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?
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.
analogy being as follows, a doorway opening is cut into a
wall which allows an individual to run through at considerable
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
Greater efficiency is having
a central convergence point prior to discharge
as opposed to trying to create varying flow paths that
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
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.
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.
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'
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.
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.
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.
result will only ever be proportional to the definition of
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.
ventilate a roof?
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
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
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.
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
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.
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
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.
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.
nice and cool outside and you're sweltering in bed inside
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.
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.
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
two are not dedicated (ducted) to each other, allowing the
pressure in the general void to escape out through the ventilator
on the roof.
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
are many variables that also need to be considered.
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.
misinformation or 'alternative facts'
need to get the moisture out of your roof'
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
does this moisture arise from?
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.
it arises from a leak in the roof, fix the leak.
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.
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
where is the need?
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.
we have 'solar powered' roof
ventilator gimmick of the decade as regards to use in hot
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?'
more money and buy a storage battery....and on and on and
gifts.....can't forget the freebies
is my favourite
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.
believing that the 'free' television set, or some such, has
not had it's cost factored into the overall price is deluding
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.