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Indoor Air Quality in a Wooden House Mikael Salonvaara*, MSc Research Scientist [email protected] Tuomo Ojanen, MSc Senior Research Scientist * VTT Building and Transport PB 1806, FIN 02044 VTT Mikael’s expertise is in heat, air, moisture and pollutant transport in building structures and in whole buildings. He has developed and used simulation models to analyse the hygrothermal performance of building envelope parts in interaction with indoor climate. Carey Simonson, PhD Associate Professor University of Saskatchewan, Saskatoon, Canada Summary This paper contains a short summary of laboratory, field and numerical studies of the indoor temperature, humidity, comfort and indoor air quality in wooden houses. Wood and wood-based materials have hygroscopic properties that are favourable for producing good indoor climate. The results show that the conditions in a room improve significantly when the hygroscopic materials replace non-hygroscopic materials. Furthermore, the diffusion of gases through a permeable building envelope can significantly increase the effective ventilation rate for poorly ventilated rooms. Hygroscopic buffering may also result in energy savings. International activities are on going to produce standardized properties of materials and products and design principles. Keywords: Indoor air quality, humidity, mass transfer, sorption, hygroscopic materials, tracer gas, ventilation, air change rate 1. Introduction Low levels of indoor pollutants (volatile organic compounds (VOC’s), radon, dust, bacteria, mould, odors, etc.) and acceptable temperature and humidity are important for indoor air quality (IAQ) and occupant health. Since occupants and buildings produce carbon dioxide (CO2), moisture, VOC's and other gases, which affect the quality of indoor air, outdoor ventilation is necessary. Typically outdoor ventilation rates are set with the assumption that the building envelope and furniture are sources of contaminants. However, for carefully selected components, it is possible that the building envelope and furniture can act as contaminant sinks and actually improve the indoor air quality [1]. If a building envelope is made from permeable materials, the diffusion of pollutant gases through the envelope can reduce the indoor concentration of pollutants. Therefore, it may be possible to provide a comparable indoor environment with a lower outdoor ventilation rate when a permeable and hygroscopic building envelope is applied. Also, since the perception of IAQ is closely linked to the humidity of indoor air, moisture transfer between the indoor air and building structures could reduce the needed ventilation rate. Reducing the ventilation rate could have a significant impact on energy consumption because up to 50% of the energy consumed in buildings is used to condition ventilation air [2]. Some research on the interactions between the indoor air and the building envelope has been carried out already decades ago but the phenomena has received increased attention in the past few years. Workshop 10 that was arranged at the Healthy Buildings 2000 conference in Helsinki dealt with the effect of wood based materials on indoor air quality and climate. Experts from many different countries agreed that the moderating effect of wood based materials on the indoor climate is likely to improve human comfort conditions compared to non hygroscopic and vapour retarding envelope materials. It was felt that more research is necessary in order to quantify and optimise the passive indoor climate control. Quite recently new research projects have been started on these issues. The new Nordtest project “Moisture buffering of building materials” is to provide a standardized method to determine dynamic moisture properties of materials and products. The IEA’s (International Energy Agency) new Annex 41 “Whole building hygrothermal performance” involves participants from approximately 20 countries. Meanwhile research on indoor climate of wooden buildings has been ongoing in Finland. The following chapter summarises some of the latest results. 