Excessive moisture in some form is often the cause of condensation problems in a house or other structure. Perhaps the most aggravating and also the most easily prevented are those caused by the movement of water vapor through walls or ceilings. Such problems may result in excessive maintenance costs, such as the need for frequent repainting and increased heating costs. However, properly installed vapor barriers, in conjunction with proper use of insulation, and adequate ventilation will avoid most of such difficulties. The results of many years of research by Forest Products Laboratory and other scientists, together with their field experience in solving condensation problems, have provided much valuable information. This publication contains recommendations based on these data. In addition to good practices in the use of vapor barriers, insulation, and ventilation, other construction details are also described and illustrated. Such practices followed in the construction of a new home can pay for themselves many times over in reduced maintenance costs. This publication is based on USDA Forest Service Research Paper FPL 132, "Condensation Problems: Their Prevention and Solution," Forest Products Laboratory, Madison, Wis.