To conserve energy in buildings, and properly size heating and cooling equipment, it is necessary to understand the thermal performance of building components including the walls and roofs under in-service, dynamic conditions. In-situ inspections show that performance of components in existing buildings can differ markedly from that predicted by design data analysis. This may be due to defective workmanship, faulty materials, deviation from the design specification, and/or deterioration in thermal behavior due to aging and moisture condensation. On-site testing is, therefore, needed to determine the actual thermal performance of building envelope systems. Determination of the thermal resistance of walls or roofs of buildings from in-situ measurements poses difficulties because field conditions are seldom in steady-state.