2. Summary of past and current research 2.1 Past research 2.1.1 Tapanila Ecological House [3-8] This research on a single-family house (gross floor area of 237 m2) was carried out at the Technical Research Centre of Finland and sponsored by the Finnish Ministry of the Environment. The wood frame house has no plastic vapour retarder and is insulated with 250 mm and 425 mm of wood fibre insulation in the walls and roof respectively. The measurements conducted in the house from 1999 to 2000 demonstrate that it is possible to design and construct a low-energy house with an airtight and vapour permeable envelope. To realize a moisture physically safe structure, the vapour permeable envelope must be airtight (e.g., 3 ach at 50 Pa) and the water vapour diffusion resistance must be greater on the warm side of the insulation (e.g., 5 times) than on the cold side. Measurement and simulation results show that moisture transfer between indoor air and a porous building envelope can reduce the maximum indoor humidity in the summer by about 20% RH and increase the minimum indoor humidity in the winter by about 10% RH when the ventilation rate is near design (0.5 ach). According to the literature, decreasing the humidity by 20% RH could possibly double the number of occupants satisfied with the indoor thermal comfort and IAQ conditions. 2.1.2 Comfortable Wooden Buildings – Phase I [9-12] This numerical study was conducted at the VTT Building and Transport using the LATENITE simulation program for Wood Focus Oy. The project investigated the effect of wood based materials on indoor climate. The results showed that it is possible to improve indoor humidity conditions when appropriately applying hygroscopic wood based materials. This is important because the literature shows that indoor humidity has a significant effect on occupant comfort, perceived air quality, occupant health, building durability, material emissions and energy consumption. Therefore, it appears possible to improve the quality of life of occupants and the energy consumption of buildings when appropriately applying hygroscopic wood based materials. Meanwhile, the risk of mould growth is low for well-designed structures. The numerical investigation concentrated on a bedroom in a wooden building located in 4 European countries (Finland, Belgium, Germany and Italy). When a well-ventilated bedroom (0.5 ach) is occupied by two adults for 9 hours each night, the indoor humidity is close to the outdoor humidity in the evening and increases during the night. Figure 1 shows the hourly temperature and humidity values for the bedroom located in Espoo. The increase in absolute humidity during the night is quite independent of the climate. However, the level of indoor temperature and humidity are very dependent on the climate. Passive methods of controlling the indoor climate are more successful in moderate climates than in hot and humid climates, even though they provide benefits in all climates. The results show that moisture transfer between indoor air and hygroscopic materials significantly reduces the peak indoor humidity (up to 35% RH) and increases the minimum indoor humidity (up to 15% RH). Based on correlations from the literature, which quantify the effect of temperature and humidity on comfort and perceived air quality for sedentary adults, hygroscopic materials can improve indoor comfort and air quality as well. According to the numerical results, it is expected that people will be more satisfied in a room with hygroscopic structures than in a room with nonhygroscopic structures, especially at the end of occupation (i.e., in the morning for a bedroom). In the morning, an average of 6 more people (out of 100) will be satisfied with the air quality in the hygroscopic bedroom than in the non-hygroscopic bedroom studied in this research. During certain times of the year (mainly summer), as many as 25% more people will be satisfied in the hygroscopic bedroom. hygroscopic non-hygroscopic 16 14 0% 10 RH 10 8 80% 6 6 0% 4 0% 4 2 0% 2 W (g/kg) 12 0 10 15 20 25 30 T (°C) Figure 1. Simulated hourly values of indoor temperature and humidity during the entire year in the hygroscopic and non-hygroscopic case in Espoo (Finland). This research also shows the connection between the moisture removal capacity of ventilation and hygroscopic materials. For example, the increase in humidity during the night was the same in the hygroscopic bedroom with a ventilation rate of 0.1 ach as in the non-hygroscopic bedroom with a ventilation rate of 0.9 ach. Since the indoor conditions at the beginning of occupation are often the same regardless of the night-time ventilation rate (due to airing or air exchange with the rest of the house during the day), a room with a permeable and hygroscopic structure will have a similar moisture performance at a significantly lower ventilation rate than a room with an impermeable or non-hygroscopic structure. This and other results suggest that the ventilation rate could be decreased slightly in a room with hygroscopic materials without degrading the indoor humidity, comfort and air quality conditions. The possible decrease typically ranges from 20% to 50% depending on the variables and criteria chosen. 