Although much information is available on the steady-state thermal performance of walls in a laboratory environment, there is little information concerning either the steady-state or dynamic performance of walls in-situ. This report describes a measuring apparatus, the Envelope Thermal Test Unit (ETTU), developed to measure the in-situ thermal performance of walls. It includes a description of EITU’s operation, its construction, and the results of tests in the laboratory and in the field. The apparatus is described in detail, including technical specifications of the hardware. The EITU measurements are compared with predictions made by computer simulation of the heat transfer through the test walls. These comparisons, based upon application of the theory of simplified thermal parameters to the interpretation of ETTU measurements, show good agreement with the computer simulations for all of the walls tested, except for a thick concrete wall. The measurements on the concrete wall were affected by large lateral heat flows.