Roof Ventilation Work?
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?
does roof ventilation work?
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
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.
is a Roof Ventilator?
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
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.
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
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
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
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
That expectation is usually based on their
level of logic, and or on the advice on ventilation systems
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
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
all of which may have either
or both, conventional, vaulted, raked or cathedral ceilings.
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,
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,
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.
because pressure has no allegiance to direction. Heat rises
only when efficient displacement can occur through dedicated
ingress and or pressure variables.
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.
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
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
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
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
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.