What is a whirlybird and does it actually do anything?

A wind-driven roof ventilator extracts air from the roof space. It does something. The question is whether what it does is worth what it costs relative to the alternatives, and for most homes the answer is that insulation comes first by a wide margin.

What it helps with

• Reducing peak roof space temperature on a hot day, which reduces the heat load on the ceiling below.
• Moving moisture out of the roof space, which matters more than the heat in cool and temperate climates.
• Reducing the risk of condensation on the underside of metal roofing.

What it does not do

• Cool the house directly. It ventilates the cavity, not the rooms.
• Replace insulation. A well-insulated ceiling isolates the living space from the roof space temperature far more effectively.
• Work without an inlet. Extraction with no low-level intake, usually eave vents, achieves very little, and this is the most common installation error.

The order that makes sense

Insulate the ceiling properly, seal the gaps around downlights and exhaust fans, make sure bathroom and kitchen extraction discharges outside, and ensure the eaves are not blocked by insulation. Then, if the roof space still runs hot or damp, ventilation is worth considering.