Every roof ventilator is an extractor, and an extractor can only remove air that is getting in somewhere else. Fit a whirlybird with no eave vents and it will spin while moving very little. Once the inlet is sorted, the choice between wind driven, solar, powered and hybrid comes down to one question: how reliable does the airflow have to be on a still, hot afternoon?
In short
- Inlet first. Eave vents are the cheapest part of the system and the most commonly missing.
- A wind driven turbine costs nothing to run and does nothing on a still day.
- Solar suits this job unusually well, because the roof needs venting most when the sun is strongest.
- Powered wins on sheltered sites, low pitch roofs and anywhere the wind is unreliable.
- Ventilation is one lever. It does not replace insulation or vapour control.
Roof ventilation gets bought on the wrong basis more often than almost anything else we sell. People compare turbine diameters and prices, fit one, and are disappointed. The unit is rarely the problem.
The inlet is the whole game
A roof space is a sealed box with a hole in the top. Put an extractor on that hole and it can only pull out as much air as can flow in. If nothing can flow in, nothing much flows out, no matter what the box says the unit can shift.
Air comes in low and leaves high. That means eave or soffit vents at the bottom of the roof space and the extractor at or near the ridge. The two work as a pair and neither does much alone.
This is worth being blunt about because eave vents are the cheapest item in the whole system. A 20 pack of poly eave vents costs a fraction of a powered ventilator, and skipping them is what makes the expensive part underperform.
The order to buy in
- First: inlets. Eave or soffit vents, spread across the low edges of the roof space.
- Then: the extractor, sized and positioned for the space.
- Check: that the roof space is actually one space. Compartmented roofs need a unit per compartment.
The four types, compared
| Type | Runs on | Airflow when still and hot | Best for |
|---|---|---|---|
| Wind driven turbine | Wind, nothing else | Very little | Exposed sites, tight budgets, small compartmented spaces |
| Solar powered | Sunlight | Good, because sun and demand coincide | Retrofit where there is no power in the roof space |
| Mains powered | Electricity | Whatever you set it to | Sheltered sites, low pitch roofs, unreliable wind |
| Hybrid commercial | Wind, with a motor when needed | Guaranteed | Warehouses and larger buildings, BMS controlled sites |
| Static vents and eave vents | Nothing, they are openings | Passive | The inlet side, and passive cross flow |
Wind driven turbines
The whirlybird is the default because it is cheap, needs no power and has almost nothing to go wrong. On an exposed site with steady breeze it does real work for no running cost.
The weakness is specific and it matters: on a still, hot afternoon it is barely turning, and that is the hour the roof space most needs air moved. If your site is sheltered by trees or neighbouring buildings, or the roof is low pitch, be honest about how much wind actually reaches it.
Size is the other trap. A 150 mm turbine like the Bradford TurboVentura is not a whole house unit. It suits small or compartmented roof spaces, extensions and garages, and the useful rule is that two small units serving two separate compartments beat one large unit that can only draw from one of them.
Solar powered
Solar suits roof ventilation better than it suits most things, and the reason is worth understanding.
The roof space needs venting hardest on a hot sunny afternoon. That is exactly when a solar panel is producing most. Supply and demand line up naturally, which is unusual, and it means a solar unit is working at full tilt in precisely the conditions where a wind turbine is standing still.
The practical advantage on a retrofit is just as large. There is no mains connection, so no electrician, no switchboard work and no circuit to run into the roof space, which on an existing house is often the expensive part of the job. There is no running cost either.
The Bradford SolarXVENT is the 150 mm unit we stock for this, in Night Sky, low profile and suited to tiled and metal roofs.
Mains powered
A powered ventilator gives you airflow you can rely on, independent of the weather. That is the whole argument for it.
The Bradford AiroMatic at 250 mm has an Air iQ auto mode that senses conditions and runs the fan when it is worth running, or a fixed speed setting for continuous ventilation. It comes in four Colorbond colours, which matters more than it should on a visible roof.
Choose powered where the site is sheltered, the pitch is low, or the wind is simply not dependable. The running cost of a low energy motor is designed to sit well below the reduction in air conditioning load, but it is a real cost and it is the trade against a turbine.
Commercial and hybrid
On a warehouse or a large building the calculation changes, because the consequence of no airflow on a still day is a much bigger heat load and a much bigger bill.
The Bradford EcoPower is a 400 mm hybrid: a Hurricane turbine with an EC motor, so it ventilates on wind alone and switches to powered extraction when the wind will not do the job. You get guaranteed airflow with the running cost of a turbine most of the time.
It also has Air iQ control responding to temperature and humidity, and 0-10 V BMS integration, so on a managed building it can be part of a ventilation strategy rather than an independent hole in the roof.
How many, and where
Three things decide the count, and none of them is the size of the house.
