INTRODUCTION Unsealed duct leakage depends on the machinery used for its fabrication@ material thickness@ assembly methods employed@ and workmanship during installation. Duct leakage tests by ASHRAE/SMACNA/TIMA (1985) and Swim and Griggs (1995) have verified that a power law relation can represent the duct leakage from longitudinal seams and transverse joints of assembled duct sections. It was also confirmed that for the same duct construction the behavior of duct leakage was almost the same under negative and positive pressure. These tests have suggested that for unsealed or unwelded ducts the joint leakage dominates@ as the longitudinal seam leakage is about 10% to 15% of the total duct leakage. There is a wide range of products and sealing methods available for ducts. A forecast of leakage class attainable by commonly used duct construction and sealing methods@ based on the data obtained by Swim and Griggs (1995) and ASHRAE/ SMACNA/TIMA (1985)@ is available in the Duct Design chapter of the ASHRAE Handbook (2009). These data do not account for the presence of fittings@ a realistic spacing of rectangular and round duct transverse joints and duct-mounted components@ such as access doors and balancing dampers. The leakage classes were calculated under the assumption of 0.82 joints per meter@ or 25 joints per 100-ft of duct length. The studies cited above suggest that duct system leakage rates are primarily a function of the geometry of the joint and seams@ the sealing used (if any)@ and the pressure difference between the inside and outside of the duct. Future leakage classes should be adjusted for typical rectangular and round transverse joint spacing@ 1.2 m (4 ft) and 3.1 m (10 ft)@ respectively. Ducts possessing a lower proportion of joints would have less leakage in both the sealed and unsealed categories. Many researchers have previously attempted to perform in-situ measurements of the effects of duct leakage on air distribution system performance and building envelope infiltration; representative studies are by Modera (1989)@ Yuill and Musser (1997)@ Proctor (1998)@ and Walker (1999). Most of these studies have tended to compare different sealing techniques. Performance studies by Xu et al. (2000@ 2004) of air distribution systems in light and large commercial buildings have reported air leakage ratios from one-quarter to one-third of the fan-supplied airflow in constant-air-volume systems. The air leakage from the ducts ?C including supply and return@ were reported in terms of the ASHRAE-defined leakage class with the reported values within a range much higher than the leakage classes predicted by the ASHRAE Handbook (2009) for unsealed ducts. Leakage measurement studies by Aydin and Ozerdem (2006) on round and rectangular ducts for positive internal pressures were conducted along with a branched duct system having different duct diameters; the results were fitted to the power law model developed by J. Stratton from ETL data (ASHRAE/ SMACNA/TIMA 1985) and confirmed by Swim and Griggs (1995). Apparently leakage data are unavailable in the literature for flat oval ducts. As a result the present project was initiated to study the leakage characteristics of spiral seam galvanized steel flat oval ducts.