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architectural notes



Roof Ventilators Explained by Isa Stralian

Bearing in mind the function of a roof ventilator is to allow and encourage the air in the roof space to refresh itself.

It is somewhat of a buffer zone against excessive heat and moisture retention and allows the area below not to be subjected to what could be considered as nature's impact in both heating and cooling.

Raked / cathedral ceilings however are another matter and require a more specialized approach regarding roof ventilation.

Let's start off with the so called wind driven Whirly, Whizbanger, Spinner, generic rotary ventilator fabricated in aluminium. Some rotate on bushes, some rotate on plastic bearings, some rotate on steel bearings.....and either way they are going to rotate easily because there is little mass (weight) to them.

The function of this type of roof ventilator is to create a negative pressure in the roof by drawing out a volume of air proportional to the rotating speed of the rotor (the spinning part)

In order for the rotor to be effective, the distance between the rotor and the spigot cannot be any more than 3mm. If it's greater then the air that would normally be drawn up from the roof by the rotor is going to be short circuited and drawn in through the gap between rotor and spigot, thereby nullifying the potential of the rotor.

So there you are in the hardware store, you spin the rotor of the roof ventilator forgetting that the potential of what you are looking at is in reality half, because the surface area of the other half has got pressure against it, and air can only escape from the area on the negative, or non pressure, side.

The best rotary ventilator in Australia was the Western Rotary which was made in steel, rotated on lubricated bushes and could suck like a hungry goat. This unit was last seen some 35 years ago and replaced by the inefficient product as seen today.

Then we have the pressure responsive roof ventilator which is made and sold under many names and referred to as a 'passive', 'static', and anything that implied that it was less dynamic than the zip zap whizbanger whereas in fact operates far more consistently than it's 'active' cousins.

The performance of this type of roof ventilator is governed by heat and pressure and the flow rate is proportional to the 'free air' area of the ventilator. Now the roof ventilator size may be a x c in size but the flow potential may well be 1/3 of that, and just like it's cousin, it's efficiency is governed by side opposite the pressure face.

The variations encountered by this type of roof ventilator are far greater and what makes them more attractive to the architect and homeowner alike is their benign appearance.

Then last but not least you have the Cupola style, as seen on many a gracious residence.
This type of roof ventilator was the precursor to those seen today although the presence of the Cupola design is quite distinctive and regarded as the finer touch on the 'crown' of the residence.

Today the Cupola is used as a services hub to where all exhaust ducting is terminated as well as performing the task of venting the roof and keeping the dwelling cool.

Usually mounted centrally and straddled across two or more planes on the roof although appears more functional than ornate as those of the past historical roofs in architecture


keeping cool

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