability and service life evaluation depends on the type of degradation mechanism considered. Since moisture and temperature are generally very important factors, we will need climate data and exposure conditions usually at the boundary of the building component. This is usually termed "microclimate", which depends on a lot of factors. The starting point here is the meteorological data defining the “regional climate” in the area where the building is situated: temperature, relative humidity, solar radiation, rain and wind intensity and duration. The next step is to define the “local climate”, i.e. the climate conditions close to the building but still undisturbed by the properties of the wood material and the shape of the structure. Ambient microclimatic conditions, especially moisture conditions, are the most important factors for durability of wood and the classification of use conditions is based on the evaluation of the water exposure during the use (like EN 335-1). There are several hygrothermal performance criteria for structures. Suitable criteria depend on materials and deterioration aspect, e.g. structural strength and durability, service life time, aesthetics, effect on indoor air, etc. One of the first biological signs of ageing is mould growth that does not affect durability as such but can cause discoloration and health problems. Discoloration is often connected also to paint and surface treatment properties. Decay is the more severe result of high moisture exposure of wooden structures when the materials are wet for long periods. Performance requirement means the minimum acceptable level of a critical property, which can be defined as limit states. This is a more or less precise definition of the limit between acceptable performance and non-acceptable performance. An example of limit states is onset of mould growth in inside surface of the building envelope, which can be regarded as non-acceptable since it may create health problems in a building and especially in indoor air. Mould growth can be evaluated using different methods: detecting the mould particles or mycelium in the materials or in indoor air or indirect evaluation mould growth using chemical analyses. The mould growth

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index is a general way to evaluate the performance of different materials under accelerated tests. These indexes could also give an evaluation on the mould attack in the real conditions. The limit states for decay can be formulated in the same way as for mechanically loaded structures considering that the capacity of the structure is reduced with time. Critical mass loss caused by fungi depends on the location of the decay and the structural properties. The evaluation of decay development is often 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 correlation between mass loss and critical parameters of wood, e.g. modulus of rupture (MOR) and strength, is high within the same wood species especially within higher mass loss values. 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 Material and structural properties Natural durability of wood is an important factor for durability and service life. EN 350-2 classifies the resistance of several wood species against decay and insects based on the evaluation in ground contact. The classification is based on heartwood only under the worst case scenario. During the use of wood, however, these conditions exist within the damage situation or use class 4 situation (wood in ground contact and continually wet). Most of wooden building products are not intended to tolerate damage conditions. There is lot of results on different treated wood products tested in the worst case condition. Combining the results to decay and hygrothermal models, an evaluation of the required durability and service life may be possible. It is also obvious, that the natural durability of wood material will not be the critical value for the durability and service life of coated plywood products used in above ground conditions (Viitanen et al. 2008). To achieve the required durability and service life, wood modification, impregnation and surface treatments are potential ways to be used, depending on the targeted use conditions. One of the most important functions of a coating for exterior use is the protection against liquid water penetration and water vapour. The degree of reduction of moisture sorption depends on the coating type. The influence of the wood species for liquid water absorption is considerable for low build coatings whereas for high build coatings with sufficient film thickness the wood species is of minor importance. The moisture properties of coated wood may change during ageing. In addition the colour of the pigment will have influence on the moisture contents. Dark colour absorbs more energy and the wood will dry faster than wood coated with coating having a lighter pigment. Structural parameters, like thickness and width of the panel, fixing (nailing) of the panels and position and contact of the panels in the structure are also needed for the practical evaluation of the durability of wood commodity. The design factors like the height of the wall, compass direction and width of eaves are also affecting the exposure condition of the wood in structure. Climatic exposure conditions connected to other environmental factors are the basic source for exposure evaluation. Humidity and temperature conditions and the exposure time of microclimate is the basic factor affecting the durability and biodeterioration of wooden products.

