No abstract
It has long been noted that interior vapor barriers in wood frame walls in hot-humid climates can lead to interstitial condensation within walls. The bases for this recognition are predictive simulations, anecdotal observations, and a limited number of experimental studies. This paper describes an experimental study conducted in a hot-humid climate that investigated the influence of an interior vapor retarder and compares observed performance with simulation predictions. The wall performance data reviewed here was gathered as part of a larger test program evaluating the performance of a range of typical wood frame, residential wall constructions in a hot-humid climate. The approach chosen was to use real-time field exposure using a ātest hutā located in Tampa, Florida. The test hut had two long sides, which provided the ability to test 16 wall specimens each. Wall specimens were instrumented with a variety of temperature, humidity, and moisture sensors. In addition to natural weather exposure, the wall specimens could be manually wetted by a water injection system to simulate rain leakage. More specifically, this paper focuses on using the data collected before and after the installation of an interior vapor barrier (vinyl wallpaper) to show the change in moisture loading and the potential condensation within the walls resulting from the installation. The field data is compared with predictions of the wall behavior using a commonly available hygrothermal model. There is increasing reliance on the use of predictive models to assess the moisture performance of building assembly designs. These predictive models need to be validated against real data to test their variance from real systems.
An electronics manufacturing facility in Colorado was constructed with typical commercial steel framing containing fiberglass batt insulation with foil-facing as the vapor retarder. A large portion of the facility required constant positive space pressurization and an indoor relative humidity (RH) of 48 % RH at 21Ā°C (70Ā°F). Due to a lack of containment of the moist interior air, condensation formed in the exterior walls, in the roofing, on the window and door systems, and on the skylights of the facility, particularly along the north and east elevations of the building, which resulted in significant damage to the wall components and the inability of the HVAC system to remain properly balanced. The as-constructed exterior wall systems were evaluated for performance relative to the existing building mechanical system. It was determined that the as-designed and as-constructed exterior wall assemblies and components could not perform satisfactorily under the necessary indoor environmental requirements. Options were explored to improve the performance of the exterior wall assemblies and related details. Due to a number of constraints, it was decided that constructing new walls inside the as-constructed exterior walls and windows, thereby creating an āinterstitialā space to separate the exterior walls from the interior building space for their full height and mechanically supplying warm, dry air to this interstitial space, was the desired method to correct the condensation problems that were occurring. These repairs have proven to be effective. This paper discusses the original design and construction, the method of evaluation of the condensation problems that occurred, and the design and results of the implemented repairs.
Most of the exterior walls of wooden houses in Japan have a vented air layer between the exterior cladding and the insulation. This vented air layer is designed to dehumidify the exterior walls by discharging humidity to the outside and allowing outdoor air to enter, thereby decreasing the risk of condensation on the exterior wall during winter. It is assumed that the source of this moisture is the indoor air, and that the outdoor air is drier. However, the outdoor air is often highly humid during the rainy season and may become a source of moisture. The vented air layer also allows rain water to drain away. Any rain water penetrating through the exterior cladding is drained away through the vented air layer. However, rain can also enter the vented air space through the air inlets. Since it takes a certain amount of time for all the rain water to drain away, water may accumulate in the vented air layer and produce high humidity in the exterior wall. In order to evaluate the effectiveness of an exterior wall with a vented air layer, its hygrothermal characteristics should be investigated, taking the effect of rain water into consideration. This paper describes a case of moisture damage where stain appeared on the outer surface of the plywood wall in a wooden residential building. Experiments were carried out in climate chambers to clarify the conditions that are causing stain. Hygrothermal conditions in the vented air layer were simulated using heat, air, and moisture model, and the causes of staining were investigated.
Condensation problems in general use (i.e., non-humidified) buildings such as offices, schools, and condominiums typically manifest themselves as visible staining on window and door perimeters or minor dripping from overhead components. The relatively low interior moisture levels that are typical of these types of buildings are generally insufficient to cause severe water damage in the short term. At the other end of the spectrum, high-humidity buildings such as museums and natatoriums can suffer extreme damage due to condensation, sometimes within weeks, not years, of completion. High interior moisture levels and, in many cases, differential air pressures, contribute to condensation in both visible locations, such as windows and curtain walls, and concealed locations within walls and roofs. Signs of concealed condensation such as dripping water or rust stains may only become visible after moderate to heavy damage has already occurred within the enclosure. These problems are typically more severe in cold climates, but high-humidity buildings may experience condensation problems in mild climates, where such issues are often not considered by designers due to the lack of prolonged cold weather during the winter. This paper will review the severe and immediate condensation problems that are unique to high-humidity buildings, including surface condensation and concealed condensation due to air leakage through the enclosure. It discusses the theoretical mechanisms by which condensation forms in building enclosures, and illustrates these concepts through various case studies in both cold and mild climates. This paper will focus on design strategies for avoiding problems, but also discusses remedial work to existing buildings, drawing on the authorās experience with the investigation and repair of high-humidity buildings.
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