The National Physical Laboratory has developed a Vacuum Guarded Hot-Plate (VGHP) for measurement of both the total hemispherical emittance and thermal conductivity of insulating materials in a vacuum or gas atmosphere. This paper describes the principles of operation of the VGHP and practical issues associated with the design and construction. Also described are features that make it a versatile and convenient device for measuring the thermal properties of a wide range of construction products.

Measurement results are presented on fumed silica powder of the type used in evacuated insulation panels, measured in a specially designed and instrumented container. Also presented are results of total hemispherical emittance measurements on reference materials of known emittance and on a low-emittance reflective insulation product. The VGHP emittance measurements are compared with the results by a spectrophotometric method, which is often not suitable for measuring insulation materials.