A MATHEMATICAL MODELING OF MOISTURE BEHAVIOUR AND MOULD GROWTH IN BUILDING ENVELOPES Kati Salminen, M. Sc. Student 1) Juha Vinha, Lic.Tech. 1) Hannu Viitanen, Ph.D. 2) 1) Laboratory of Structural Engineering, Tampere University of Technology, Finland 2) Building and Transport, VTT/ Technical Research Centre of Finland, Finland ABSRACT This paper deals with the research project a Mathematical modeling of moisture behaviour and mould growth in building envelopes. The research project will be done in co-operation between Tampere University of Technology and Technical Research Centre of Finland. A mathematical model of mould growth on wooden material has been created by Ph.D. Hannu Viitanen. One objective in this research is to test usage of the model by doing experiments for exterior wall structures and materials in laboratory and in field conditions. This paper concentrates on the experiments of the research. INTRODUCTION Tampere University of Technology and Technical Research Centre of Finland have started a study A mathematical modeling of moisture behaviour and mould growth in building envelopes. The research project is funded by Tekes (The National Technology Agency of Finland) and eight Finnish companies and associations. The research has started on 1 st May 2005 and it will end on 30 th August 2008. Mathematical modeling of mould growth has been researched in the VTT Technical Research Centre of Finland for several years. The recent model of mould growth consists of a mathematical basic model that takes into account the delay and influence of fluctuating humidity conditions. In addition to wood, the model also contains correction coefficients for some other materials. In this research, laboratory and field experiments will be also done for seven construction materials other than wood. Future research is needed to reduce some limitations and unpunctuality in the recent model. The improved calculation model will be an effective tool for analysing the behaviour of different types of structures. OBJECTIVES The objectives of the research are: - to develop reliability and range of use of the recent mathematical model in fluctuating temperature and humidity conditions - to increase the number of material choices for the model - to test usage of the model by doing experiments for structures and materials in laboratory and in field conditions
- to estimate usability of the model in calculations of the durability of structures - to formulate more diversified application of the mathematical model that construction companies can use in the future BACKGROUND Ph.D. Hannu Viitanen has created regression models for a mathematical model on simulation of growth of fungi on wooden material in his dissertation (Viitanen 1996). The mathematical model was generated by Hukka and Viitanen (1999) for calculating the development of mould growth, which is expressed as mould index. The conditions favourable for initiation and temperature dependent critical relative humidity are shown in figure 1. Figure 1 Conditions favourable for initiation of mould growth on wooden material (left) and temperature dependent critical relative humidity needed for mould growth at different values of mould index (right) after Hukka and Viitanen (1999). The mathematical model of mould growth is based on the regression model for mould growth. In figure 2 predicted mould growth is expressed with a characteristic curve as a function of time (Viitanen et al 2000). Mould index 6 5 4 3 2 1 0 5 C RH 100 % RH 97 % RH 90 % RH 80 % 0 28 56 84 112 140 168 Time (days) 6 5 4 3 2 1 0 40 C RH 100 % RH 97 % RH 90 % RH 80 % 0 28 56 84 112 140 168 Time (days) Figure 2 The predicted mould growth at +5 and 40 C in different humidity conditions (Viitanen et al 2000) Mould growth on materials is detected using microscopy or visually and expressed as mould indexes developed by Viitanen (1996): Index for mould growth on materials - 0 = no growth - 1 = some growth (microscopy) - 2 = moderate growth (microscopy) (coverage > 10 %)
- 3 = some growth (visually detected) - 4 = visual coverage > 10 % - 5 = coverage > 50 % - 6 = tight coverage 100 % The mould growth model is combined with building physic models for analysing the critical humidity and temperature conditions (Viitanen and Salonvaara 2001, Viitanen et al 2003) EXPERIMENTS In earlier studies the measurements were performed in different static and fluctuating conditions and the test material was wood. In this research the experiments for materials and structures will be performed in laboratory and in field conditions. Seven different construction materials, four insulation materials and three stone materials, will be measured. Insulation materials are polyurethane, glass wool, expanded polystyrene and polyester wool. Stone materials are concrete, light concrete and expanded clay aggregate concrete. The reference material is wood. MATERIAL EXPERIMENTS IN LABORATORY CONDITIONS The material experiments in laboratory will be performed at + 22 C, + 4-5 C and at temperatures below zero. The specimens will be placed in climate chambers. The certain relative humidity will be obtained with using the saturated salt solution. The material experiments performed at + 22 C - specimens from three production runs - four parallel specimens from each production run - one test series of stone material will be wet - specimen s edges will be treated with mould suspension - the duration of experiments two years - three different test conditions: 1) 97 %-RH/+ 22 C 2) 90 %-RH/+ 22 C 3) cyclical conditions: 97 %-RH/+ 22 C and 50 %-RH/+ 22 C o only for wooden material o two test series with four parallel specimen o duration of 97 %-RH/+ 22 C period two weeks o duration of 50 %-RH/+ 22 C period one week or eight weeks The material experiments performed at + 4-5 C - relative humidity 98 % - specimens from three production runs - four parallel specimens from each production run - one test series of stone material will be wet - specimen s edges will be treated with mould suspension - the duration of experiments two years
