Pressure/flow relationships of crawl space foundation vents were determined and used in evaluating crawl space ventilation of three houses. Vent effective leakage area (ELA) ranged from 8% greater than net free area (NFA) for the smallest NFA vent to an ELA of 50% less than NFA for some larger NFA vents. Vents with low NFA provide more ventilation 'per gross ventilation area' than vents with high NFA; two 0.015 m² NFA vents supplied the same ventilation as one 0.048 m² NFA vent. Vents installed according to building code requirements using state NFA may be providing up to 50% less ventilation than calculated by ELA. Fan pressurisation tests on three houses indicated that the ELA of the crawl space with existing vents closed and sealed was greater than 1/1500 of the gross floor area in all test cases. When existing vents were opened, ELA increased by 100% to 350%. ELA of the existing vents indicated less ventilation compared to NFA.

KEYWORDS: crawl spaces, ventilation, air flow, vents, pressurisation, measuring, housing, testing.