FAQ – How effective is a whirly bird at cooling my roof?

I get enquiries very often about installing a ‘whirly bird’ (roof ventilator) to cool the house.

Whirly birds do not do any cooling. That is: they will not lower the temperature and more than the ambient.

Then just ventilate.

But ventilation is better than no ventilation and if installed correctly, they can provide airflow (sucking in cooler air and expelling warmer air).

roof attic ventilation diagramThe diagram shows how a whirly bird can ventilate the ceiling space. Whirly birds just by themselves are not very effective. They require vents (either in the eaves or in the ceiling of the house) to let cooler air in as the warm air is expelled through the turbine. So, if you are getting whirly birds installed, make sure that there are adequate ceiling vents to allow air replacement.

Also, whirlybirds are less effective if you have no sarking under the roof. A closed attic space is needed to force the whirly bird to draw air through the vents. Otherwise the whirly bird will just draw air through the gaps in the roof tiles locally and not produce much cross ventilation.

So, how effective a whirly bird will be for your roof depends on what sort of roof you have…

…if you don’t have sarking under the roof tiles or a decent roof overhang to install eaves vents – then you may be wasting your money on whirly birds.

The key is in installing a ventilation system containing whirlybirds rather than just installing whirlybirds by themselves,.

 

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 Update:  16th October 2013

Last week, we put in some “new’ roof ventilators on a roof in Greenacre, Western Sydney.

They were not whirly birds. These were the first “Universal Tile Ventilators” we have installed. It was an easy installation and we are waiting for the client (who sourced the ventilators himself and asked us to put them in) to give us some feedback on it’s effectiveness in providing cross ventilation to his ceiling cavity.

Update 18th October 2013

We had a hot windy day on yesterday and my Greenacre client reckons that his house is cooler due to the Universal Tile ventilators….

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12 Responses to "FAQ – How effective is a whirly bird at cooling my roof?"
  1. would the bathroom ceiling ventilator would help even if switch off? or is it better to have a eaves vent? and how many vents? 2 on each side of the house? or that is too much?

    • Hi Roger,

      Like most things in life, there is no definitive answer to your questions.
      A bathroom ceiling ventilator – if it is ducted through the ceiling cavity and out the roof, will let hot air escape from the bathroom to the outside even if switched off.

      Eaves vents allow air to enter the ceiling cavity only. They have no effect on air circulation in any inside rooms/spaces. They work in conjunction with whirly birds (usually 2 vents per whirly bird) to circulate air through the ceiling cavity area.

      Hope this helps.

      Jack

  2. Whirlybird and twista seem to do the same job with only minor differences. Is one more effective and better made than the other ? When being installed will the tradesman also instal eave vents ?

    • Hi Pat,

      I have not done any testing between the brands. So, I don’t know which ones are more effective in drawing the hot air out.
      A tradesman who understands how whirly birds work (and reads the instructions from the manufacturer!) will allow for installation of eaves vents.

      There is a catch.

      There is no sure way of easily finding out if the eaves are fibre cement sheeting or asbestos. (Although the age of the house will give a clue). Tradesman should not be cutting holes in asbestos eaves.

      That is why the ‘universal tile ventilators’ (even though they are more expensive per pair) are perhaps the better solution if you have asbestos eaves.

      Hope this helps.

      Jack

  3. We have a house with raked (cathedral) ceilings. I am not sure of the setup of the roof (insulation etc.), but we have to rely a lot on using air-conditioners to keep the house cool. Would whirlybirds be useful in helping to expel that hot air that accumulates in our ceiling and make us a little less reliant on using airconditioners? It’s only a one-storey house as well.

    • Hi Jon,

      Whirlybirds are designed to vent the ceiling cavity – rather than the living space. Air conditioning is still the most effective cooling for living spaces.
      However, Edmonds make a cathedral ceiling venting system that can help vent living spaces. Details at: http://www.edmonds.com.au/~/media/edmonds/files/cathedral-ceiling-vent-kit.pdf

      Edmonds also do ‘Odyssey’ – which is a more high tech ventilation system for living spaces – although this can almost be a expensive to install as an air conditioner.
      http://www.edmonds.com.au/home-ventilation/living-space/odyssey

      Hope this helps.

      Jack

      • I have a double storey home with a large 3 storey open area with west facing full length windows. Area gets frightfully hot in summer even with window shades and double glazed coated glass. Problem is the hot air rises to the top and can’t be ventilated. We are debating between Odyssey to suck the air out or putting a fresh air filtered intake to the upstairs HVAC and run fan mode to draw in outside air (we have multiple ducted vents to the open area) interested to hear your thoughts on what’s more effective

        • Hi Michael,
          I may be good at fixing roof leaks.
          But I am hopeless trying to be an air conditioning engineer.
          I can only offer a few comments that may be helpful…

          … When I get into my ute on a hot day, I open the windows or doors to let the trapped hot air out.
          Then as I drive off, I put the A/C on and turn the vents to recirculating.
          The theory is that it is more efficient to cool recirculating air than it is to try to cool a stream of fresh air.

          … If the day is not too hot, I can just open both windows and this cools the cab down without A/C. This is what the Odyssey is supposed to do to the house (although the analogy will be with the ute windows both opened a fraction).

          If the day is stinking hot, then the only way to keep cool is air conditioning.
          If the ambient outside air is not too hot, then just leaving the windows open may create a draft and allow the hot air to escape…

          Jack

  4. We have a 30 year old cedar one storey home with cathedral ceilings in the lounge. We have whirly birds on the roof and the ceiling vents are in the lounge. It is very hard to heat the home.
    Can we cover the vents so that we stop heat loss in winter.

    • Hi Glenys,

      Most people have the opposite problem – too hot in the summer time.
      You can cover the vents to stop the warm air from escaping in the winter.

      Also, some cathedral ceilings do not allow for space to insulate the roof effectively (or maybe substandard insulation has been installed). This will make a huge difference.

      Hope this helps.

      Jack

  5. Just found out that out bathroom exhaust fan are not connected to anything. All the steam from the bathroom released into the ceiling cavity. My question is I have someone suggested to install a whirlybird on the roof. Is this enough to remove the steam from the ceiling without connecting it to an aluminium flexible duct?

    • Hi Cynthia,
      A ‘closed’ system like using a flexible duct connection is the optimum solution.
      A whirly bird installed vertically above the bathroom exhaust is a good compromise – since warm air always want to travel upwards and a whirlybird is the least form of resistance for the escape of the moist air.

      Jack

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