The vapor retarder facing should be installed toward the warm-in-winter side of the construction except where noted. This means that in ceilings the vapor retarder faces down, in walls it faces the inside, and in floors over unheated spaces it faces up. With the exception of reverse-flange insulation, the stapling flanges are on the same side as the vapor retarder. In warm, humid climates local practice should be followed regarding the use of and/or placement of vapor retarders.

Note: The ASHRAE Handbook of Fundamentals, Chapter 23, Thermal and Moisture Control in Insulated Assembly Application, contains information on the placement and use of vapor retarders in various climates including humid climates such as Florida, the Gulf Coast, or Hawaii.