Question: I have a project specified with De Boer Duo membrane. Does this membrane require venting?
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.
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.