Water, in all of its states, is the major cause of damage to a building and affects its durability. Moisture can cause concrete to crumble, wood to rot and paint to peel; it can also damage plaster, ruin carpets and encourage mould growth. Moisture can appear as a solid (ice), a liquid or a gas (water vapour). It can originate from the outside of a building as surface runoff, ground water, ice, snow, rain or fog. It can also originate from the inside as water vapour produced by the occupants and their activities like washing, cleaning and cooking, and direct sources like houseplants, aquariums and humidifiers. Moisture can also come from plumbing leaks, open sumps and damp or leaky foundations.

In its different forms, moisture can move through the envelope in a number of ways:

• Gravity: Water running down a roof or condensation running down a windowpane shows how gravity causes water to move downward.
• Capillary action: Water can move sideways or upward by capillary action. Capillary action depends on the presence of very narrow spaces, as with lapped siding or porous materials such as concrete or soil. Think of how a paper towel wicks water.
• Diffusion: Water vapour can move directly through materials by diffusion. Diffusion depends on a difference in water vapour pressure and the material's resistance to this pressure (e.g. some paints help reduce diffusion through drywall).
• Air movement: Moisture in the form of water vapour is carried by moving air, for example, where there is an opening in the house envelope.

Far more moisture can be carried by airflow through a small hole in the envelope than by diffusion through the building materials.