Roof Ventilation Work?
does roof ventilation work?
Just as one accepts
that water will simply pour out of a hole in the bottom of a bucket,
just as easily as heat through a hole at the top of a roof.
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. Heat venting can be easily achieved by simply placing an appropriate
hole in your roof. Nature does the rest.
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
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 particularly during adverse
A good motionless
cupola / roof ventilator is part of good design, similar to those seen
on the roof peaks of gentleman's residences built in Australia at the
turn of the 1900's.
A Cupola is seen
straddled across the ridge on both sides whereas a roof ventilator is
off to one side.
Most of us would
prefer something of benign appearance that is unobtrusive and blends
into the overall visual composition and not take flight during a storm
as the rotating type do.
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.
The efficiency of the hole is proportional to the resistance created
by the roof ventilator fitted over the hole.
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 erratic 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.
This industry has
it's share of misinformed and deceptive conduct.
Of course the homeowner
is also guilty of purchasing on the ''never
mind the quality, feel the width' disposition and
then using cost to justify selection.
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'
as to performance is 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. Why? because the dynamics as
exist have not been understood. Therefore the solution implemented will
simply parallel the understanding.
Money will have been spent with little or no satisfactory result.
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.
ventilate a roof?
ventilation is sought as a means to improving comfort levels and 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
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 pressure differentials, the natural
rise and fall of pressure around a building. Not wind velocity, because
that movement will not always be there.
extension to the natural mode of roof venting is the independent venting
of individual rooms as desired by means of a closeable ceiling vent/register,
eg 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 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
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.
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 roofer etc.
are many variables that also need to be considered.
can present a flow diagram with the arrows going in the desirable direction
but only an industry professional can outline the consequential advantages
/disadvantages as may exist
misinformation or 'alternative facts'
need to get the moisture out of your roof'
cautious of the entity promoting this necessity as they clearly have
no idea in promoting such misconception which has no basis in today's
building technology, 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
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 attic situations.
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 on
gifts.....can't forget the freebies
is my favourite
roofing company that offers free whirlies with every roof construction
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 themselves.
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.