The overall hygrothermal behavior of wall assemblies and their related performance is dependent on a number of factors, including properties of the materials used in the assembly as well as the boundary conditions to which the assembly is subjected. Performance assessment, in terms of the rate of energy transfer through the assembly, requires determining heat loss as a function of time in steady-state conditions. Assessing hygrothermal performance, on the other hand, requires, in addition to determining heat transfer constants, knowledge of the mass transfer of air and moisture through the assembly. In small-scale tests, these transient effects can be monitored in real time to offer a ready means of calculating the physical constants of given materials or components. In tests of full-scale wall assemblies, for example, monitoring the change in mass over time can present certain technical challenges if measurable and reproducible data are to be obtained. To assist the benchmarking of computer models that simulate such types of hygrothermal effects, a balance is required that can assess minute changes in mass. Ideally, the balance should measure the effects on a continuous basis such that changes in weight in relation to time can be readily determined. This paper describes the design of, and experimental results derived from, a weighing system for full-scale wall assemblies. The system is capable of continuously monitoring weight changes of 2.5-by-2.5-m walls having nominal weights of up to 225 kg (to the nearest 1g). The weight data have been used to determine weight loss over time in wood sheathing affixed to a wood frame when exposed to steady-state laboratory conditions. The data were used as a basis for helping benchmark an advanced hygrothermal computer model--hygIRC.

Authors: Wahid Maref, Ph.D.; Michael A. Lacasse, Ph.D., P.E.; Nicholas Krouglicof, Ph.D., P.E.

Citation: Thermal Performance of the Exterior Envelopes of Buildings VIII

Keywords: December, Florida, 2001