Energy and Moisture Impact of Exterior Insulation and Finish System Walls in the United States

Abstract

This paper summarizes a family of computer simulations undertaken to assess the hygrothermal performance of exterior insulation finish systems, and it discusses the validation of this simulation model prior to its use. This research deals with performance evaluation of selected wall systems in all eight International Energy Conservation Code (IECC) U.S. climate zones. The performance ranges are representative of generic exterior insulation and finish wall systems but not of a specific manufactured product. A comparison was performed among three different exterior insulation and finish system (EIFS) walls and a brick wall system. Two steps were required to perform this work. The first step required the model validation of eight distinct wall systems previously measured in Charleston, SC. The validation was performed using WUFI-ORNL (Version 5) software, and a comparison of the measured versus simulated results from these wall systems indicated that the simulation model accurately captured the hygrothermal response of the wall systems. The second step was to simulate four wall systems in the eight climate zones. Each wall system was modeled with and without a vapor retarder and with and without a “leak.” For this study, a leak is defined as the percentage of rainwater that strikes a wall's façade and that is assumed to get past the cladding as well as the water-resistive barrier and into the exterior sheathing. All of the simulations followed the recommendations prescribed in ASHRAE Standard 160, Design Criteria for Moisture Control in Buildings. Two specific criteria were extracted from the simulations to assess the hygrothermal properties of the wall systems. The average monthly heat flow into the building interior was used to evaluate the energy efficiency, and the average monthly moisture content of the exterior sheathings was studied to measure the moisture performance of the wall systems.