Civil Engineering Reference
In-Depth Information
Condensation
Condensation will occur when the
surface temperature of the wall (or,
for that matter, window or door) is
below the dewpoint temperature for a
sustained period of time. The
dewpoint will vary according to the
air temperature and the relative
humidity. Condensation occurs when
the relative humidity of the air in
direct contact with the cold surface
rises to 100%.
The two most common situations
in which condensation occurs on the
vertical external envelope are:
●
barrier', to emphasise that the
function of the layer is to control the
amount of water vapour entering the
construction. As the achieved vapour
resistance will depend at least as
much on workmanship as on the
design and integrity of the materials
used, it is not realistic to specify a
minimum vapour resistance to be
achieved for the layer as a whole,
though for the material to qualify as a
vapour control layer it should have a
vapour resistance greater than
200 MNs/gm.
Plastics films are the most usual
materials for forming a vapour
control layer in a wall construction.
Joints in a flexible sheet vapour
control layer should be kept to a
minimum. Where they occur, they
should either be overlapped by a
minimum of 100 mm and taped, or
sealed with an appropriate sealant,
and should be made over a solid
backing. Tears and splits should
always be repaired with jointing as
above.
Penetrations by services should be
kept to a minimum and carefully
sealed where they are inevitable.
Draughts of moisture laden air
through gaps in vapour control layers
are more significant than normal still
air diffusion through materials, even
if there are splits in the vapour control
layer, and it is therefore much more
important to provide an air seal than
to take elaborate precautions for
making a total seal of the vapour
control layer.
Thermal bridges, which may be so
localised as to make little
contribution to the total heat loss
from the building, can lead though to
surface temperatures low enough to
promote the growth of troublesome
moulds. Walls and their associated
windows and doors should be
designed with as continuous a layer
of thermal insulation as possible,
with special consideration being
given to junctions between elements
such as occur at window to wall joints
or at junctions between external walls
where more dense materials may
overlap or penetrate thermal
insulation.
Where a group of buildings is
showing problems due to deficiencies
Plasterboard lining
Timber studs
Breather
membrane on
sheathing
Vapour control
layer
Thermal
insulation
Sheathing
Facing brickwork
single glazed windows
●
walls with high thermal capacity
where the temperature is unable to
follow rapid changes to the air
temperature, and can often fall
below the dewpoint.
Cavity
Ventilation
Figure 1.37
Typical timber frame construction, with the
cavity, for example, ventilated by open
perpends
There are five main ways in which the
risk of condensation on or in the
vertical external envelope can be
reduced:
●
installing thermal insulation in
cavity wall construction to reduce
heat losses
in the thermal insulation, it may be
worthwhile carrying out a survey
using infrared thermography to
indicate parts of the structure needing
attention (Figure 0.41).
installing a vapour control layer
(see next section below) on the
warm side of thermal insulation to
restrict moisture which diffuses
through the insulation from
condensing on any colder outer
surface
●
ensuring cross-ventilation of any
cavity or void to remove excessive
moisture (Figure 1.37)
●
Summer condensation
During cold weather, interstitial
condensation occurring within
internally insulated solid walls is
normally prevented by a vapour
control layer on the warm (internal)
side of the insulation. However, in
spring or early summer, strong sun on
unprotected walls which have
become wetted by rainfall can drive
moisture towards the inside of the
building and through permeable
insulation or through gaps between
less permeable insulation, to
condense on the outside face of the
vapour control layer
(42)
.
Walls at risk face within 60
installing extra sheets of glazing in
the form of either double (or triple)
glazing or double windows
●
providing thermally insulated
external doors
●
In BRE experience, run-off of heavy
condensation on single glazing can be
mistaken by building occupants for,
and confused with, rainwater leakage
of windows.
°
of due
south. Moisture contents of the
external wall need not be high for
summer condensation to occur. Once
it has occurred it stays for many days,
as transfer of moisture out is much
slower than transfer in. The
condensation can be prevented by
internal heating, but this is not an
option for the summer. Since the
condensation occurs behind a
waterproof vapour control layer it is
often not noticed. The risk of summer
condensation occurring should be
Vapour control layers
It will be necessary in many cases to
provide a vapour control layer within
the construction of the wall to control
water vapour movement.
Of the various terms that are used,
'vapour control layer' is preferred to
either 'vapour check' or 'vapour
