The Determination of the Dynamic Performance of Walls

Measuring the thermal performance of walls in-situ poses two separate problems: 1) how to measure time-varying surface temperatures and heat fluxes on both sides of the test wall and 2) how to reduce this data set into a minimal number of parameters that effectively characterize the wall. In this paper we present a methodology for interpreting field measurements of wall performance and describe an instrument developed for carrying out such measurements. The method is a simplified dynamic model that uses a small number of simplified thermal parameters (STP) —a steady-state conductance, a time constant and a few surface storage terms —to describe the thermal performance of a wall. We demonstrate the ability of this model to simulate actual wall performance by comparing model predictions with results generated from conventional response-factor methods. The instrument developed for field measurements is the Envelope Thermal Teat Unit (ETTU), which consists of two four-foot by six-foot blankets placed on either side of the test wall that are used to both measure and control the surface heat fluxes and surface temperatures of the wall. During a typical test, which lasts about 12 hours, one blanket imposes a specified flux through one surface of the test wall while the resulting heat flux on the other surface and the surface temperatures on both sides are measured. The model presented here can be used for both laboratory and field measurements and may be applied to any component of the building envelope.