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MODELLING Data generated using laboratory work and field tests For generating critical humidity and temperature conditions needed for mould and decay development, first step is using laboratory conditions. There is always a wide variation within the growth conditions of different fungus species, and we need an overall evaluation of the growth activity and decay development of a “typical” example fungi (mixture of mould/bluestain fungi) or typical decay fungi (e.g. Coniophora puteana or Gloeophyllum sepiarium). There are different types of models concerning the development of biodeterioration, mould growth or decay development. Some of the models are based on the laboratory studies, some on the field studies and some on the hygrothermal properties of materials (Fig. 1). The first isoplets or models were based on the laboratory studies on agar culture using different mould and decay fungi (Ayerst 1969, Block 1953, Smith and Hill 1982, Grant et al. 1989). On the base of results, different functions or s.c. isoplets on critical humidity and temperature levels were developed. Later also time-of-wetness and varied humidity and temperature conditions were analysed (Adan 1994, Viitanen 1996). The TOW is defined by the ratio of the cyclic wet period (RH>= 80 %) and the cyclic dry period. The hygrothermal models are based on osmotic potentials for the spore germination and start of the mould growth in different materials (Sedlbauer 2001). The models developed by Hukka and Viitanen (1997) and Viitanen et al. (2000) are based on laboratory work using Scots pine sapwood exposed to mould fungi and decay fungi under different humidity and temperature conditions for varied periods. The basic regression models are general isoplets for mould growth and decay development in untreated pine sapwood Laboratory studies mould / cecay / insect Isoplets on mould and decay development Critical humidity / moisture conditions and temperature for organisms to develope - effect of exposure conditions - material parameters - sample size - treatments

Field studies Performance of different material under varied natural conditions Most often high moisture exposure using critical details - material parameters - effect of exposure conditions and climate - sample size and form - treatments

Modelling Data from laboratory and field studies Humidity, temperature, exposure time, exposure conditions, materials, treatments

Exposure under normal use conditions Conditions in different part of the building Effect of environmental factors, climate, other buildings, materials, details, etc.

Figure 1. Use of data from tests to model the durability of materials under different exposure conditions.

Mould growth and decay development are separated processes and also the models are different. For mould development, the ambient critical humidity level of microclimate is between RH 80 and 95 % (Fig. 2a) and for decay development between RH 95 – 100 % (Fig. 2b). For the mould 88

Cost Action E37 Final Conference in Bordeaux 2008 Socio-economic perspectives of treated wood for the common European market

growth model, also varied cyclic conditions can be taken under consideration. For decay development, the model is based on constant conditions. According to experience, the decay will develop when moisture content of wood excess the fibre saturation point (RH above 99.9 % or wood moisture content 30 %. Morris et al. (2006) have modelled decay development in wooden sheathing and found the critical ambient humidity condition for decay development is around RH 98 - 99 %, depending on the temperature and exposure time. 100 100

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Figure 2: RH and temperature isoplets as a function of time for start of mould growth (left) and early stage of decay development (right) in pine sapwood (Viitanen 1996, 1997a,b).

When analyzing and comparing the results from different tests, dose-response functions can be used to evaluate the implication of different aging factors. In Fig. 3 an evaluation model of development of decay in pine sapwood as mass loss of wood relation with accelerated test under “worst case scenario” and exposure periods of high humidity conditions of microclimate in different temperature is shown. Temperature has a significant effect on the decay development, especially at lower humidity conditions. Pine sapwood, RH 97 % 80

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Figure 3: Development of decay of Coniophora puteana in accelerated decay test (EN 113) and at RH 100 % (left) and RH 95 % (right) under different temperature. Development of decay is evaluated using mass loss value caused by the fungus in Pine sapwood. The modelled results are based on laboratory tests (Viitanen 1996).

