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Does Roof Ventilation Work?

by Isa Stralian


... A man installs a 100mm roof ventilator,…another installs a 300mm roof ventilator…both can claim to have roof ventilation….so which works?...both do, .....but how well?

How does roof ventilation work?

Just as one accepts that water will simply pour out of a hole in the bottom of a bucket, one also needs to realise that hot air will escape just as easily through a hole at the top of a roof.

Therefore there is no justification in using energy, to remove energy (heat) out of the attic / roof void by using an electric fan, whether it be powered by solar ventilator or not, when venting can be easily achieved by simply providing an appropriate hole in your roof.

What is a Roof Ventilator?

Roof ventilation begins with the hole in the roof. Not just any hole, but a hole size which is proportional to the area and with consideration to roof dynamics.
Not only is the hole required to be of proportional size, but it's position on the roof in relationship to air ingress points (eaves vents) are relatively equidistant to the roof ventilator's location, so you are venting the area between ingress and egress efficiently without a high flow rate being necessary

The device fitted to the open hole is commonly referred to as a motionless 'cupola or roof ventilator', and is designed to ensure the hole works efficiently and without any undesirable ingress of water etc especially during adverse weather conditions.

A good roof ventilator is more a roof modification, as opposed to an appendage, similar to the cupola seen at the roof peaks of gentleman's residences built in Australia at the turn of the 1900's

Most of us would prefer something with a roof ventilator with a benign appearance that is unobtrusive and blends into the overall presence of the building as opposed to a feature akin to a child on the roof waving its arms at you, especially if it's inclined to take flight during a storm as the whirly type often do.

How efficient is Roof Ventilation?

The efficiency of roof ventilation parallels a bank account, you can get it out as fast as you put it in - no more no less
The efficiency of the hole is proportional to the resistance created by the roof ventilator fitted to the hole.

An analogy being as follows, a doorway opening is cut into a wall which allows an individual to run through at considerable speed.
If a door were to then be fixed in the half closed position, then the speed of passage (efficiency) would be severely reduced.

In a common residential attic space /roof area it is best to have one large hole than several smaller holes. Put it simply one is better off by having one larger roof ventilator than two or more smaller.
Greater efficiency is having a central convergence point prior to discharge as opposed to trying to create varying flow paths that are inefficient

This is where the design of the roof ventilator will dictate the exhaust efficiency of the hole in the roof. The word 'ventilator' may be the same in describing the product, but the individual performance and life value of the product will be quite something else.

It is said that, 'In the land of the blind, the one eyed man is king', and this industry has it's share of misinformed and deceptive conduct albeit through ignorance.

Of course the homeowner is also guilty of purchasing on the ''look at the quantity, never mind the quality' disposition and then using cost to justify selection with the claim that all roof ventilators are the same.

Then you get those comparing apples with bananas on the basis that it's all fruit.
The proverb, 'you get what you pay for', applies only too well in the ventilation industry, along with 'fools are self made'

When one decides install ventilation into a building or simple vent the roof space, they do it for a purpose. An intended one time only exercise that has expectations attached.

That expectation is usually based on their level of logic, and or on the advice on ventilation systems and products.
The advice will be proportional to the experience of the advisor. This is not to say the next door neighbour doesn't know, but merely that his/her situation will be different to yours.

The truth is that, for the majority attempting to avail themselves of the benefits of roof ventilation will not realise their goal simply because the dynamics at hand have not been defined and or understood and therefore the solution implemented will ultimately parallel the understanding.
They will have spent their money and got no closer to achieving that which they desired.

The end result will only ever be proportional to the definition of the problem.

Now lets be fair here, the builder, or roofer, is not likely to be a misplaced rocket scientist who decided to downgrade their vocation. Expecting them to have a hobby in micro fluid dynamics is flattering, to say the least, but rather naive. Especially given that you're about to spend money on the basis of what is anticipated to being a desirable outcome.

Why ventilate a roof?

Ventilation, and in particular natural roof ventilation is usually sought as a means to improving comfort levels wherever possible, preferably with no running or maintenance costs. This is nothing new as all buildings prior to the 1960's had some form of ventilation present within the design of the building

There are predominantly three types of residential buildings that benefit from varying forms of ventilation and each of them will vary according to their design and geographical location

single level

double storey

multi level

all of which may have either or both, conventional, vaulted, raked or cathedral ceilings.

To the conventional ceilings, whose roof attic space consists of that between the horizontal ceiling and the roof pitch as it may be typically represented by the letter 'A' which will be either a gable or hip roof.
This is the volumetric area which is being sought to be vented,

In summer months, for the purpose of reducing the heat load gain of the day, and to assist in the evening purging of same. Temperature values in the roof space average out to approximately 68 degrees C on a day of 35 degrees, depending on the relative humidity.

