Vapor Retarders Vapor retarders are normally specified where the heat flow direction and/or humidity within the building, during any climate or occupancy cycle, might cause the dew point or condensation to occur under the built-up roofing within the insulation chamber. Koppers endorses the following NRCA recommendations concerning vapor retarders: The outside average January temperature is below 40 degrees Fahrenheit (5 degrees Celsius). The expected winter interior relative humidity is 45 percent or more. The decision on the use of a vapor retarder is the responsibility of the building designer/engineer. It is important that the vapor retarder design provide a continuous seal (including ventilators, skylights, peripheral walls, and other terminations) from the source of water vapor. Koppers may require certain insulation types and/or combinations be installed for warranted projects over vapor retarders.  Return to General Requirements and Considerations for the Designer