A comprehensive program to investigate the thermal performance of insulated flat roofs has been developed at Drexel University in Philadelphia. A number of 1.22-m (4-ft)-square roof systems have been sealed into expanded polystyrene (EPS) modules and placed in a horizontal array on the roof of a Drexel building. The roof specimens are exposed to the outdoor urban environment and at the same time are connected to a “life support system” that maintains the undersides of the roofs exposed to a typical indoor atmosphere. A 1.22-m (4-ft)-square temperature-controlled test plate, also enclosed in EPS, is used with heat flow sensors to determine periodically the thermal resistances (R-values) of the roof systems. The test plate system has been calibrated with fibrous glass board material from the National Bureau of Standards and preliminary test results have been obtained for a single-ply roof system with EPS insulation. The application of the test plate approach to in situ R-value measurements of roofing systems at remote sites is also discussed.