Vapour Insulation

The dampness that sometimes occurs inside buildings can be caused by penetration of moisture from the outside or by condensation of water vapor generated from inside. 

Protecting the interior of buildings against moisture penetration from outside involves well-known procedures. Protection against condensation of water vapor produced inside the building is a different matter, and is perhaps not so well understood. Protection from the outside is provided by water repellent materials which turn water aside and force it to return down to the earth outside the building. Moisture vapour, on the other hand, can permeate most ordinary building materials such as wood, paper, lath, plaster, untreated brick etc. 

In addition, this moisture  vapour will condense into water when its temperature is reduced by contact with a cool surface or cool air. Hence, high humidity in a building may result in condensation of water not only on the inside surface of walls and windows, but also on the outside or within the exterior walls, ceiling or roof. 

Generation of water vapour is inevitable in a building. Occupants, cooking activities, laundering, invented fuel-burning devices, humidifiers and evaporation from basement floors or earth in crawl spaces, all produce moisture related vapor. Some moisture can be eliminated at its source, and adequate ventilation will help carry it away. But this does not provide the answer to the problem of condensation on the inside of buildings or within the walls. If the inside surfaces of walls and windows are kept warm enough, condensation will not take place on then Vapor can be kept from penetrating through the exterior walls from inside by an effective vapor barrier (vapor insulation) placed on the inside of exterior walls.