Roof Ventilation or Whirly Birds - Do They Work?

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Roof Ventilation or Whirly Birds - Do They Work?

Postby Johnny » Thu May 05, 2011 4:33 pm

I was just talking with my new boss and he mentioned he is going to install some Twista roof vents, it reminded me of this forum, it's been a while!

I then thought maybe I should put some in at home??
Does anyone have experience with these? Before and after? Did they work?
Are they energy efficient? Did they reduce the bills?
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Re: Roof Ventilation or Whirly Birds - Do They Work?

Postby Gordon-Loomberah » Thu May 05, 2011 5:26 pm

I put in Supavent last year, and I have another for the other end of the roof ( 2 are recommended for this size roof), but havent gotten around to installing it yet.
There is certainly some hot air going up out of the hole, but I've come to the conclusion that the whirly bit is more of a triumph of marketing over reality- the hot air was going up and out at about the same rate whether or not it is spinning. I reckon just as much air is being forced into one side of it as exits the other side when the wind makes it spin, so no real "suction" of extra hot air from the roof cavity.

So, I decided to modify it with some cut up pieces of 1.5l plastic mineral water bottles to make 3 makeshift blades that we glued to the spokes in an attempt to force more air upwards. It appears to make a slight improvement, as in the air does rise faster when it is spinning... noticeable, but not a huge improvement. We tested it indoors with feathers to measure updraughts ;) . I think that more significant modifications could be done to make it shift more air with better blades, but thats a long way down my to do list.

I didn't do any before and after temperature measurements in the roof space, so nothing quantitative, other than, yes some hot air was venting and cooler air was being drawn up through the access hole, so it would have lowered the temperature a bit. Once I get around to putting the other one up I may log some temperatures outside air temp from Wx station, vs roof space vs inside on says of similar weather... but thats also a way down the to do list... and depends a bit on how much the thermocouples for my data logger might cost.

I installed one on my power shed (garden shed from Bunnings), and that definitely made it cooler in there, but it is a much greater vent size proportionally, than what is on the house.
They look like they might allow rain in, but having seen it in a 240mm/hr downpour, I'm happy to say it doesnt leak. Also, mine are very quiet as they have decent bearings, we just dont hear the one on the house roof, no matter how fast it spins.

A fan would definitely improve its effectiveness, however, Its hard to know if the power used by the fan would be offset by less AC use in a building. Maybe one of the new low power ceiling fans would do the job nicely at minimal power cost. Actually a fan into a covered vent without the spinning bit would be better.

Is it energy efficient? for sure, it uses none!
Does it lower the bills? hard to say, as havent paid an electricity bill since 1991 :)

I've just thought of another example of the whirly bit not doing anything useful- I have a smaller one on my composting toilet vent pipe. Sure it spins in a breeze, but didnt do much in the way of sucking up the fumes. Painting the PVC vent pipe black was a lot more effective in drawing air upwards, but only worked when the sun was out, so I now have a 24VDC 3.6W fan running continuously there. Since it's venting the composting loo chamber, its a damn good test (with my nose) of how much air is being drawn upwards by the whirly bit... I reckon it is bugger all!
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Re: Roof Ventilation or Whirly Birds - Do They Work?

Postby Johnny » Thu May 05, 2011 6:17 pm

:lol: Love the "Loo Scent Test" :lol:
Sounds like you got all the gadget measuring too! What about the old feel test - did you reckon it felt cooler since you had it installed?
Is it energy efficient? for sure, it uses none!

I wondered if in winter it might be better to keep the roof space hot? - Maybe you would need to use more heating if the roof space got cooler? That wouldn't be very efficient even thought the whirly bird is not costing anything to run.
Also as you point out it may only be effective if you have a fan installed - and then it would cost something (unless it it a solar powered fan :idea: ).
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Re: Roof Ventilation or Whirly Birds - Do They Work?

Postby Gordon-Loomberah » Thu May 05, 2011 7:16 pm

Johnny wrote::lol: Love the "Loo Scent Test" :lol:
Sounds like you got all the gadget measuring too! What about the old feel test - did you reckon it felt cooler since you had it installed?


All I can really say is that it made a big difference in the power shed, but in the house... well not that I could tell, given the variation in outside temperature from hour to hour and day to day, any effect was lost in the noise for a"feel test". I'm sure some thermocouples could pick the difference though:) I have a lot of insulation under the roof and on top of the ceiling, about R5.9 total, so that will partly mask any effects, unlike the uninsulated power shed.

I wondered if in winter it might be better to keep the roof space hot? -


Not sure how much difference it is going to make, but we did plan to block it off once it got cold...we just haven't been up there to do it yet. There is the theory that keeping the air moving in winter helps keep the roof space drier too.