The models on biodeterioration can be used as a tool for building physic performance and service life evaluation. Sedlbauer (2001) has 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. This model has been incorporated in a hygrothermal calculation tool Wufi (Wufi). Ojanen and Salonvaara (2000) have used the “VTT mould growth model” implemented in another building physic simulation model TCCC2D for evaluate the risk of mould growth in different humidity exposure conditions in building envelope. Isaksson (2008) has presented the state of art situation on methods to predict wood durability.

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Cost Action E37 Final Conference in Bordeaux 2008 Socio-economic perspectives of treated wood for the common European market

For using the biodeterioration models, the limit state or mass loss level has first to be defined. The tests have been performed with small samples, and a sample size correction factor may be needed. Using the basic experimental data, a second version of the model on decay development was generated by Viitanen et al. (2008). This model is a time stepping scheme. The development of decay is modelled with two 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 under unfavourable conditions for the decay process (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 0C and the relative humidity is 95% or above. Outside these condition bounds, the activation process may recover, while the mass loss process is simply stopped. Using results of field tests, another type of models were generated by Australian scientists (Leicester et al. 2002, Mackenzie et al. 2007, Wang et al. 2008). The results of L-joints tests, coated and uncoated wood samples performed in different district of Australia were used as a basic data connected to climatic data. The data (mass loss and decay of samples) was used as a base for modelling decay development under different climatic conditions. The decay model is based on the lag-phase and active decay developing phase. Also a software for calculating the service life in different exposure conditions was developed (htpp://www.timber.org.au). The decay development above ground has been modelled (Wang et al. 2008). Different parameters include: prediction model, decay rate and parameters for wood, climate, paint, thickness, connection and geometry of the wood structure. Evaluation of decay development in different climatic exposure conditions In the USA, the Scheffer (1971) index is developed to evaluate the decay risk in different parts of the USA based on temperature and distribution of rainfall. In Australia the decay development in above ground in different climatic conditions is modelled and also a software for calculating the decay risk is developed (Wang et al. 2008). The climatic data used for the modelling (Wang and Leicester 2008) were: annual rainfall (mm/year), number of dry months (months/year), number of rain days (days/year), time of wetness (hours/years), dry-bulb temperature (°C), wetbulb temperature (°C), wind speed (km/hr) and wind direction (degrees from the north). Also the data on decay in ground contact and termite attack connected to climatic data is used for modelling (Leicester et al. 2003a,b). In Europe, Brischke and Rapp (2005, 2007) and Brischke (2007) have evaluated the effect of different test conditions on the decay development in different wood species. In sapwood and heartwood, the decay development was different depending on the exposure conditions, and the average weather data was not sufficient for estimation decay development. Brischke and Rapp proposed a roadmap to specify the performance of wood durability, and a COST decay index has been started to develop. An example of the European work is the on-going Woodexter project, where a new format of the basic decay model according to Viitanen (1997) was developed (Viitanen et al. 2009). The 90

Cost Action E37 Final Conference in Bordeaux 2008 Socio-economic perspectives of treated wood for the common European market

empirical wood decay model can be used for the ERA-40 data for air temperature, humidity and precipitation at 6 hour intervals. 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 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 (Fig. 4). In these calculations, the α–factor of the empirical wood decay model was reduced during nondecay periods by the rate that corresponds to the recovery time of two years. In the first simulation, the calculation was based on the relative humidity and temperature in air only (Fig. 4a) and in the second simulation, a modification was made so that the humidity of air was set to 100% during precipitation, at non-freezing temperatures (Fig. 4b). The maps are based on different data than that generated in Australia, and the threshold level may be different. However, they will give evaluation on the effect climatic conditions on the decay development in different geographical area.

Figure 4: Modelled mass loss (in %) of small pieces of pine wood that are protected from rain (left) or exposed to rain (right) in 10 years in Europe (according to Viitanen et al 2009).

The resistance of wood species and coated wood products are often evaluated using accelerated tests simulating the “worst case scenario”. When comparing the test results between treated and untreated pine sapwood, evaluation of the resistance of wood products under different actual conditions can be performed using the decay development model modified with this “resistance factor” (Brischke 2007).

