The unglazed transpired solar collector is now an established solar air heater for heating outside air directly. Sample applications include pre-heating ventilation air and heating air for I:rop drying. The outside air in question is drawn straight from ambient. uniformly, through the whole surface of a perforated blackened plate (the absorber plate) exposed to the sun. An important parameter in fixing the collector's efficiency is the heat exchange effectiveness, epsilon. Once epsilon is known, finding the collector's efficiency is straightforward. This paper presents measurements of e for the case where the plate is perforated with circular holes on either a square or triangular layout, covering a range of wind speeds extending from zero to 5 m/s. These data extend the earlier measurements of Kutscher to a wider range of plate thicknesses, hole spacings (pitch). suction velocities, and to include a square layout of the holes. In the region where the two experiments overlap. agreement between the two is excellent. A new predictive modal is developed that is based on breaking down the total heat transfer into contributions from each of the plate sections: the front, the holt: and the back. Excellent agreement a as found between modelled and measured e: the new measured data were predicted with a 4.2% root mean squared error (RMSE) and Kutscher's measured data is predicted within 6.3% RMSE.