Fiber-reinforced plastic (FRP) materials are extensively used in building construction, primarily due to their superior structural performance characteristics. Recently, a newly-developed concept consisting of a prefabricated, interlocking fiberglass composite panel system has been adopted for use in the construction of building envelope systems. The structural characteristics of these panels allow, among other things, expeditious construction. Other performance advantages include corrosion resistance, reduced maintenance, electric insulation characteristics, and electromagnetic transparency. Since little information is available on the thermal performance characteristics of such a panel system, a testing program was developed to investigate the thermal insulation characteristics of FRP panels that are commercially available at the present. Two full-scale 1.2 m by 1.2 m (4 ft by 4 ft) FRP panels were tested, Two panel thicknesses were considered: 25 mm (1 in.) and 75 mm (3 in.). The thermal characteristics of the panels were measured including the effects of the presence of the joints between the panels. A temperature-controlled test plate, calibrated with fibrous glass board material of known thermal conductivity, was used with heat Row sensors to determine the thermal resistance of the FRP panels at the mid-sections of panels and at the interfaces (i.e. joints) between two adjoining panels. Two conditions were simulated; ＇dry joint＇ which includes only mechanical interlocking at the joints, and ＇sealed joint＇ in which the joints were sealed with a commercially available sealant. The R values of the tested panels were approximately 5% to 46% higher in the sealed-joint condition, Sealed joints decrease heat exchange across the envelope system, thereby increasing the thermal resistance values of the panel system. The relatively high R value of the 75 mm panel system (2.0 (m(2) K)/W(11.36 (h ft(2) degrees F)/Btu)) is encouraging, and makes this envelope system a potential candidate for wider use in energy-conscious commercial buildings.