![]() The temperature gradient, or amount of heat loss, can be illustrated on a roof assembly diagram. One method to accomplish this is to graphically illustrate the temperature change. It is a roof system designer's goal to make sure the temperature at the vapor retarder membrane is warmer than dew-point temperature, which, in effect, causes dew-point temperature to occur within the insulation that is installed above the vapor retarder. The rate at which the air cools depends on the material it is passing through because every building material has distinct thermal-resistance properties. ![]() To ensure the temperature at the vapor retarder remains higher than the dew-point temperature, sufficient insulation amounts must be determined and installed above the vapor retarder to maintain the vapor retarder at a warm enough temperature.ĭuring winter, warm interior air will cool as it passes through or across a roof assembly. Once it has been determined a vapor retarder is to be used, the design temperature at the bottom side of the vapor retarder should be calculated and the design dew-point temperature determined. In any case, NRCA considers the roof system designer to be responsible for determining whether a vapor retarder is necessary.įor a vapor retarder to perform its intended function, the temperature at a vapor retarder's level must be warmer than the dew-point temperature. Army Corps of Engineers-Cold Regions Research and Engineering Laboratory (CRREL) or American Society of Heating, Refrigerating and Air-Conditioning Engineers (ASRHAE) Inc. ![]() Further support for the inclusion of vapor retarders can be obtained from the U.S. NRCA's guidelines for determining the need for a vapor retarder have the benefit of simplicity they are not supported by a great deal of scientifically developed data. For existing buildings, the design interior relative humidity sometimes can be obtained from a knowledgeable building maintenance engineer or whoever maintains the building's HVAC and mechanical control systems. This value typically is determined by the designer of the building's heating, ventilating and air-conditioning (HVAC) system. The expected interior winter relative humidity of a particular building can be determined from the design relative humidity value. If local historical climatic data are not available, the map in Figure 1 can be used to determine the general regions that have outside average January temperatures below 40 F (4 C). ![]() Local climatic data also may be available from private weather services, local television or radio meteorologists, or local weather observers. The outside average temperature for a specific location can be determined from historical climatic data compiled by the National Weather Service. NRCA has maintained a vapor retarder should be considered by roof system designers when the following two conditions are met: the outside average January temperature is below 40 F (4 C) and the expected interior winter relative humidity is 45 percent or greater. The use of a vapor retarder has been debated in the roofing industry for years. Vapor retarders primarily are used in low-slope roof assemblies to prevent large quantities of moisture-laden air from a building's interior from moving into the roof assembly where condensation may occur. This article will focus specifically on vapor retarders. There are three primary methods used to prevent moisture from accumulating in low-slope roof assemblies: use of a vapor retarder, ventilation of interior space and self-drying roof assembly design. If a roof assembly is not sufficiently insulated, warm, moist air will rise into the roof assembly and may cool to its dew-point temperature, causing condensation to occur. Roof assemblies create a barrier between these areas of differing vapor pressures. Warmer interior air exerts a higher vapor pressure than cooler outside air. These conditions will occur most often during winter months in cold climate regions. Vapor drive from a building's interior to exterior is likely to be strongest when the exterior temperature and relative humidity are low and the interior temperature and relative humidity are high. The climate in which a building is located significantly will affect the type, direction of flow and degree of moisture migration and vapor drive that will occur into and out of a building. However, moist air within a building can enter a roof assembly and condense into water. Low-slope roof assemblies, when properly designed and constructed, perform this function well. The fundamental purpose of a roof assembly is to keep water from entering a building through the roof.
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