Also as you point out it may only be effective if you have a fan installed - and then it would cost something (unless it it a solar powered fan :idea: ).


There are solar powered fan vents for sale (very expensive though!) which have a small solar cell embedded in their tops, and they look pretty nifty too, like big mushrooms :) but a similar arrangement could be easily done DIY for much less cost. If you are producing your own solar power anyway, a few tens of watts for a few fans isnt a big cost, and I reckon a small fan will shift many times the hot air vented by convection... and the undetectable "whirly gig effect" ;)
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Re: Roof Ventilation or Whirly Birds - Do They Work?

Postby Bthree » Fri May 06, 2011 10:49 am

Thinks about a computer fan (2-5watts) and a plug pack
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Re: Roof Ventilation or Whirly Birds - Do They Work?

Postby variflex » Fri May 06, 2011 4:59 pm

I have no acutal measurements of temps or data however, when using our ducted A/C I would always set the temp to 18 degrees as thats the lowest setting but it never really felt cold, I then installed one whirly bird and noticed the A/C started feeling cooler, I then fitted a 2nd whirly bird and now find that i am setting to A/C to 20 -24 as it now feels cold when set at 18 degrees, all this was Before I had my roofing insualtion installed,
A few friends of mine fitted whirly birds and said that it made no difference however they already had insulation in their roof
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Re: Roof Ventilation or Whirly Birds - Do They Work?

Postby Gordon-Loomberah » Fri May 06, 2011 5:14 pm

variflex wrote:...all this was Before I had my roofing insualtion installed,
A few friends of mine fitted whirly birds and said that it made no difference however they already had insulation in their roof


This seems to be their main feature, they work well in poorly or uninsulated buildings like corrugated iron factory sheds, garden sheds etc, but are much less effective in well insulated buildings.
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Re: Roof Ventilation or Whirly Birds - Do They Work?

Postby 470rigby » Sun May 08, 2011 1:28 pm

Gordon-Loomberah wrote:I've come to the conclusion that the whirly bit is more of a triumph of marketing over reality- the hot air was going up and out at about the same rate whether or not it is spinning. I reckon just as much air is being forced into one side of it as exits the other side when the wind makes it spin, so no real "suction" of extra hot air from the roof cavity.


Can't but agree! A simple weatherproof vent will work just as well. IMO..they are a solution in search of a problem, but marketed using the "shotgun" or "one size fits all" approach; meaning most people install them without thinking through what it is that they are trying to acheive....eg.why would you install one in a cathedral ceiling in winter when you are trying to keep the room warm??...or in a summer when it is hotter outside???

An ununsulated dark colourbond clad shed on a hot day...yes. To cool down a hot house in the evening (after the outside air temp drops below the inside temp) after a hot day..yes.
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Re: Roof Ventilation or Whirly Birds - Do They Work?

Postby Johnny » Tue May 10, 2011 3:24 pm

Guys thanks for all the comments guys, it's given me plenty to think about, it looks like there are a number of options for roof vents.
Gordon-Loomberah wrote:I think that more significant modifications could be done to make it shift more air with better blades

The ones I was looking at have many blades on them - I grabbed a diagram off their site:
ImageImage Credit: Twista Roof Vents
So I guess these might work a bit better than ones with no blades.

Gordon-Loomberah wrote:There are solar powered fan vents for sale (every expensive though!) which have a small solar cell embedded in their tops, and they look pretty nifty too, like big mushrooms :) but a similar arrangement could be easily done DIY for much less cost.

I like the sound of those solar vents - other than the price! I checked some out on-line but the price still does seem a little high, I guess they may be a little cheaper in time (with the increasing amounts of solar panels being produced), but I reckon they would do the trick - also they don't operate at night which has both positive and negatives about it.
Gordon-Loomberah wrote:Post by Gordon-Loomberah » Fri May 06, 2011 4:14 pm
variflex wrote:...all this was Before I had my roofing insualtion installed,
A few friends of mine fitted whirly birds and said that it made no difference however they already had insulation in their roof

This seems to be their main feature, they work well in poorly or uninsulated buildings like corrugated iron factory sheds, garden sheds etc, but are much less effective in well insulated buildings.

And I guess this is the other thing for me to checkout - roof insulation. Since the whole government scheme went bad it's been given a lot of press - I guess it''s possibly worth looking into this too.
Meanwhile the boss gets them installed here - I'll see how they operate and make a decision before next summer!

Thanks again for the input guys.
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Re: Roof Ventilation or Whirly Birds - Do They Work?

Postby variflex » Tue May 10, 2011 5:51 pm

Just on a side note, I went for the whirly birds with the clear perspex vent to allow light into my ceiling, my theory was that the light within the roof would help keep vermon out and also assist me when i have to get into the roof space.
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