Mechanical Venting

Supply-driven and exhaust-driven mechanical ventilation systems can pressurise or depressurise internal atmospheres in different areas of the building. Supply-driven systems can be problematic as increased internal pressures can drive moist air into the attic through openings in the ceiling.

Exhaust-driven systems can de-pressurise internal areas and increase the intake of moist external air, e.g. ground moisture via a vented cavity. Unbalanced mechanical ventilation can also encourage moisture migration by creating negative pressures in the ceiling cavity. These systems need to be well designed and maintained to avoid the risk of affecting internal moisture. A balanced mechanical ventilation system, where both intake and exhaust are connected to the outside, is the preferred system and will be most effective when the thermal envelope of the building is airtight.