Due to climate change and rising global temperatures, energy demand associated with commercial and office building cooling, is projected to increase. Passive-cooling, based on phase change materials, can assist to moderate this increase, however relatively few modelling equations to describe its operating behaviour exists. In this experimental investigation the melting performance of three commercially available encapsulated phase change materials were evaluated for passive cooling applications in air ducts systems. Two paraffin type phase change materials and one salt hydrate phase change material with melting temperatures in the range of 22 °C and 28 °C were considered. Vertical orientated plate type encapsulations with a thickness of 10 mm and a pitch of 15 mm were tested for air inlet temperatures ranging from 30 °C to 35 °C and upstream air velocities ranging from 0.4 m/s and 0.9 m/s. The average effectiveness, cooling power, energy absorption, and phase transformation durations were determined. It was found that the average thermal effectiveness decreased with increased velocities and that the cooling power, which was inversely proportional to the phase transformation duration, increased with air flow rate and inlet air temperature. Based on the data a new empirical correlation model was developed which describes the cooling capacity of the in-duct phase change material plates.