Civil Engineering Reference
In-Depth Information
thinning of applied finishes, with
reduced durability.
When rain penetration through
panel joints does occur, its source can
be particularly troublesome to
diagnose. This is because the water
can percolate down through the many
cavities in the external envelope
before appearing on the inside of the
building, perhaps some distance from
the point or points of entry. Water
trapped within the existing external
envelope can cause serious damage to
the fabric of the building, causing
steel reinforcement to rust as well as
saturating insulation and, hence,
degrading its thermal properties.
In any face-sealed system of
cladding the jointing products used in
them (eg sealants and gaskets) will be
subject to movements etc which must
be accommodated. There are exacting
raintightness requirements for such
joints; they need to be virtually
perfect. Rainwater will be drawn in
by capillary action if any gap is less
than a millimetre or so, and any gap
greater than that will have water
pumped through it by differential air
pressure.
It may become necessary to
provide cavity trays and weep pipes
within cladding of this kind to allow
rainwater penetrating the façade to
escape without percolating to the
interior of the building.
the inside or to the outside. External
insulation is not always feasible
(Figure 2.54a). Where there is a
sound and weatherproof structure, or
where the relatively simple
replacement of weather seals in panel
joints would ensure weathertightness,
internal insulation may be achieved
relatively simply and economically
compared with external insulation
and overcladding. However, it is
rarely possible to provide for a
complete elimination of thermal
bridges at junctions of the external
wall with internal walls and floors,
since returning the insulation along
the cross-wall may not be practical
(Figure 2.54b).
Insulating the outside of the
building enables the insulation to
cover all such potential thermal
bridges. It is expected that such
insulation would be protected by
overcladding.
BRE is sometimes asked whether it
is reasonable to fill the cavities of
LPS dwellings with thermal
insulation. Although experience of
the performance of such filled
cavities is limited, nevertheless
condensation on thermal bridges has
been observed. A filled cavity also
limits visibility if optical probe
inspections on metal cleats are
required. External insulation and
overcladding is probably the best
option, albeit more expensive (Figure
2.55).
a
a
b
Thermal bridge?
b
Figure 2.54
Thermal bridging at the junction of internal
and external walls
Thermal properties
Many large panel systems were
designed and constructed before the
1973 fuel price increases, and by
today's standards may have
unacceptable levels of thermal
insulation. Depending on the precise
details of construction, an
unrefurbished dense concrete panel
wall with an internal lining of lightly
insulated plasterboard might give a
U value of around 0.85 W/m
2
K. Air
leakage rates measured in LPS
dwellings show, on average, only half
the leakage rates of other dwellings,
so this is not generally a problem,
though it does affect the tendency to
condensation
(117)
.
There are main two options
available for upgrading thermal
insulation levels in the walls of LPS
construction: applying insulation to
Figure 2.55
Overcladding on a system built high-rise
block