2.1.3 Comfortable Wooden Buildings – Phase II [13-16] The second phase of the project focused on verifying the results found in phase I. In Phase II VTT, Helsinki University of Technology and Fraunhofer Institute for Building Physics (Holzkirchen, Germany) worked together carrying out detailed material property testing, small- and full-scale laboratory tests, some field tests and numerical analyses. The results of the second phase in the project supported the findings of phase I. Material testing in the laboratory proved the importance of surface treatment of materials on the ability of the material to interact with the indoor climate. Surface treatments or paints that are more permeable that those used today would be needed in order to effectively exploit the phenomena. New simulations show that the moisture safety of building structures can be improved by adding hygroscopic capacity in contact with indoor air. The hygroscopic buffer lowers the maximum humidity in indoor air, which reduces the risk of condensation on or immediately below the surface materials of the structures. Different surface materials were tested in Holzkirchen with constant ventilation rate in the room and the results were compared to those of the reference room. The reference room had plastered and painted surfaces. Wood as a surface material performed even better than expected by the simulations as a moisture buffer dampening the variations in indoor humidity efficiently. The numerical simulations compared well with the experiments and thus allows us to assume that the results of phase I simulations are valid. Preliminary results from field measurements in Germany show that wooden houses with hygroscopic mass or moisture capacity have less varying and more constant indoor humidity throughout the year than houses with less active moisture capacity (e.g., stone houses with painted plaster walls). 3. Conclusions Moisture transfer between indoor air and structures is important and several wood based materials are appropriate. One of the most important findings is that the vapour resistance of the interior coating and the active area are very important and can be used to compensate each other. This means that in new and retrofit buildings it may be possible to apply surface texturing to increase the active area or small but highly active modules. These modules could employ natural or forced convection and could take up a small fraction of the internal surface area of the room, but have an internal surface area that would be comparable to the entire surface area of the room. Since the local airflow and mixing will have a large impact on the performance of very small modules, large rooms with poor mixing may need several modules distributed throughout the room. Moisture is an important comfort and indoor air quality parameter and the comfort and perceived indoor air quality can be improved when applying permeable and hygroscopic materials. It appears possible to provide similar indoor climate and perceived air quality conditions with a smaller ventilation rate when permeable and hygroscopic materials are correctly applied. To quantify the amount that ventilation can be reduced when applying wood based materials in real buildings would be a long and difficult task, but future research in the area of indoor climate and air quality could focus on defining appropriate ways to quantify the effect of humidity on IAQ. It is expected that health affects would create additional arguments for the application of hygroscopic wood based materials. It is known that the durability of building materials and the risk of mould growth are affected by moisture. Results show that it is possible to design a permeable envelope with good moisture performance. In fact a permeable envelope made of wood based materials is less susceptible to condensation and mould growth at the internal surface of thermal bridges because the peak indoor humidity is lower when applying hygroscopic wood based materials. The commercial application and exploitation of the moisture buffering effect requires the introduction of the phenomenon with correct argumentation and solid design methods. Demonstration of the designed performance in real and occupied buildings is still needed. References [1] [2] [3] [4] Salonvaara, Mikael. “Prediction of hygrothermal performance of building envelope parts coupled with indoor climate”. 