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Title: Vents and vapor retarders for roofs
Authors: Tobiasson, Wayne
Keywords: Vents
Publisher: Cold Regions Research Engineering Laboratory (U.S.)
Engineer Research and Development Center (U.S.)
Series/Report no.: Miscellaneous Paper;2246
Abstract: Introduction: Vents and vapor retarders are features incorporated into roofing systems to prevent moisture from condensing within the roof. The need for vents and vapor retarders in building envelopes depends on the climate of the place, the temperature and moisture conditions within the building, and the type of materials and systems used for the envelope. Condensation problems in roofs are usually caused by moisture in indoor air that moves upward into the roofing system in cold weather. However, problems can also occur in hot , humid regions when moisture in outdoor air condenses within a roof, particularly above air-conditioned spaces. Moat of this paper is concerned with condensation in cold weather, but the warm. weather problem is also considered. Moisture in the wrong place can rot wood, corrode metal, cause leaching, efflorescence, and spalling of concrete and masonry, and delaminate or disintegrate other building components. Moisture causes insulation to lose some of its insulating ability, adds unwanted dead load to a roof, and can result in annoying, damaging leaks into the rooms below. Roofs suffer more than their share of moisture problems but most of the problems are due to flaws in the exterior waterproofing system of the roof, not to improper control of condensation. Roofs are waterproofed by water-shedding surface such as shingles and metal panels or by water-tight membrane and flashing systems made of bitumens, elastomers, or flexible plastics. Membrane and water- shedding roofing systems are shown in Exhibit 1. Flaws at flashings and penetrations are the primary cause of roof leaks for low- slope membrane roofs. In cold regions, ice damming at the eaves of steeply sloped water-shedding roofs is a major cause of leak . Since most types of roofs do not suffer condensation problems, it appears that current measures used to prevent condensation in roofs are doing their job well. Some recommendations for condensation control are not being used and since no adverse effects are occur ring , I believe those recommendations are unnecessary. However, a few kinda of roofs suffer chronic condensation problems that need to be eliminated. Valuable guidance on vapor retarders and vents is presented in Chapters 20 and 21 of the ASHRAE Fundamentals Handbook. I will overview that guidance as it applies to roofs, expand on ways to ensure that it is incorporated into the as-built world of buildings, and take exception to one aspect of it on venting of compact membrane roofs
Description: Miscellaneous Paper
Rights: Approved for public release; distribution is unlimited.
Appears in Collections:Miscellaneous Paper

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