Past and Future Research on Whole Building Heat, Air and Moisture Transfer Carey Simonson Associate Professor Department of Mechanical Engineering University of Saskatchewan
[email protected] November 21, 2003
This paper summarizes the findings of three projects related to whole building heat, air and moisture transfer and outlines future research which could be part of IEA/ECBCS Annex 41. PAST RESEARCH Tapanila Ecological House [1-6] 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 vapor 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 vapor permeable envelope. To realize a moisture physically safe structure, the vapor permeable envelope must be airtight (e.g., 3 ach at 50 Pa) and the water vapor 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. Comfortable Wooden Buildings – Phase I [7-10] 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.
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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. 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 non-hygroscopic 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. 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 0.9 ach as in the non-hygroscopic bedroom with a ventilation rate of 0.1 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. Hygroscopic Materials for HVAC Systems [11-17] This research on air-to-air heat exchangers that are coated with hygroscopic desiccants has been conducted at the University of Saskatchewan. Since these exchangers transfer moisture between the supply and exhaust air streams of buildings, they moderate the minimum and maximum indoor humidity in a similar way as hygroscopic envelopes and furnishings. Therefore, moisture exchangers can increase the effective hygroscopic mass of a building. Furthermore, by controlling HVAC systems, which include hygroscopic materials (i.e., moisture exchangers), it may be possible to reduce the peak cooling loads of buildings and also reduce the energy consumption of buildings. This is an important aspect of Annex 41. A preliminary study estimated that energy savings in the range of 5% to 30% are possible when hygroscopic building materials are combined with well-controlled HVAC systems [10].
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FUTURE RESEARCH Current research and future projects that could be undertaken at the University of Saskatchewan within the Annex 41 subtasks are listed below. Funds for the current research are from the Natural Science and Engineering Research Council of Canada (NSERC) and Canadian Industry. Future research funds will be sought from similar sources. • Depth of moisture penetration and moisture storage capacity of hygroscopic materials during transient boundary conditions, Current research, Subtask 2. • New methods for heat and moisture exchange between ventilation and exhaust air flows for buildings, Current research, Subtask 2. • Potential for hygroscopic materials in North American climate and buildings, Current research, Subtask 4. • Quantifying the convective mass transfer coefficient in buildings and its effect on moisture transfer between indoor air and hygroscopic building materials, Future research, Subtask 1 or 2. • Numerical simulation of the effect of hygroscopic materials in the building envelope and HVAC system on thermal comfort, IAQ, required HVAC capacity and energy consumption, Future research, Subtask 4. • Whole building HAM applications for remote, northern housing, Future research, Subtask 4. • Building constructions that can produce an enhanced indoor environment and yet provide a moisture durable envelope (e.g., a wooden frame envelope with insulation external to the stud cavity), Future research, Subtask 4. Experimental research will be conducted using the transient moisture accumulation facility and heat and moisture exchanger facility at the University of Saskatchewan. Detailed numerical models for specific components (insulation, desiccants and energy exchangers) will be developed to support and extrapolate the experimental measurements. Whole building modeling will be accomplished using the commercial simulation package TRNSYS and the research tool LATENITE (in collaboration with VTT Building and Transport). TRNSYS has the advantage of modeling the building and various HVAC systems, while LATENITE has a more detailed and accurate HAM model. REFERENCES 1. 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. 2. 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. 3. 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.
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4. 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. 5. 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. 6. 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. 7. 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. 8. 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. 9. 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. 10. 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. 11. Besant, R.W. and Simonson, C.J., 2003. Air-to-air exchangers, ASHRAE J., 45(4), 42-52. 12. Besant, R.W. and Simonson, C.J., 2000. Air-to-air energy recovery, ASHRAE J., 42(5), 31-42. 13. Simonson, C.J., Shang, W. and Besant, R.W., 2000. Part-load performance of energy wheels: Part I - Wheel speed control and Part II - Bypass control and correlations, ASHRAE Trans., 106(1), 286-310. 14. Simonson, C.J. and Besant, R.W., 1999. Energy wheel effectiveness. Part I − Development of dimensionless groups and Part II − Correlations, Int. J. Heat Mass Transfer, 42(12), 2161-2185. 15. Simonson, C.J., Ciepliski, D.L. and Besant, R.W., 1999. Determining the performance of energy wheels: Part I - Experimental and numerical methods and Part II - Experimental data and numerical validation, ASHRAE Trans., 105(1), 174-205. 16. Simonson, C.J. and Besant, R.W., 1998. Heat and moisture transfer in energy wheels during sorption, condensation and frosting conditions, ASME J. Heat Transfer, 120(3), 699-708. 17. Simonson, C.J. and Besant, R.W., 1997. Heat and moisture transfer in desiccant coated rotary energy exchangers: Part I - Numerical model and Part II - Validation and sensitivity studies Int. J. HVAC&R Research, 3(4), 325-368.
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