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Cladding and Curtain Wall Construction 449
The open drained joint is designed to collect most of the rain in the outer zone of the joint
in front of the baffle, which acts as a barrier to rain that may run or be forced into the joint
by wind pressure. The baffle is hung in the joint so that to an extent there is a degree of air
pressure equalisation on each side of the baffle due to the air seal at the back of the joint.
This air pressure equalisation acts as a check to wind-driven rain that would otherwise be
forced past the baffle if it were a close fit and there were no air seal at the back of the joint.
At the base of each open drained joint, there is a lead flashing, illustrated in Figure 7.28
and Figure 7.29, which serves as a barrier to rain at the most vulnerable point of the intersection
of horizontal and vertical joints. As cladding panels are fixed, the baffle in the upper
joints is overlapped outside the baffle of the lower units.
Where there is a cavity between the back of the cladding units and an inner system of
solid block walls or framing for insulation, air seals can be fitted between the frame and
the cladding units. It is accepted that the system of open joints between units is not a
complete barrier to rain. The effectiveness of the joint depends on the degree of exposure
to driving rain, the degree of accuracy in the manufacture and assembly of the system of
walling, and the surface finish of the cladding units. Smooth faced units will tend to encourage
driven rain to sheet across and up the face of the units, and so cause a greater pressure
of rain in joints than there would be with a coarse textured finish, which will disperse
driven rain and wind, and so reduce pressure on joints. The backs of cladding panels will
tend to collect moisture by possible penetration of rain through joints and from condensation
of moisture-laden air from outside and warm moist air from inside by vapour pressure,
which will condense on the inner face of panels. Condensation can be reduced by the use
of a moisture vapour check on the warm side of insulation as a protection against interstitial
condensation in the insulation and as a check to warm moist air penetrating to the cold
inner face of panels. Precast concrete cladding panels are sometimes cast with narrow
weepholes, from the top edge of the lower horizontal ribs out to the face, in the anticipation
that condensate water from the back of the units will drain down and outside. The near
certainty of these small holes becoming blocked by wind-blown debris makes their use
questionable.
Attempts have been made to include insulating material in the construction of precast
cladding, either as a sandwich with the insulation cast between two skins of concrete
or as an inner lining fixed to the back of the cladding. These methods of improving
the thermal properties of concrete are rarely successful because of the considerable
section of the thermal bridge of the dense concrete horizontal and vertical ribs that are
unavoidable, and the likelihood of condensate water adversely affecting some insulating
materials.
It has to be accepted that there will be a thermal bridge across the horizontal support
rib of each cladding panel that has to be in contact with the structural frame. The most
straightforward and effective method of improving the thermal properties of a wall structure
clad with precast concrete panels is to accept the precast cladding as a solid, strong,
durable barrier to rain with good acoustic and fire-resistance properties and to build a
separate system of inside finish with good thermal properties. Lightweight concrete blocks
by themselves, or with the addition of an insulating lining, at once provide an acceptable
internal finish and thermal properties. Block wall inner linings should be constructed
independently of the cladding panels and structural members, as far as practical, to reduce
interruptions of the inner lining, as illustrated in Figure 7.3.