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Cost Action E37 Final Conference in Bordeaux 2008 Socio-economic perspectives of treated wood for the common European market

Modelling the hygrothermal conditions of a façade and decay development The developed mathematical model for decay is suitable to be used to post-process the results of hygrothermal dynamic simulations of building components. As example a well insulated cavity wall with untreated wooden exterior sheating is used (Fig. 5). The decay (mass loss) of exterior surface is in focus. The geographical location and orientation, together with different assumptions for driving range can be used as variables. The simulations are performed with a hygrothermal simulation program Wufi 4.1, which solves the dynamic temperature and moisture conditions in a construction and can e.g. take into account the amount of water absorbed to construction due to the driving rain. The driving rain load Sd, is calculated as in Eq 1: S d = s ⋅ (R1 + R2 ⋅ v )

(1)

where s is free rain, v is wind velocity in orthogonal direction to surface. R1 and R2 are coefficients, e.g. R1 = 0 for vertical facades and R2 = 0,2 for in-disturbed rain (Wufi).

Figure 5: A cross section of a well-insulated (320mm) cavity wall with ventilated air cap (30mm). Exterior climate is on the left hand side.

The case is studied for west oriented wall in 3 very different locations in Europe: Grenoble, Bergen and Sodankylä. The impact of the driving rain is studied by assuming either free driving rain (R2 = 0,2) on façade or reduced driving rain (R2 = 0,07) on the centre of the facade due to the pressure field around a typical building. Following results show some examples on the output and the use of the developed model for decay in assessment of the building structures. Figure 6 shows the impact of different climates on the decay development. The mass loss is much higher in a humid location as Bergen than in Central France. A cold location in Northern Finland gives no decay during 3 years. Other parameters When applying the models for evaluation of service life in certain microclimatic conditions, the great natural variability of materials, structures, different treatments and organisms should also be taken into consideration. Different type of microbial growth will be found on stone based material and on insulation material than that on wood material. The ageing of material and accumulation of dust and other material on the surface of building material will change the response of the material to moisture and biological processes.

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The susceptibility of different materials to biological attack mainly depends upon the water activity and nutrient content of the substrate. During manufacturing of different wooden products, the properties of materials and also the equilibrium moisture content (EMC) can be changed. For example the EMC of particleboard is lower than the EMC of solid wood. Norway spruce sapwood has often proved to be a less susceptible to mould than Scots pine sapwood. It has been shown, that after fast kiln drying, the amounts of nitrogen and low-molecular hydrocarbon compounds on the surface layers of sawn sapwood timber can be higher than inside the wood and this may promote the mould growth (Theander et al. 1993). Heartwood of several wood species is often more resistant than sapwood (Van Acker et al. 1999). In buildings, the material is most often coated, treated or painted with different products and treatments. In such cases, the surface treatment has important role for the durability and life time of substrate. In the present stage, the durability of different wood products is not well defined. In the model of Wang and Leicester (2008) several different factors of wood durability has been included. Several different tests have been performed, but modelling of the durability and service life of wood products in different end use condition is still partly an open question.

CONCLUSIONS The developed mould and decay models are tools for risk assessment and service life evaluation. Different decay models are suitable for durability and service life time analysis of wooden structures. Other hygrothermal performance criteria can be used for lower humidity conditions; i.e. for mould growth etc. Other materials have different deterioration phenomena, which should be taken into account.

