2004 ASHRAE. ABSTRACT Many enclosure assemblies in many different climates have traditionally been ventilated with exterior air. The nature and magnitude of the benefits of providing a ventilated wall system have been debated, but little quantitative research has been conducted. This paper presents some of the results of a series of detailed ASHRAE-supported full-scale field test house studies that explored the role of ventilation and sheathing membranes in the drying of wood-framed walls. Several companion papers describe the technical background and the results of computer modeling and laboratory testing. The studies involved five different 1.2 m wide and 2.4 m high wood-framed wall systems—three clad with brick and two with vinyl siding. Along with comprehensive boundary conditions (including driving rain), a total of over 20 sensors were used to moni- tor the moisture content, RH, and temperature within each wall system. Air velocity within the ventilation space of the brick veneer was also measured directly. A unique intra-wall wetting system was developed to uniformly and repeatedly wet the sheathing of the walls in a uniform manner. The walls were wetted three times at different times of the first year and their response-monitored. Changes were made to the size of the ventilation space in all walls, and a second year and three more wetting-drying cycles were imposed. Results included: all walls allowed a very significant amount of drying, some of the walls exhibited no damage at all at the end of the experiment, drying rates varied significantly during different weather conditions, ventilation increased the drying poten- tial of some walls, and the nature of the sheathing membrane influences the drying rate. It was also shown that solar-driven vapor diffusion acts to redistribute vapor from within the wall to the interior (where it did cause damage) and that ventilation reduces the magnitude of this flow. This reduction of inward drives due to ventilation had a larger effect for the absorptive brick cladding tested. The location of brick vents—both top and bottom—was clearly shown to be beneficial to drying. The vinyl siding profile tested allowed significant ventilation-induced drying, whether applied with or without furring. INTRODUCTION Moisture is one of the most important factors affecting building enclosure durability and performance, especially in cold climates. The design of moisture-tolerant enclosures should involve the simultaneous consideration and balancing of the potentials for wetting, storage, and drying. Design guidelines may stress the avoidance of wetting, but increased safe moisture storage capacity or drying potential can also improve the moisture tolerance of an assembly. Drainage is usually regarded as the most important mois- ture removal mechanism, and internal drainage has recently received much attention for walls clad with EIFS, wood siding, stucco, etc. Drainage, however, does not necessarily remove sufficient moisture to ensure proper enclosure perfor- mance: other drying mechanisms must be provided. One drying mechanism that has not received the attention it is due is ventilation. Many enclosure assemblies in many different climates have traditionally been ventilated with exterior air. North American building codes also contain requirements for the venting (and, hence, presumably ventilation) of roofs, walls, and crawlspaces. An increasing proportion of wall systems incorporate a space behind the outer cladding (or screen). This space, which Field Studies of Ventilation Drying John F. Straube Randy van Straaten Eric Burnett John F. Straube is an assistant professor and Randy van Straaten is a graduate student in the Civil Engineering Department, University of Waterloo, Ontario, Canada. Eric Burnett is a professor and Hankin Chair in the Civil and Environmental Engineering and Architectural Engi- neering Departments, Penn State University, University Park, Pa. (RP-1019)