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Moisture can cause problems with a building in a few ways:

• Driven water - rain and snow, blown in the wind and entering cracks and holes
• Condensation - moisture turns from vapour to water as it cools (a.k.a DEWPOINT), sometimes inside assemblies
• Excess moisture in the air - steam from a shower on a wall or ceiling, moisture collects on surfaces.

An airtight building can prevent the flow of water vapour into built assemblies - water molecules and air molecules are about the same size, and many water problems are caused by moisture following air that leaks out through poorly sealed areas.

This also means that airtight buildings need to take care of moisture levels in the air - too much or too little - with mechanical means.

Furnaces, HRV / ERVs and air conditioners can be coupled with dehumidifying equipment to regulate moisture.