Insulate Magazine Issue 11

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- Hygrothermal performance of building components
and building elements - may appear to
show no risk. However, the fact that BS 5250
does not support the construction type means
that we believe insulation manufacturers should
label any condensation risk analysis for a hybrid
flat roof as ‘not recommended’.
Consider as well another reason to doubt the
appropriateness of this analysis of a hybrid flat
roof solution. Section 4.2 of BS 5250 states: ‘BS
EN ISO 13788 considers only the risks
arising from the diffusion of water vapour through
the building fabric; it does not take account of
the much greater risk of condensation occurring
as a result of air leakage, which
transports water vapour through gaps, joints and
cracks in the building fabric.’
By its very nature, a hybrid roof that features
insulation fitted between timber joists
introduces the potential for air gaps. Section
A.3 is clear that, ‘the rate at which moisture is
transported by air movement, where it occurs,
is much greater than that of transportation by
diffusion.’ A condensation risk analysis cannot
adequately allow for the standard of
workmanship, and a high level of workmanship
in itself would not mitigate potential issues.
If anybody is to make a judgement on the
levels of workmanship, and whether it makes the
construction method any more acceptable for a
particular project, it should be the
designer, building inspector or end user. The
insulation manufacturer can advise on the
interpretation of a condensation risk analysis
for a hybrid roof, but first and foremost should
promote the fundamental principles of good roof
design outlined in BS 5250.
the correct VCL position, some might ask if a
hybrid roof is acceptable if the VCL is moved to
ceiling level behind the internal plasterboard
finish. While technically that might satisfy the
basic requirements, we would urge practical
thinking: are light fittings or other services going
to be installed in the ceiling? Can the continuity
of the VCL – vital to its performance – be
guaranteed?
It only takes the building’s next owner to want to
change something and that VCL could be
compromised as part of any works to the ceiling.
Will anybody make sure it is restored to its
original condition? If a VCL cannot be relied upon
to mitigate the risk of condensation then the roof
design cannot be considered appropriate, and
there are precious few alternative
solutions.
Despite the best efforts of many to promote the
advantages of service voids – airspaces between
ceiling and VCL, where services can be installed
and the ceiling altered without risking the
integrity of the VCL – few seem willing to employ
them. There are several reasons why: extra time
and materials to construct, restrictions on
headroom etc; all of which serve to highlight why
it is best to keep things simple.
One of the positive aspects of a warm roof is that
the VCL can be installed with the
confidence that it will perform for as long as the
roof performs – hopefully for the lifetime of the
building. That’s why we referred to it as a good
example of fabric first construction, and it’s why
we believe it should always be preferred over a
hybrid alternative.
Considering one of those principles is to ensure
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