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REFERENCES Adan, O.C.G. (1994) On the fungal defacement of interior finishes. Eindhoven University of Technology. Thesis. Eindhoven. pp 83 – 185. Ayerst, G. (1969) The effects of moisture and temperature on growth and spore germination in some fungi. J. Stored Prod. Res. 5:127-141. Block, S.S. (1953) Humidity requirements for mould growth. Applied Microbiology 1(6), 287-293. Brischke, C. (2007) Investigation of decay influencing factors for service life prediction of exposed wooden components. Dissertation. In German. University of Hamburg. 321 p. Brischke, C. and Rapp, AO. (2005) Relation between lab tests, field tests, and in service performance and their contribution to SLP. Proceedings of COST E37 Workshop, Olso, Norway. Brischke, C. and Rapp, AO. (2007) Dose-response relationships for service life prediction of wood. Wood Science and Technology (In Brischke C. 2007). EN 113 (1997) Wood preservatives – The method for determining the protective effectiveness against wood destro¬ying Basidiomycetes. Determination of toxic values. CEN. European committee for standardization. EN 335-1 (2006) Durability of wood and wood based products – Definition of use classes – Part 1: General. CEN. European committee for standardization EN 350-2 (1994) Durability of wood and wood based products. Part 2: Guide to natural durability and treatability of selected wood species ot importance of Europe. CEN. European committee for standardization Grant, C., Hunter, C.A., Flannigan, B. and Bravery, A.F. (1989) The moisture requirements of moulds isolated from domestic dwellings. Internat. Biodet. 25, 259-284. Hukka A. & Viitanen H. (1999) A mathematical model of mould growth on wooden material. Wood Science and Technology 33(6), 475-485. Isaksson, T. (2008) Methods for predicting durability of wood. State of the art. Draft report. TVBK -30XX. Division of Structural Engineering. Lunds Institute of Technology, Lund University. ISO 15686-1 Building and construction assets – Service life planning – Part 1: General principles. Leicester, R.H., Wang, C-H. and Cookson, L.J. (2003) A risk model for termite attack in Australia. Document No IRGWP 03-10468. 15 p. Stockholm, International Research Group on Wood Protection.

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Leicester, R.H., Wang, C-H. Ngyen M.N., Thornton, J.D., Johnson G., Gardner, D., Foliente G.C., and MacKenzie C. (2003) An engineering model for the decay in timber in ground contact. Document No IRGWP 03-20260. 21 p. Stockholm, International Research Group on Wood Protection. Mackenzie, C.E., Wang, C-H., Leicester, R.H., Foliente, G.C. and Ngyen, M.N. (2007) Timber service life design guide. Report Project No PN07.1052. Forest and Wood Products Australia Limited. 109 p. Morris, P., Symons P, and Clark, J. (2006) Resistance of wood sheating to decay. Wood protection 2006. March 21-23, 2006. New Orleans, Lousiana, USA. Ojanen, T and Salonvaara, M. (2000) Numerical simulation of mould growth in timber frame walls. Healthy Buildings. Espoo, 6 - 10 Aug. 2000. Seppänen, O. & Säteri, J. (ed). Vol. 1. FiSIAQ. Scheffer, TC. (1971) A climate index for estimating potential for decay in wood structure above ground. Forest Products Journal 21, 25-31. Sedlbauer K. (2001) Prediction of mould fungus formation on the surface of/and inside building components. University of Stuttgart, Fraunhofer Institute for building Physics, Doctoral thesis. Stuttgart. Germany. Smith, S.L. and Hill, S.T. (1982) Influence of temperature and water activity on germination and growth of Aspergillus restrictus and Aspergillus versicolor. Transactions of the British Mycological Society Vol 79. H 3 p. 558 – 560. Theander, O., Bjurman, J and Boutelje, J. (1993) Increase in the content of low-molecular carbofydrates at lumber surfaces during drying and correlation with nitrogen content, yellowing and mould growth. Wood Science and Technology 27, 381-389. Uppala, S.M., Kållberg, P.W., Simmons, A.J., Andrae, U., da Costa Bechtold, V., Fiorino, M., Gibson, J.K., Haseler, J., Hernandez, A., Kelly, G.A., Li, X., Onogi, K., Saarinen, S., Sokka, N., Allan, R.P., Andersson, E., Arpe, K., Balmaseda, M.A., Beljaars, A.C.M., van de Berg, L., Bidlot, J., Bormann, N., Caires, S., Chevallier, F., Dethof, A., Dragosavac, M., Fisher, M., Fuentes, M., Hagemann, S., Hólm, E., Hoskins, B.J., Isaksen, L., Janssen, P.A.E.M., Jenne, R., McNally, A.P., Mahfouf, J.-F., Morcrette, J.-J., Rayner, N.A., Saunders, R.W., Simon, P., Sterl, A., Trenberth, K.E., Untch, A., Vasiljevic, D., Viterbo, P., and Woollen, J. (2005) The ERA-40 re-analysis. Quarterly Journal of the Royal Meteoroogical Society, 131, 2961-3012. Van Acker, J. Militz, H. and Stevens, M. (1999) The significance of accelerated laboratory testing methods determining the natural durability of wood. Holzforschung 53(5), 449-458. Viitanen, H. (1996) Factors affecting the development of mould and brown rot decay in wooden material and wooden structures. Effect of humidity, temperature and exposure time. Doctoral Thesis. Uppsala. The Swedish University of Agricultural Sciences, Department of Forest Products. 58 p.

