Thermal bridges, localized areas of building enclosures that allow higher heat flow, are recognized as an important source of heat loss/gain. Computermodels are now widely used to quantify the impact of thermal bridges. However, few full-scale measurements of heat flow through thermal bridges havebeen undertaken to confirm and validate computer models. To allow for the measurement of specimens with complex 2D and 3D heat flows (i.e. thermalbridges) a special large-scale guarded hot plate apparatus based on ASTM C177 was designed and constructed. This apparatus is capable of acceptingspecimens measuring 4 ft (1219mm) by 4 ft (1219mm) and up to 16 inches (406mm) thick. This paper describes the apparatus, its design, and theresults of commissioning tests.Computer modelling was used to estimate heat flow errors describing the characteristics and limitations of the apparatus. Once constructed, the apparatuswas validated against a heat flow meter calibrated using a NIST-traceable calibration specimen. The capability of the guarded hot plate to measurelayered specimens with thermal bridges was demonstrated through the measurement of three specimens resembling typical wall assemblies. Measurementsfrom the apparatus were compared against results from computer models and good agreement was found.