- Is the roof space one volume? Hips, valleys, bulkheads and split levels often divide a roof into compartments that do not share air. Each compartment needs its own extractor and its own inlets, or it does nothing.
- Where is the extractor sitting? Hot air collects high, so units go as close to the ridge as the roof allows, not halfway down a plane.
- Is the inlet area big enough? Extract capacity beyond what the inlets can feed is capacity you have paid for and will not get.
What the code asks for
Roof space ventilation is not only a comfort item. NCC 2022 Housing Provisions clause 10.8.3 sets roof space ventilation requirements in climate zones 6, 7 and 8, with opening areas that vary by roof pitch and a minimum roof space height of 20 mm.
The reason it sits in Part 10.8, Condensation management, rather than in the energy provisions, tells you what it is for. Trapped moist air in a roof space is what drives the condensation that rusts sheeting and soaks insulation. We covered the diagnostic side in diagnosing a sweating roof, and the design side in condensation in metal roofs.
Ventilation on its own is not the whole answer to condensation. It sits alongside cutting the moisture at source and controlling the vapour path. Fitting a ventilator and calling the problem solved is a common and expensive mistake.
What goes wrong
- No inlets. The single most common fault. The turbine spins and the roof space stays hot.
- One unit on a compartmented roof. It ventilates the compartment it sits in and nothing else.
- Choosing wind driven on a sheltered site. Trees and neighbouring buildings take the wind you were counting on.
- Insulation blocking the eaves. Batts pushed hard into the eave line seal the inlet you just paid to create.
- Polymer vents in a bushfire area. Check the site rating. The metal eave vent exists for exactly this.
- Treating ventilation as a condensation cure. It is one of the levers, not the fix on its own.
We supply it, and we install batts, wall wrap and sarking
AWBM supplies the whole roof space package across Australia: eave vents, wind driven, solar, powered and commercial ventilators, plus the insulation and membranes that go with them. Ventilators are a supply item, your roofer cuts them in. Batts, wall wrap and sarking we install.
Or browse accessories, ceiling insulation and building membrane.
Frequently asked questions
Which roof ventilator is best: a whirlybird, a solar vent or a powered fan?
It depends on how reliable the airflow needs to be. A wind driven turbine is cheapest and free to run but does almost nothing on a still day. Solar performs when the sun is out, which is when the roof needs it most. Powered gives airflow regardless of weather and suits sheltered sites and low pitch roofs.
Do I need eave vents as well as a roof ventilator?
Yes. An extractor can only remove air that is entering somewhere else. Eave or soffit vents let air in low so the ventilator can push it out high. Without them the unit moves very little air.
How many whirlybirds do I need?
It depends on whether the roof space is one volume or several. Hips, valleys and bulkheads often split a roof into compartments that do not share air, and each compartment needs its own extractor and inlets. Size of house alone does not answer it.
Is a solar roof vent worth it?
The timing works in its favour: the roof space needs venting most on a hot sunny afternoon, which is when the panel is producing most. On a retrofit it also avoids running a circuit into the roof space, which is often the expensive part.
Do roof ventilators help with condensation?
They help, as one lever of several. Moving trapped moist air out of the roof space reduces condensation risk, but it works alongside cutting the moisture at source and controlling the vapour path, not instead of them.
Does the NCC require roof space ventilation?
NCC 2022 Housing Provisions clause 10.8.3 sets roof space ventilation requirements in climate zones 6, 7 and 8, with opening areas that vary by roof pitch and a minimum roof space height of 20 mm. Check the clause against your building.
Can I use plastic eave vents in a bushfire area?
Check the site rating first. Metal eave vents exist for bushfire applications where a polymer vent is not appropriate, so this is not a decision to make on price.
What size ventilator do I need?
A 150 mm unit suits small or compartmented spaces, extensions and garages. Larger spaces need a bigger unit or several, and commercial buildings usually justify a 400 mm hybrid. The inlet area has to match whatever you choose.
What does AWBM supply and install for a roof space?
We supply eave vents, ventilators, insulation and membranes across Australia. We install batts, wall wrap and sarking. Ventilators go through the roof sheeting, so your roofer fits those. Send the roof details and we will work out the combination and quote it.
Sources
- NCC 2022 Housing Provisions, Part 10.8 Condensation management, clause 10.8.3 roof space ventilation
- BlueScope Corrosion Technical Bulletin CTB-11, Roof Condensation and Insulation, October 2022
- Product specifications, sizes, colours and control features from the CSR Bradford product data for the ventilators listed above
General information. Ventilator performance depends on the building, the site exposure and the inlet area available, so treat the comparison above as how to choose rather than a performance guarantee. Bushfire applications and NCC ventilation requirements vary by site and climate zone, so confirm both against your own project.