Geographical locations have a great deal to do with selection of ventilation systems. In Australia southern regions have dryer conditions whereas northern have greater humidity and therefore the air mass is more sluggish, just as insulation is an advantage in the cooler south and somewhat a disadvantage in the north

Insulation has a plus/minus factor in that it provides resistance to heat transfer but also stores heat as a consequence making it more difficult to shed during the evening purge cycle.

The claim that winter venting is to remove moisture.
The removal of precipitation during frost conditions with ventilation is never successful and can be achieved easier by installing a vapour or foil membrane

The ideal venting mode is performed by atmospheric pressure and thermal load, (natural air movement) the natural rise and fall of air currents and not by wind velocity alone because that velocity will not always be there

An extension to the natural mode of venting is that you are able to vent heat out of the rooms independently by means of a closeable ceiling vent/register in each individual area,, such as bedrooms, with the rule being, open ceiling vent, open window...closed ceiling vent, closed window.

There's nothing worse than trying to get some sleep whilst the house is having difficulty in shedding it's heat load during the evening purge cycle.

It's nice and cool outside and you're sweltering in bed inside

However this refinement has another positive side in that during the winter day, ceiling vent open/window closed has heat in the roof coming back down into the room so as long as someone is there to close the register before 4pm you will have a naturally heated room that is going to cost less to keep warm during the evening.

How?, because pressure has no allegiance to direction. Heat rises only when efficient displacement can occur through dedicated ingress and or pressure variables.

To vaulted, raked or cathedral ceilings, where the void between the two building fabrics has an unobstructed common airspace is less than 300mm, and over a distance greater than 5 metres from ingress to egress.

The greatest problem with this types of building is that the heat trapped above door heights is greater in volume
These rooms each require venting direct to atmosphere by means of a suitable roof ventilator, on the roof, and a closeable ceiling vent directly underneath, so the natural flow can be controlled from within the building.

The advantage to this type of building, is that the roof ventilator can be left open when the windows are closed and thus allow the room to vent in pressure responsive mode. (where the area breathes in and out through the roof ventilator providing the roof ventilator is not a whirly rotating type

The two are not dedicated (ducted) to each other, allowing the pressure in the general void to escape out through the ventilator on the roof.

So obviously the approach to each roof type is different because the existing dynamics due to the building characteristics are unique to the situation, and it's obvious that you are not going to get this degree of assessment by a builder or roofer etc.

There are many variables that also need to be considered.

Anyone can paint a rosy picture but only an industry professional can outline the advantages/disadvantages as a consequence of introducing roof ventilation, and ventilation in general, to your home.

The misinformation or 'alternative facts'

'You need to get the moisture out of your roof'

Stay away from the entity promoting this necessity as they clearly have no idea in promoting such misconception which has no basis and is used to create fear

Where does this moisture arise from?

If it's moisture precipitation under a steel roof during frost conditions (zero air movement) then ventilation is not going to solve the problem. A moisture barrier/ foil will.

If it arises from a leak in the roof, fix the leak.

If it's vapour from the fan in the shower. In an average household of 4-5 people there is insufficient moisture to warrant concern however this would be best vented direct to atmosphere via eaves, or roof.
Establishing a trickle flow effect (via window) is necessary for a wet area.

If it's cooking moisture then you have a fat residue problem as well, so it needs to be vented direct to atmosphere.
Recirculating kitchen exhaust ventilation systems are a high maintenance scenario...and not efficient

So where is the need?

During the summer months the air in the attic is cooler than ambient during the early morning hours, so to strip it out and replace it with warmer ambient is self defeating to say the least.

Then we have 'solar powered' roof ventilator gimmick of the decade as regards to use in hot attic situations.

During the day for most months of the year you have a roof attic area bursting with energy (pressure) just looking for any hole to escape from.
So the question is 'how is this gimmick going to work when the sun disappears and you need to purge the heat from the roof during the night cycle of the summer months?'

Spend more money and buy a storage battery....and on and on and on


The gifts.....can't forget the freebies

This is my favourite

The roofing company that offers free whirlies with every roof construction or restoration.
Free! the magic word...Something for nothing,..... as some people choose to believe and cannot resist the thought of missing out.

Anyone believing that the 'free' television set, or some such, has not had it's cost factored into the overall price is deluding themselves.

Irrespective of how the deal is packaged and promoted......
There are no freebies

The comments on roof ventilation are based on Australian building technology, climate, geographical aspects and logic based on physical principles. They are not to be confused with rationale and methodology as used in wishful thinking although some aspects may seem similar.

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