Civil Engineering Reference
In-Depth Information
The Solution
Two possible solutions exist, at least in theory. Firstly, prevent the moisture-laden air en-
tering the roof space or, secondly, ventilate the moisture-laden air before it can condense
and cause any harm. Examining the first option, this at first seems the most simple meth-
od, and in theory this can be achieved by placing a vapour barrier of polythene or similar
vapour proof sheeting immediately above the ceiling finish, yet beneath the insulation.
This would contain the moisture-laden air within the building below. In practice, however,
there are numerous small holes through this vapour barrier in the form of electrical ser-
vices, hot and cold water services and soil and vent pipes. There is also the problem of ef-
fectively sealing joints between the sheets of polythene used. The major drawback to this
option in the writer's opinion is that a vapour barrier immediately above the ceiling finish
effectively traps some of the vapour, and can lead to unsightly mould growth which can be
extremely difficult to eradicate once it has appeared. It is virtually impossible to contain
moisture within rooms such as bathrooms, shower rooms and kitchens and therefore this
first option of a vapour barrier is not a practical one.
We are therefore left with the option of ventilating the roof void. The British Standard
5268: Part 3 no longer sets out minimum requirements as this is considered the responsib-
ility of the 'building' designer as distinct from the roof structure designer. BS 5250 gives
guidance on prevention of condensation in roofs. NHBC cover the subject in clause 7.2/
S11(a),
Ventilation.
The Building Regulations themselves in approved document F clause
f2,
Condensation
, also require the designer to take account of the possibility of condensa-
tion within the roof space. The TRA technical bulletin gives details of how this ventilation
should be provided in conjunction with trussed rafter roofs.
