Attached garages can pose a threat to a home’s indoor air quality because the garage air, which often contains some contaminants such as particulate matter, carbon monoxide, and volatile organic compounds, can migrate into living spaces. In this paper, a model is developed to characterize the transport of contaminants from a garage to a living space based on a measurement of single tracer gas decays without limiting the numbers of zones in the building. Unlike conventional multi-tracer gas methods, this model considers time delays of contaminant transport, and is much simpler, less costly and less time-consuming. It interprets the test results with clear physical meanings of the coefficients in the model equations, and is helpful for understanding the transport process of contaminants. An example of the model’s applications shows the goodness of fit of the model to test data and its effectiveness at interpreting results of tracer gas decay experiments.