Short answer:
We do not require venting for the De Boer Duo membrane.
However, it may be deemed by an HVAC engineer that venting may
be required for the building structure. If roof venting is
required, Equus Industries can assist with detailing for the
proposed vent.
Long answer:
It has been Equus Industries’ policy, and indeed is the policy
of De Boer in Europe, that venting of membrane roofs is not
required for the performance of the DuO Membrane. Interestingly
there is little information on this subject at all. De Boer’s
engineering department has recently carried out their own
research and have released a document about the subject.
However, to answer your question, we do not require venting for
the membrane. Venting is a complex issue and we have found that
there is not one single formula for this. Venting is done for
the roof space not for the membrane. Basically the research out
of Europe says that vents on a cold roof do not work because at
some point when the conditions are right humidity enters the
roof structure through the vent. It stands to reason that if
humidity is allowed to freely escape through a roof vent then it
is also allowed to freely enter through the same opening under
the right conditions. This has been our experience also. The
other issue that complicates matters is that the roof space must
allow the humidity free passage to get to a vent. This is not
always the case, or possible, without using multiple vents and
again, generally, humidity is allowed to go the other way.
Venting should only be required if activities within the space
below are such that high humidity may be a factor. If this is
the case this humidity can be dealt with in other ways which
will allow that humidity to be expelled to the outside or to
dissipate naturally through the internal structure over time.
If roof space ventilation is required, and forced or natural
flow ventilation through soffit vents cannot be introduced, then
the alternative exists to vent through the roof skin/membrane
itself. If this is the case, it is an architectural services
design issue, and should be referred to the designer working in
conjunction with an appropriate heating/ventilating services
engineer to ascertain the placement and quantity of vents .
If venting is required then it should first be done by looking
at safer alternative sites rather than on the roof proper, such
as at the parapet edge upstands as one example and it should be
allowed to cross vent from one side to the other or, where
possible, allow under soffit vents, again ensuring that there is
adequate cross flow ventilation allowance.
We have, over the years, found that vents on the roof often
leak. They are also readily vandalised. The only vent which has
provided us with a level of comfort is the Viking vent supplied
by Skellerup. This vent has a one way valve at the top which
allows the humidity to escape, and should stop any humidity
getting in. However, we have also found these can leak under the
right circumstances .They have the ability then to allow
humidity back into the roof structure under the right
conditions.
This is one of the reasons we support the use of Warm Roof
concepts, where we are applying the insulation on top of the
roof structure with the membrane being applied over this. In
essence, we would have plywood, insulation and then membrane.
This ultimately prevents condensation risks from the outside as
there is no opportunity for external moisture to affect the roof
space and gives a consistent R value to the roof skin, ensuring
humidity is always on the outside. In this case the condensation
risk is only applicable if there are uses within that space
which will generate excessive humidity issues.
The following points should be noted:-
1) It is not the roof membrane which determines whether vents
are required or not, but the construction of the roof, the roof
space between the outer skin and the inner lining and the use
of those areas immediately beneath the roof, which determine
whether venting is required or not.
2) The purpose of venting is to ensure that there is no build-up
of deleterious moisture within the space below the roof skin.
Such excess moisture can cause deterioration in framing members,
equipment/services within the roof space, and the roof underlay
itself. Excessive condensation can also cause deterioration to
internal linings beneath the roof space.
Such moisture can come from habitable and wet service areas
below the roof space or from equipment and services within the
roof space.
3) The most effective means of ensuring there is no such
moisture build-up is to create cross flow ventilation through
the internal roof spaces either by forced circulation of air, or
natural ventilation through soffit vents not exposed to weather.
4) If roof space ventilation is required, and forced or natural
flow ventilation through soffit vents cannot be introduced by
the designer, then the alternative exists to vent through the
roof skin/membrane itself. If this is the case, then the
designer must determine number and spacing of vents concomitant
with the need to ventilate the space beneath. This is an
architectural services design issue, and should be referred to
the designer working in conjunction with his heating/ventilating
services engineer.
5) Bathrooms should be vented through stacks through the roof
with the roof membrane dressed to the stacks to maintain
waterproof integrity of the roof surface. This will certainly
reduce pressure on the roof space from moisture generated within
the building.
6) It should be noted that with designs that do not have a high
moisture generation potential in areas adjacent to the roof
space, natural dissipation of moisture to the interior of the
building may remove the need for roof space, ventilation and/or
venting.
In conclusion we would reiterate that provision of vents through
a membrane roof is not required for the performance of the
membrane roof, but because the roof space beneath may be
naturally sealed from the exterior, some means of ventilation
may be required for the structure. The parameters involved in
determining the requirement for venting through the roof are
complex and relate to:-
a) internal dimensions of the roof space
b) potential for generation of moisture/vapour within/adjacent
to the roof space
c) availability of natural cross flow ventilation within the
roof space.
The decision on provision of vents must be made by the designer
who has an understanding of the structure and its use. Given the
correct information, we can assist in this by way of comment.
