A unique guarded hot box designed for thermal testing of fenestration products incorporates several new design concepts from guarded hot plates, namely wall and edge guards, is described in detail. The wall and edge guarded hot box was built to meet the test methodologies specified in the American Society for Testing and Materials (ASTM) Standard Test Methods C 236-89 [1], “Standard Test Method for Measuring the Steady-State Thermal Performance of Building Assemblies by Means of a Guarded Hot Box”, C 976-90 [2], “Standard Test Method for Measuring the Steady-State Thermal Performance of Building Assemblies by Means of a Calibrated Hot Box” and C 1199-91 [3], “Standard Test Method for Measuring the Steady-State Thermal Transmittance of Fenestration Systems Using Hot Box Methods”.

Calibration test results for the box wall and specimen frame flanking are presented. Results from calibration transfer standard tests are compared with theoretical calculations and discussed.