Civil Engineering Reference
In-Depth Information
Existing thermal insulation
removed
Steel cladding functions as a
vapour control layer on the
correct (warm) side of the new
thermal insulation
However, water flows are bound to be
concentrated at vertical ribs, where
the run-off rate can be many times the
average. Because of this, where there
is a choice, it is preferable not to
specify vertical joints in re-entrant
corners unless there are nearby
vertical ribs to deflect sideways flow
downwards.
Horizontal projections at regular
intervals over the height of the
building will also help to throw water
clear of the façade, and therefore will
reduce water load on horizontal joints
(Figure 3.61c). Much of this run-off
falls as a curtain of water some
300-600 mm clear of the façades,
depending of course on wind
conditions.
correct side of the thermal insulation
(Figure 3.62). Factors influencing the
choice of external insulation include
the degree of overhangs of verges,
eaves and sills which can master the
additional thickness.
Hawthorne Leslies are prone to
thermal bridges in gable ends, with
ensuing condensation. In 5Ms,
sagging of the mineral fibre thermal
insulation blanket has occurred,
leaving areas uninsulated.
Most of the systems designed in
the interwar period will be deficient
in thermal insulation. Filling the
cavity is definitely not advisable,
since the insulation could easily form
a damp poultice round the steel,
accelerating the rate of corrosion.
New external insulation
Thermal properties
In the case of systems employing
steel sheet cladding, the risk in using
cavity insulation lies in the fact that
the steel forms a very effective
vapour control layer on the incorrect
(ie cold) side of the thermal
insulation, thus risking severe
condensation on the inside of the
cladding. In theory the risk can be
reduced by providing a vapour
control layer on the inner (warm) side
of the insulation, but these layers
need to function as complete barriers
and this is very difficult to achieve in
practice. Several installations have
been inspected by BRE where the
cavities of steel system dwellings
have been filled with blown fibre, but
the long term prognosis is not good.
Internal insulation is not
recommended for use where the wall
is clad with sheet steel unless there is
a ventilated cavity behind the
impermeable cladding.
Many Atholls have been insulated,
some internally and some externally,
with apparent success and no further
corrosion of the steel sheeting being
observed. The internally insulated
examples had a polythene sheet
vapour control layer on the warm side
of a 60 mm insulation quilt, replacing
the original inner lining.
Perhaps the lowest risk of
condensation is experienced using
exterior insulation. In this case the
existing steel cladding functioning as
a vapour control layer is on the
Fire
Dwellings built with industrialised
systems of construction have been
particularly identified as needing
careful consideration of likely
performance in fire situations. For
example steelwork fire protection has
been found missing in a number of
Arcal system buildings.
Large areas of cladding on the side
walls of buildings which are close to
a boundary may represent a risk of
fire spread to adjacent buildings, and
are subject to control under building
regulations. However, there is no
current control on timber cladding if
it is at least 1 m from the boundary.
Where timber cladding has been used
in buildings closer than 1 m, they
probably were built under the old
byelaws.
The performance of light
cladding in fire is difficult to predict.
There is generally little risk when
non-combustible materials are used
for additional insulation or
overcladding except where the
existing structure of the building
being overclad presents a risk.
A risk of increased vertical (and
also horizontal) fire spread has been
identified during laboratory testing of
cladding systems incorporating
combustible insulants (Figure 3.63).
Sheeted systems usually have
designed or fortuitous cavities behind
the cladding. Where the cladding is
sheet aluminium, the laboratory tests
have shown that a fire within the
Figure 3.62
External insulation of steel sheeted walls of
a BISF house. The cavity should not be filled
on any account, and the existing thermal
insulation should be removed
Risk of fire in cavity, hence the need for cavity
barriers
Fairly low
risk of fire
through
windows
Figure 3.63
Fire risk from combustible thermal
insulation in overcladding systems
