The doctoral dissertations of the former Helsinki University of Technology (TKK) and Aalto University Schools of Technology (CHEM, ELEC, ENG, SCI) published in electronic format are available in the electronic publications archive of Aalto University - Aaltodoc.

Moisture and Fungal Spore Transport in Outdoor Air-Ventilated Crawl Spaces in a Cold Climate

Miimu Airaksinen

Dissertation for the degree of Doctor of Science in Technology to be presented with due permission of the Department of Mechanical Engineering for public examination and debate in Auditorium K216 at Helsinki University of Technology (Espoo, Finland) on the 21st of November, 2003, at 12 o'clock noon.

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A crawl space foundation is widely used in buildings and detached houses in northern countries. The relative humidity of the air in crawl spaces is the most critical factor concerning mould growth in the structures of a crawl space. Possible contamination in the crawl space might be transported indoors if the pressure inside the apartment is lower. The objective of the study was to find out the important properties of ground covers and the optimal air change rates for the controlling of moisture conditions in an outdoor air-ventilated crawl space in a cold climate and to estimate the acceptability of current moisture conditions in respect of material durability. In addition, factors affecting the transport indoors of possible pollutants from crawl spaces were studied.

The moisture conditions were calculated with a dynamic simulation model, which was validated against measured data. The moisture and thermal capacity and resistance of the ground cover were varied, as was the air change rate in the crawl space. The acceptability of moisture conditions was evaluated using a mould growth index. The concentration of fungal spores was measured both through field measurements and full-scale laboratory measurements. The penetration of inert particles of different sizes through a building envelope was studied by means of full-scale laboratory measurements. The airtightness of the building envelope and the pressure difference across the envelope were varied.

It was shown that in a relatively warm crawl space moisture problems were easy to avoid – ground soil should be covered so as to prevent moisture flow from the ground and an air change of at least 0.5 ach is enough to keep relative humidity at a low level. A relatively cold crawl space needs a ground cover with moisture and thermal resistance. A ground cover with a moderate thermal resistance, such as 15 cm lightweight aggregate, needs a higher ventilation rate, at least 2.0 ach, to warm up the crawl space in the summer. A ground cover with a high moisture capacity can stabilise the fluctuation of relative humidity in a crawl space, and thus avoid critical peaks of relative humidity in respect of mould growth. The safest ground cover solution is a thick cover with a high thermal resistance and a low air change rate of 0.5 ach; with this approach natural ventilation can be used. Heating a crawl space in summer is an excellent way to avoid mould growth. The advantage of heating is greatest if the ground cover has a high thermal conductivity. The energy consumption of heating is strongly dependent on the set point value for the relative humidity. However, if the set point value is kept reasonable and the ventilation rate remains low the specific annual energy consumption is within the range of 1.4-3.6 kWh / m2 of the crawl space area.

Results from field measurements showed a correlation between microbes in the crawl space and indoors. In the full-scale laboratory measurements it was established that inert particles and fungal spores in a size range 0.6-2.5 µm penetrate a wooden structure at moderate pressure differences. Laboratory measurements showed that the penetration was highly dependent on pressure difference and not dependent on holes in the surface boards of the structure. The results are likely to show that the surface contacts of mineral wool in the floor structure may have an important role in penetration. It is clearly difficult to control the penetration of fungal spores by sealing the building envelope. The only effective way to avoid penetration seems to be balancing the building; however, in cold climates the moisture condensation risk should be taken into account. The results indicate that mechanical exhaust ventilation causing an under-pressure in the building may cause health risks if some contamination exists in the building envelope.

This thesis consists of an overview and of the following 6 publications:

  1. Kurnitski J. and Matilainen M., 2000. Moisture conditions of outdoor air-ventilated crawl spaces in apartment buildings in a cold climate. Energy and Buildings 33, No. 1, pages 15-29. © 2000 Elsevier Science. By permission.
  2. Airaksinen M., Kurnitski J. and Seppänen O., 2003. On the crawl space moisture control in buildings. Proceedings of the Estonian Academy of Sciences: Engineering 9, No. 1, pages 34-58. © 2003 Estonian Academy Publishers. By permission.
  3. Matilainen M. and Kurnitski J., 2003. Moisture conditions in highly insulated outdoor ventilated crawl spaces in cold climates. Energy and Buildings 35, No. 2, pages 175-187. © 2003 Elsevier Science. By permission.
  4. Matilainen M., Kurnitski J. and Seppänen O., 2003. Moisture conditions and energy consumption in heated crawl spaces in cold climates. Energy and Buildings 35, No. 2, pages 203-216. © 2003 Elsevier Science. By permission.
  5. Airaksinen M., Pasanen P., Kurnitski J. and Seppänen O., Microbial contamination of indoor air due to leakages from crawl space – a field study. Indoor Air, accepted for publication. © 2003 by authors and © 2003 Blackwell Publishing. By permission.
  6. Airaksinen M., Kurnitski J., Pasanen P. and Seppänen O., Fungal spore transport through a building structure. Indoor Air, accepted for publication. © 2003 by authors and © 2003 Blackwell Publishing. By permission.

Keywords: crawl space, mould growth, moisture control, ground covers, ventilation

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© 2003 Helsinki University of Technology

Last update 2011-05-26