The dynamics of Roof Ventilation
by Isa Stralian
One of the reasons you’re reading this post is to either plagiarize the content or you really want to know.
It’s not so much a matter of whether roof ventilation works, but it’s suitability to your needs and expectations.
Your expectations are positive benefits, by way of well being and household cost efficiency.
… You install a 100mm roof ventilator,…your neighbour installs a 300mm roof ventilator…you can both claim to have roof ventilation…both will work! …..but performance results will be different?
How does roof ventilation work in summer months?
Heat rises however the simple fundamental of air out equals air in is to be observed, much like a bank account.
Using the same size ventilator, the venting a high pitched roof is better than a low pitch because the vertical distance between ingress, (eaves), and egress (roof ventilator) is greater.
Therefore the air out is going to be at a greater velocity.
There is no justification in using energy, to remove energy (heat) out of the attic / roof void by using an electric fan, solar powered or otherwise, as 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 overall roof area.
The hole is to be of proportional size to the roof, and it’s position on the roof is also important.
The eaves vents are to be relatively equidistant to the roof ventilator location on the roof, so efficient air washing between ingress and egress occurs.
A high flow rate is not necessary, as it is the volume removal that is important, and that there is a balance between ingress and egress.
Passive Ventilation
A device commonly known as a roof ventilator is fitted over the open hole in the roof and is designed to ensure the hole works efficiently and without any undesirable ingress of water and debris
Today a good roof ventilator is a version of 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 historically do.
How efficient is Roof Ventilation?
The efficiency of roof ventilation parallels a bank account, you can get it out as only as fast as you put it in.
The efficiency of the hole is only as per 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 having one larger roof ventilator than two or more smaller.
Efficiency
It is said that, ‘In the land of the blind, the one eyed man is king’, and this industry has it’s share of one eyed individuals either misinformed or using deceptive conduct. (Flim Flam)
Of course the homeowner is also guilty of purchasing on the ”look at the quantity, using cost to justify selection, whilst telling themselves that all roof ventilators are the same.
Only in as far as allowing air to move through them,…not their suitability.
Then you get those comparing apples with bananas on the basis that it’s all fruit.
So how is it that when the result doesn’t meet expectations, it’s someone else’s fault?
Ignorance
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.
Expectations are based on level of experience, and or on the claims associated with 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 their situation is likely be different to yours.
The truth is that a lack of understanding will deprive the majority to avail themselves of the benefits of roof ventilation.
Solutions can only parallel the level of understanding.
Some will have spent their money and gotten no closer to achieving their expectations.
The end result will only ever be proportional to the total understanding 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 fluid dynamics is flattering, to say the least, but rather naive.
Especially given that you’re about to spend money on 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.
A house in Queensland, where the humidity level is higher, will need a different approach than one in Victoria.
Geographical location makes a difference.
- Single level
- Double storey
- Multi level
All of which may have either or both, conventional, vaulted, raked or cathedral ceilings.
Conventional Attic Void
To the conventional ceilings, whose roof attic space consists of that area between the horizontal ceiling and the roof pitch, will most likely be a gable or hip roof.
This is the volumetric area which is being sought to be vented.
Skillion, Raked, Vaulted Roof
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.
This type of build has little or no void, certainly not sufficient to allow for a resistance free air corridor.
Venting in these cases is done ‘room to atmosphere’.
These rooms each require venting direct to atmosphere by means of a suitable roof ventilator, on the roof, and a closable ceiling vent directly underneath, so the natural flow can be controlled from within the building.
All Weather
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
Understanding the Exercise
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 norther regions.
Insulation
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 is erroneous.
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 the wind will not always be there.
Cool outside and you’re sweltering inside
An extension to the passive mode of venting is that you are able to vent heat out of the rooms independently by means of a closable ceiling Purge Vent 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.
It’s nice and cool outside and you’re sweltering in the bedroom.
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.
Cross flow ventilation
The greatest problem with this types of building is that the heat trapped above door heights is greater in volume.
Then you have the dust factor which will be proportional to volume, the geater the volume the greater the dust ingress.
You cannot have one without the other and apart from that cross ventilation cannot be controlled.
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.
Moisture in the roof
The misinformation or ‘alternative facts’
‘You need to get the moisture out of your roof’
In today’s day and age of building technology this is not a concern.
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.
Vapour Moisture
If it’s vapour from the fan over the shower. In an average household of 4-5 people there is insufficient moisture to warrant concern.
However it would be best vented direct to atmosphere via eaves, or roof.
All fans can be ducted to within 300mm of roof ventilator and volume will take the path of least restistance…out the roof ventilator.
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…should be avoided at all cost..
So where is the need?
So let’s look at the Whirly, Zip bang type
In the summer months the air in the attic is cool during the early morning hours, so to strip it out and replace it with warmer ambient is self defeating to say the least.
And that’s not all, during the winter months you’re stripping out the dry air in the roof void and replace it with ambient moist air.
So what happened to wanting to vent the moisture out?
Then we have ‘solar powered’ roof ventilator gimmick of the decade as regards to use in hot attic situation
During the summer months you have a roof attic area bursting with energy (pressure) just looking for any hole to escape from.
So you think putting in a solar powered fan is going to be the just the answer you need.
So how is it gong to work when the sun disappears?, given that you need to purge the heat from the roof during the night cycle in the summer months.
If you install a battery backup or switchover, who is going to ensure everything is working properly?
And to top it off, a solar powered fan is not going to work anywhere near as efficiently as a mains powered unit.
So after you’ve spent the money you have one more thing to continuously monitor.
Then you have the freebies….
FREE….The most powerful marketing word in the world
It was once suggested that a high percentage of the community would line up to get a shot of the plague simply because it was free.
Then you have roofing company that offers free whirlies with every roof construction or restoration.
After it’s installed you have no choice but to accept any noise as well as maintain and or replace it.
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……
Get used to it! There are no freebies…..somehow, somewhere you’re paying for it.
The comments on roof ventilation are based on Australian building technology, climate, geographical aspects and physical principles.
They are not to be confused with wishful thinking.
Different solutions are applied to varying circumstances, given the various climatic regions in Australia.