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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.

As buildings get more airtight, we MUST include mechanical ventilation with heat recovery. This is done with an HRV or ERV. These are also known as "fresh air machines" and they help to keep air flowing throughout a building, exhausting stale air and bringing in fresh air from outside.

HRV = Heat Recovery Ventilator: Exchanges heat from outgoing exhaust air to incoming cooler fresh air.

ERV = Energy Recovery Ventilator: Does the same thing as an HRV, but also exchanges