November 2018

Perfectly Pitched
THE RIGHT INFORMATION TO
CONTROL CONDENSATION
In his latest column, pitched roofing expert and consultant John Mercer – writing on behalf
of EDILIANS (formerly Imerys Roof Tiles) – explains how to control and prevent
condensation in pitched roofs.
As we strive to improve the energy
efficiency of our homes, greater levels of
thermal insulation and air tightness reduce
the average temperatures within the roof
structure. Warm, moist air generated within the
living spaces can find its way through the ceiling
into the cold roof space. Controlling condensation
within our buildings remains one of our key goals
to ensure we live and work comfortably, healthily
and to prevent damage to the building fabric.
The ability of air to hold moisture reduces as it
cools and it will then deposit the moisture onto
cold surfaces in the form of condensation. For
example, in a two-storey house with a floor plan
of 100m², there is around 420 cu metres of air,
which, at 20 degrees, could potentially hold up to
8.4 litres of water vapour. If this warm air passes
through the ceiling into the roof space and cools
down to 10 degrees, it can then only hold 3.4
litres of water vapour. This means that 5 litres of
water will be deposited somewhere if it is not
allowed to escape from the roof space. It may not
be as dramatic as that in practice, but it
illustrates the potential risks.
The development of vapour-permeable and airpermeable
roofing underlays has been greatly
beneficial in helping to prevent harmful levels of
condensation from building up in the roof space.
However, it is important to use these products
correctly, in accordance with the guidance given in
BS 5250 and with the information contained in the
underlay manufacturer’s accreditation certificate.
Two ways to control the risk
Essentially, in simple terms, there are two ways
we can control the risk of condensation build up
in the roofspace; i.e we either
prevent water vapour from
reaching the loft space in the
first place, or we remove it once
it gets there before it has a chance
to build up to harmful levels.
“We either prevent
water vapour from
reaching the loft space
in the first place, or we
remove it once it gets
there before it has a
chance to build up”
To prevent the water vapour passing from the
living space into the cold roof space, it is
necessary to install effective vapour barriers. It is
difficult, if not impossible to construct a totally air
and vapour-tight ceiling, so British Standard BS
9250 gives guidance on minimising air leakage
through junctions and penetrations such as light
fittings, loft hatches etc. to create a ‘continuous’
ceiling. We now have well-documented methods
to achieve this in new buildings, though it is
generally more difficult in existing buildings.
Greater energy efficiency is achieved and the risk
Left: John Mercer, Roofing Consultant.
Below: Warm to cold illustration.
of condensation reduced if we
prevent air leakage through the
ceiling. However, where this is not
practical we must use adequate
ventilation to remove the water vapour from
the roof space.
“We now have welldocumented
methods
to achieve this in new
buildings, though it is
generally more
difficult in existing
buildings”
Minimum requirements: BS 5250
BS 5250 sets out the minimum requirements for
roof space ventilation. For example, in the
average house with a cold loft, there should be a
minimum of a 7mm continuous ventilation gap at
eaves when using a vapour permeable underlay
with a normal, or ‘discontinuous’ ceiling. With all
the well documented benefits of dry fix systems,
it makes perfect sense to supplement eaves
ventilation with high-level ventilation using a dry
ridge system. Although eaves to eaves ventilation
works well in theory, it relies on external air
movement and may not work so well in practice if
the house is too close to adjacent properties or if
the loft is full of items blocking the air flow.
Alternatively, air-permeable underlays can be used,
in many circumstances without any traditional
ventilation, though it is important to follow the
manufacturer’s installation guidance.
38 TC NOVEMBER 2018