Contaminant transport by air infiltration from crawl space to occupant area: Numerical simulations and field measurements in Swedish schools

Licentiate thesis, 2020

Some Swedish school buildings built in the 1960’s and 1970’s have indoor air quality problems. Many of these buildings have a crawl space from which contaminants are suspected to originate. The poor indoor air quality cause discomfort among pupils and teachers and a solution to the problem is not always found. This thesis summarizes the work done on investigating contaminant transport driven by air leakage from the crawl space to the classroom in such buildings. Field measurements of temperature, wind, and pressure difference across the floor construction between classroom and crawl space has been  conducted in two school buildings. A method in which frozen carbon dioxide is used to determine if air leakage to the classroom originates from the crawl space is also successfully tested. Also, a numerical infiltration model is developed in MATLAB and used to investigate how temperature, wind and air permeability distribution affect the pressure difference across the floor construction and contaminant concentrations. The numerical model is also used with the Monte Carlo method to investigate, for example, correlations between model parameters, such as air permeability and temperature, and to analyze measures, such as increased ventilation or use of an exhaust fan in the crawl space.

Results presented in this thesis shows that outdoor temperature and wind has a
stronger influence on the concentration levels indoors and the pressure difference
across the floor than for example the building airtightness. For buildings with an imbalanced ventilation system, where the exhaust airflow is larger than the supply airflow the most critical weather case, in terms of high concentrations of contaminants indoors, is during mild and calm days. Numerical simulations also show that the pressure difference across the floor construction is positive (so that air leaks from the crawl space to the classroom) for most weather cases and building configurations.