Civil Engineering Reference
In-Depth Information
Aluminium frames
Aluminium is a good conductor of
heat (ie a poor insulator) and thus
aluminium frames need to
incorporate a 'thermal break' plastics
insert to reduce the amount of heat
which is transmitted to the outside of
the building (Figure 4.5). This helps
prevent condensation on the frames in
cold weather.
Since the late 1980s, aluminium
frames fitted directly into openings
have largely superseded the types
which required timber subframes.
The normal finish for aluminium
frames is a white polyester powder
coating which should be durable
and need no maintenance other
than washing. Other colours may
be available.
The current standard for
aluminium windows is BS 4873
(160)
and for finishes BS 6496
(161)
.
Most types of window can be
obtained in PVC-U: inward or
outward opening, sliding sashes, bay
windows, door frames and
conservatories. They do not usually
require subframes, but suitable
loadbearing elements will be needed
for bay windows.
Basic formulations of PVC-U are
rather brittle, particularly when
cold. PVC-U for windows is
especially formulated for improved
toughness. Other additives are used to
limit the deterioration caused by
ultraviolet sunlight. Different
manufacturers use different
formulations, and so the quality of the
PVC-U is important. PVC-U is not as
stiff as wood or metal, so profiles are
made with a fairly large cross section.
Metal reinforcement may be placed
within the profile to stiffen it. The
corner welds are a potential
weakness.
Many materials can have their
colour changed by exposure to
sunlight. It is a potential problem for
white PVC-U and imitation wood
grain, just as for paints or polymeric
finishes on window frames of other
materials. In all cases, manufacturers
need to test their materials for how
well they hold colours.
Ultraviolet light from the sun will
tend to break down the surface layer
of white PVC-U. At first it will
become rather more brittle. Then a
very thin layer will 'chalk' away. The
window will lose its shine and fresh
white pigment will be revealed. If
suitable stabilisers have been added
to the PVC-U, the loss of thickness
will be too slow to matter.
For wood-grain finishes, the
coloured film on the surface must last
the whole life of the window. It is
important that the dyes that give the
wood-grain appearance do not fade.
Ultraviolet light can make the film
more brittle, further weakening the
whole profile.
The dark colour of some
windows causes them to become
quite hot in summer sunshine.
PVC-U expands substantially when
heated, and so dark windows must be
designed so that this expansion will
not cause their opening lights
to jam. Temperatures can approach
levels at which PVC-U begins to
soften and distort. Profiles must be
resistant to this, and reinforcement is
suggested for all dark PVC-U
windows.
Windows should have profiles
made to BS 7413
(162)
, to BS 7414
(163)
if white, or to BS 7722
(164)
for a
wood-grain finish. White windows
should also be made to BS 7412
(165)
.
For further information, see BRE
Digest 404
(166)
.
PVC-U Frames
Since the 1970s the majority of the
replacement domestic window units
installed are made of PVC-U. The
reasons for this popularity include
good overall performance, with
hardly any maintenance required.
PVC-U compound is extruded into
lengths of 'profile', which are then
cut to length and assembled into
window frames. Corner joints are
heat welded; other joints may also be
welded or may use screw fixings.
Glazing materials
A brief mention of the historical
development of glass was given in
Chapter 0. Annealed glass has been,
and still is, the commonest glazing
medium, in spite of its vulnerability
to breakage. However, safety
glasses are increasingly being
specified, particularly after the
introduction of legislation on safety
in the workplace.
Ordinary laminated glass is,
thickness for thickness, no stronger
than annealed glass; its virtue is that
broken shards tend to be held by the
interlayer and, if recommended
glazing techniques have been
followed, the glass should be retained
until safe removal is possible. Wired
glass is weaker than unwired
annealed glass of corresponding
thickness but, as with laminated, the
shards tend to be held. Thermally
toughened glass is stronger than
Figure 4.6
Sealed double glazed units mounted in hardwood frames