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Viitanen, H. (1997a) Modelling the time factor in the development of mould fungi in wood - the effect of critical humidity and temperature conditions. Holzforschung 51 (1), 6-14 Viitanen, H. (1997b) Critical time of different humidity and temperature conditions for the development of brown rot decay in pine and spruce. Holzforschung 51 (2), 99-106. Viitanen H., Hanhijärvi A., Hukka A. & Koskela K. (2000) Modelling mould growth and decay damages Healthy Buildings. Espoo, 6 - 10 August 2000. Vol. 3. FISIAQ, 2000, p. 341–346. Viitanen, H., Ritschkoff, A-C, Ojanen, T., Salonvaara, M. (2003) Moisture conditions and biodeterioration risk of building materials and structure. Proceedings of the 2nd International Symbosium ILCDES 2003. Integrated Lifetime Engineering of Buildings and Civil Infrastructures, Kuopio, 1-3 Dec. 2003 RIL, VTT, RILEM, IABSE, ECCE, ASCE. Espoo , 151 – 156. Viitanen, H., Suttie, E., Van Acker, J., Militz, H., Bollmus, S., Thelandersson, S., Jermer, J, Bengtsson, C., Gruell, G., Heisel, E. (2007) Feasibility Study on Wood Durability and Performance Service Life. CEI-Bois. Viitanen, H., Toratti, T., Peuhkuri, R., Ojanen, T. and Makkonen, L. (2008) Woodexter, WP1: Exposure conditions. Draft report. Viitanen, H., Toratti, T., Peuhkuri, R, Makkonen, L, Ojanen, T., Saila, J., Ruokolainen, L. and Räisänen, J. (2009) Modelling the durability of wooden structures. Proceedings to 4th International Building Physics Conference. Istanbul. June 2009. Viitanen, H., Nurmi, A., Metsä-Kortelainen, S., Jämsä, S and Paajanen, L. (2008) Effect of coatings on the performance and durability of birch plywood – Results after outdoor weathering and accelerated decay resistance assessment. IPPS conference 24.-26. September, Helsinki 2008. Wang, C.H., Leicester, R.H. and Ngyen, M.N. (2008) Timber Service life design. Manual No 4. Decay above ground. CSIRO. 109 p. Wang, C-H. and Leicester, R.H. (2008) Timber Service life design. Manual No 1. Processed climate data for timber service life prediction modelling. CSIRO. 144 p. Wufi (Wärme und Feuchte instationär - Transient Heat and Moisture) 4.1 Pro software, The Fraunhofer Institute for Building Physics IBP.

ACKNOWLEDGEMENTS The authors gratefully acknowledge the financial support of WoodWisdom-Net and the wood industry partnership Building with Wood for funding. The “WoodExter” project partners are thanked for their collaboration in this project. The ERA-40 data have been provided by ECMWF.

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