Annex 41 working meeting, October 26-28 2005, Trondheim The material experiments performed below 0 C - only for wooden materials - two test series with four parallel specimen - specimen s edges will be treated with mould suspension - the duration of experiments two years - three different test conditions: 1) cyclical conditions: +22 C/ 97 %-RH and 5 C/ 98 %-RH o duration of 97 %-RH/+ 22 C period two weeks o duration of 98 %-RH/- 5 C period one week or eight weeks 2) cyclical conditions: +22 C/ 97 %-RH and 20 C/ 98 %-RH o duration of 97 %-RH/+ 22 C period two weeks o duration of 98 %-RH/- 5 C period one week or eight weeks 3) statistical conditions: -20 C/ 98 %-RH o in beginning 97 %-RH/+ 22 C period two weeks IN FIELD CONDITIONS In field conditions the specimens will be placed in a shelter (see figure 3), where the specimens will be sheltered against rain and snow. In the shelter the temperature and relative humidity are same as in open-air. The wall structure of the shelter is designed so that air can flow into the shelter but rain and snow cannot wet the specimens. The specimens will be laid down on the steel shelves. The material experiments in the shelter - four test series with four parallel specimens - specimen s edges will be covered with mould suspension - a part of the specimens will be dry - a part of the specimens will be wet - a part of the specimens will be piled up (like in a storage) - control of mould growth every 2-3 months - the duration of the experiments about 2.5 years Figure 3 On the right side is picture of shelter. On the left side is shown the double- structure of exterior wall.
STRUCTURE EXPERIMENTS IN LABORATORY CONDITIONS The experiments will be performed in the weatherization equipment (figure 4). The equipment consists of two chambers, one for modeling inside conditions and the other for modeling outside conditions. In these experiments sprinklers and radiation heaters will not be used. There will be three weather periods; fall, winter and spring. In fall and winter period outer temperature will be constant, but in spring period outer temperature will range between night temperature and day temperature. The duration of a period is one month. Cooling units Heaters Structure to be tested Evaporator INSIDE OUTSIDE Evaporator Sprinklers and radiation heaterd Figure 4 The principle of weatherization equipment The wall-structures will be composed of a frontal element, insulation material and a gypsum board. The objective of structure experiments is to test mould growth on interface of frontal material and insulation material. The relative humidity inside must be high enough so that the intended moisture content in the structure will be achieved. The moisture behaviour of the structure is analysed with a building physical modeling tool. The specimen will be cut from the structure with a bore. Three parallel specimens will be cut from every structure element. The specimen will be composed of frontal material and insulation material.
Annex 41 working meeting, October 26-28 2005, Trondheim IN FIELD CONDITIONS The structure experiments will be performed in the test house on the test field of Tampere University of Technology. Three exterior-wall modules can be installed to the test house. Each module will be divided into 1-4 structure elements. The relative humidity on the interface of frontal material and wind shield board will be detected every 2-3 months. The entire duration of the experiments is about two and a half years. The test house is shown in figure 5. Figure 5 The test house REFERENCES Hukka, A, and Viitanen, H. 1999. A mathematical model of mould growth on wooden material. Wood Science and Technology. 33 (6) 475-485. 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. Dissertation. Uppsala. The Swedish University of Agricultural Sciences, Department of Forest Products. Thesis. 58 p Viitanen, Hannu; Hanhijärvi, Antti; Hukka, Antti; Koskela, Kyösti. 2000. Modelling mould growth and decay damages Healthy Buildings 2000: Design and Operation of HVAC Proceedings. Espoo, 6-10 August 2000. Vol. 3. SIY Indoor Air Information Oy (2000), s. 341-346 Viitanen, H and Salonvaara, M. 2001. Failure criteria. In Trechsel, H. ed. Moisture analysis and condensation control in building envelopes, MNL40. ASTM USA. pp. 66-80 Viitanen H, Ritschkoff A-C, Ojanen T and Salonvaara M. Moisture conditions and biodeterioration risk of building materials and structure. Proc. 2nd Int. Symp. ILCDES 2003 Integrated Life-time Engineering of Buildings and Civil Infrastructures. Kuopio, 1-3. 12. 2003