How moisture can affect your building
Water, one of the most plentiful substances on the planet, can wreak havoc on most building materials. It is a requirement for living things and its presence usually brings some sort of flora or fauna. While most building materials are affected by water those which are based on natural materials generally more susceptible to water damage.
Since building envelopes are meant to protect its inhabitants from unwanted environmental effects designs should either keep water away from materials sensitive to water or provide an easy way out should water get in.
In cold climates the importance of keeping the water out of a building is greater because of the likelihood of condensation. this can occur inside the wall or on its surface and can degrade the material or worse still foster mould growth on the surface and it is believed that the toxins associated with certain moulds can cause negative health effects.
A common way to prevent unwanted water entering is by using plastic (polythene) sheeting. Placed under a floor or in a wall it is an effective way to stop unwanted water. Care must be taken however to prevent holes or punctures in the sheeting or its purpose may be defeated because moist air or water can get through the holes. Sealing the edges is also important for the same reason.
The location of sheeting is important because we don’t want to trap water in the wall or floor by putting the sheeting in the wrong location. In walls the sheeting is placed closest to the warm side. Why? Because condensation is formed by the cooling of warm moist air. By keeping the plastic on the warmest side we keep the moist air warm and prevent it from getting to the cooler side of the wall and avoid it cooling enough to cause condensation.