Description

BRANZ was called into a Dunedin school where too much ventilation caused excessive condensation in a large roof space. Understanding why this happened points to warm roofs being better in some locations.

Ventilating roofs is generally considered to be a good approach to managing moisture in the roof space and to make the overall roof assembly more robust, reducing the risk of condensation.

A valid question is, can there be too much roof ventilation? We recently saw a school where too much ventilation caused excessive condensation in a large roof space. To find out why this happened, we looked at climatic conditions, location, orientation and building design.

Scope

This article includes:

• Building design, orientation, solar gains
• Roof fitted with wind-driven ventilators
• High risk of condensation in this case
• Air condenses on cladding and underlay
• Extractor fan brings in more moist air
• Air barrier may help but not enough
• Even passive vents could be risky here
• Warm roof a better solution here
• Ventilating cold roofs usually helpful