A comprehensive monitoring system has been implemented in the Archetype Sustainable Twin Houses at Kortright Center in Vaughan, Ontario, Canada to investigate the thermal performance of a variety of sustainable housing technologies. Among these technologies includes a two-stage variable capacity air source heat pump (ASHP) system in House A of the twin houses. Using monitored data from a data acquisition system (DAQ) which collected data every five seconds for a one month period in August 2010, the part load cooling efficiency was investigated. To investigate the cooling performance for a variable speed compressor air source heat pump, data for various operating conditions including the outdoor temperature and relative humidity, the ASHP supply and return air temperatures and relative humidities, the supply air velocity, and the power consumption of the entire system was measured.