R values determined in laboratory settings, according to adopted standard methods are reproducible quantities that predict the thermal resistance of various building materials under specified conditions. Insulation materials installed in walls, floors, and ceilings, however, are subject to a wide range of variables, including air and moisture ingress, biological challenges, settling, and other physical changes and temperature variations. Hence, there is a need for accurate in situ thermal measurements to quantify building system thermal resistance under actual environmental conditions. A self-supporting portable instrument that satisfies the requirements of ASTM C1046 has been developed to measure heat flux and R values in situ. The instrument consists of a housing that automatically positions a heat-flux transducer (HFT) on the region to be tested. The heat-flux transducer is a bismuth-telluride Peltier device that generates a voltage that is proportional to the heat flux. A temperature sensor is attached to the heat-flux transducer. A fan circulates air from the surroundings through the instrument housing to ensure that the interior of the housing is in thermal equilibrium with the surroundings. The fan also creates a pressure difference that is sufficient to press the instrument firmly against the surface of a smooth wall or ceiling. A second temperature sensor, located on the opposite side of the building envelope, sends data to the instrument electronically to provide for a calculation of the temperature difference across the test region. A microcomputer receives the heat flux and temperature data needed for R-value calculations. This instrument can be used to evaluate heat flow and thermal resistance for walls, ceilings, floors, and other components that are part of the building envelope. In situ R values and heat-flux data obtained with this instrument will be discussed in this paper.