Moisture problems in residences generally occur in seasons when the outdoor temperature and vapor pressure are low and there are many indoor vapor sources. As mentioned earlier, these may include cooking, laundering, bathing, breathing, and perspiration for the occupants, as well as automatic washers and dryers, dishwashers, and humidifiers. All these sources combine to cause vapor pressure indoors to be much
higher than outdoors so that the vapor tends to migrate outward through the building envelope.
In an ideal world, construction water should be evaporated before the building is occupied. Concrete, plaster, even water-based paints all evaporate water and could contribute to potential condensation problems. Construction water is usually removed within a year, but other sources still exist. Moisture is constantly being generated within a home by the users after occupancy as well. For example, in the winter months, V2 lb of moisture is generated with each shower, whereas 0.12 lb is generated per bath. Appliances generate 5.66 lb per day, including dishwashing, and a person generates 11 lb in a 24-hour period.4 Venting of moisture-laden air from bathrooms, laundry rooms, and kitchens will reduce indoor vapor pressure, as will the introduction of outdoor air with low moisture content. Finally, undrained and unvented crawl spaces, as well as wet basements or bare-earth floors, will continue to be a problem unless corrective measures are performed.4
Moisture in building materials usually increases their thermal conductance significantly and unpredictably. Porous materials that become saturated with moisture lose most of their insulating capability and may not regain it when they dry out. Dust, which usually settles in airspaces, may become permanently affixed to originally reflective surfaces. Moisture migration by evaporation, vapor flow, and condensation can transport significant quantities of latent heat, particularly through fibrous insulating materials.
More than 90 percent of the moisture entering a perimeter structural cavity is from air leakage. The other 10 percent or less occurs as vapor diffusion. This happens because air, by nature, moves toward low pressure through any possible pathway. It acts as a fluid, draining through any imperfection. Moisture diffusion directly through a material is usually a much slower process. Most materials of any density retard this flow somewhat. Standard gypsum wallboard or plaster assemblies, once painted, seriously impede moisture diffusion.  
There are several ways to prevent condensation inside and outside a home:
Figure 4.1 Moisture migration.
(.Better Homes and Gardens)
4. Water vapor within the construction assembly should be allowed to escape by using vapor-porous materials.
5. Vapor traps (i. e., double vapor barriers) formed between two vapor – resistant components must be avoided4 (Fig. 4.1).