Because of the up-front costs of add-on solar collection systems, the cost-effectiveness of solar energy is still in doubt. Building integrated solar thermal systems may improve the cost-effectiveness of add-on collection systems given their ability to expand to cover the entire area of a roof at a reasonable cost. The objective of this study was to evaluate the effectiveness of a newly developed flat plate integrated solar collector system. The developed collector system consists of a low-temperature flat plate collector integrated within a concrete building envelope. To evaluate this system, a full-scale test prototype of the solar collector was constructed and instrumented with thermocouples. Measurements were conducted over a 7-month period in which the solar collector was evaluated over different climatic conditions. On the basis of the results of the experimental program, it was determined that the solar collector provided an efficiency ranging from 49-75% in the range of ambient temperature, solar radiation, and inlet temperature evaluated in this experiment. The estimated efficiency of the system was on the high range as compared with the reported efficiency of similar systems. Depending on the extent of the collection area, the developed solar collector can provide a significant portion of the building's space heating and hot water needs, especially in the spring and summer seasons. DOI: 10.1061/(ASCE)EY.1943-7897.0000093. (C) 2013 American Society of Civil Engineers.