1998 ASHRAE Annual Meeting. Toronto, CA, 20 - 24 June 1998 CD-ROM: The Transactions CD. Technical and Symposium Papers. American Society of Heating; Refrigerating and Air-Conditioning Engineers (1998). Salonvaara, Mikael; Kokko, Erkki. Heat and mass transfer in cellulose fibre insulation structures. Espoo. VTT, 1999. VTT Tiedotteita - Meddelanden - Research Notes 1946. 51 s. http://www.vtt.fi/inf/pdf/tiedotteet/1999/T1946.pdf VTT Building Technology. Building Physics, Building Services and Fire Technology. In Finnish. Kokko, Erkki; Ojanen, Tuomo; Salonvaara, Mikael; Hukka, Antti; Viitanen, Hannu. Moisture physical behaviour of wooden structures. Espoo. VTT, 1999. VTT Tiedotteita - Meddelanden – Research Notes 1991. 160 s. http://www.vtt.fi/inf/pdf/tiedotteet/1999/T1991.pdf VTT Building Technology. Building Physics, Building Services and Fire Technology. In Finnish. Liddament, M.W. and Orme, M., 1998. Energy and ventilation, Applied Thermal Eng., 18, 11011109. [5] [6] [7] [8] [9] [10] [11] [12] [13] [14] [15] [16] Simonson, C.J., Salonvaara, M. and Ojanen, T., 2001. “Moisture content of indoor air and structures in buildings with vapor permeable envelopes”, Proceedings (CD) of Performance of Exterior Envelopes of Whole Buildings VIII: Integration of Building Envelopes, 16 pages, Clearwater Beach, Florida, ASHRAE. Simonson, C.J., 2001. “Airtightness and ventilation of a naturally ventilated house in Finland”, Proceedings of the 22nd AIVC Conference, 1.1-1.12, Bath, UK. Simonson, C.J., 2000. Moisture, thermal and ventilation performance of Tapanila ecological house, Espoo, VTT Research Notes, 2069, 143 pages + App. 5 pages, http://www.vtt.fi/inf/pdf/tiedotteet/2000/T2069.pdf. Simonson, C.J. and Salonvaara, M.H., 2000. “Mass transfer between indoor air and a porous building envelope: Part I - Field measurements”. Proceedings of Healthy Buildings 2000, Vol. 3, (edited by O. Seppänen and J. Säteri), Finnish Society of Indoor Air Quality and Climate (FiSIAQ), 117-122. Salonvaara, M.H. and Simonson, C.J., 2000. “Mass transfer between indoor air and a porous building envelope: Part II - Validation and numerical studies.” Proceedings of Healthy Buildings 2000, Vol. 3, (edited by O. Seppänen and J. Säteri), Finnish Society of Indoor Air Quality and Climate, 123-128. Simonson, C.J. and Ojanen, T., 2000. “Moisture performance of buildings with no plastic vapour retarder in cold climates.” Proceedings of Healthy Buildings 2000, Vol. 3, (edited by O. Seppänen and J. Säteri), Finnish Society of Indoor Air Quality and Climate, 477-482. Simonson, C.J., Salonvaara, M. and Ojanen, T., 2002. “The effect of structures on indoor humidity - possibility to improve comfort and perceived air quality”, Indoor Air, 12, 1-9. Simonson, C.J., Salonvaara, M. and Ojanen, T., 2002. “Humidity, comfort and air quality in a bedroom with hygroscopic wooden structures”, Proceedings of the 6th Symposium on Building Physics in the Nordic Countries, Trondheim, Norway. Simonson, C.J., Salonvaara, M. and Ojanen, T., 2001. Improving indoor climate and comfort with wooden structures, Espoo, VTT Publications, 431, 200 pages + App. 91 pages, http://www.vtt.fi/inf/pdf/publications/2001/P431.pdf. Simonson, C.J., Salonvaara, M. and Ojanen, T., 2001. “Effect of hygroscopic materials on energy consumption”, Report for Wood Focus Oy, Technical Research Centre of Finland, 11 pages, May. Ojanen, Tuomo; Salonvaara, Mikael. “A Method to Determine the Moisture Buffering Effect of Structures During Diurnal Cycles of Indoor Air Moisture Loads”. Building Simulation 2003. Eindhoven, The Netherlands, 11-14 Aug. 2003. Research in Building Physics Proceedings of the 2nd International Conference on Building Physics. Leuven, Belgium, 14.18. Sept. 2003. Katholieke Universiteit Leuven (2003), p. 353 - 362. Salonvaara, Mikael; Ojanen, Tuomo; Karagiozis, Achilles. “Indoor air humidity variations and its effects on the moisture performance of building envelope”. Building Simulation 2003. Eindhoven, The Netherlands, 11-14.Aug.2003. Proceedings of the Eighth IBPSA Conference and Exhibition (CD). International Building Performance Simulation Association (IBPSA) (2003).