Field Performance of Unvented Cathedralized (UC) Attics in the USA

Abstract

This article reports on field experience of unvented cathedralized (UC) attics in several environments in the United States. Traditionally, in some regions of the country, because of high water tables or the risk of flash flooding and lower cost, slab on grade construction is a preferred mode of construction. Mechanical equipment for air conditioning and distribution ducts are usually located in the attic spaces to conserve space. Conventional construction involves providing insulation on the floor of the attic and venting the attic space to the outside. The loss in efficiency in operation of the equipment and through duct leakage is no longer sustainable. Insulating the attic roof itself and blocking of ventilation to the outside transfers the air and thermal energy controls from the boundary with the living space to the plane of the roof. The air distribution systems now fall within conditioned space, which increases their efficiency, durability, and maintainability. While design criteria vary for different climatic regions, UC attics can be insulated in various ways and by using different vapor diffusion resistance strategies of the roof assemblies depending on the climate. The field data presented in this article include measured temperature of asphalt shingles, and thermal and moisture conditions of attic spaces and roof sheathing, as well as air leakage rates. This is of interest for determining probable roofing durability. A more complete understanding of the hygrothermal performance of the assemblies was gained through these measurements.