Civil Engineering Reference
In-Depth Information
BS 7543
(74)
, on the durability of
building components, states that a
design life of a sealed unit is intended
to be 30 years, but notes that their
service life can vary between 5 and
35 years. In a survey carried out for
BRE, some 30% of units had failed
prematurely.
The most common failure is
misting between the glass panes.
Some misting failures have been
found to be the result of volatile
emissions from components of the
double glazing unit itself or from
glazing compounds, but most are a
result of the condensation of
moisture within the unit. Failures can
also include glass breakages.
Any weakness in the edge seal
which allows water or water vapour
to penetrate into the unit will cause
the dew point of the air within the
double glazing unit to rise, and
failure can ensue. Silicone based
seals are prone to failure when used
alone in single sealed systems, as
also are linseed oil putty fully bedded
systems which allow water to lie in
contact with the units.
Water may condense on the inner
face of the outer pane when the
outside temperature is low. This
misting may not be present during the
day when the building is occupied
and the temperature is higher.
Nevertheless, regular intermittent
condensation can lead to changes in
the inner surface of the outer pane
known as scumming. Condensation
between the panes of a double
glazing unit at any time is
unacceptable
(167)
.
Manufacturers' warranties
generally cover either 5 or 15 years.
However, the conditions of the
warranties should be fully checked
as failure to maintain windows could
be a cause of default.
If a double-glazing unit fails,
resulting in condensation between
the panes, it cannot be repaired and
needs to be replaced. However, the
cause of failure must first be
determined so that the same mistake
is not repeated. Once the cause has
been established, replacement can
involve either a better quality unit or
glazing in such a way that premature
failure will not recur.
It is important that consumers
demand BSI kitemarked
(183)
, or
similarly quality-assured, double
glazing units. If they are kitemarked
they will bear BS 5713, the
manufacturer's name and the year of
manufacture all stamped on the
spacer bar. This can easily be
checked as they arrive on site. To
gain a kitemark the manufacturer
must have a quality control system in
place in the factory (ISO 9000) and
have units audit tested to the
requirements of BS 5713. In future,
European Standards will replace
BS 5713 and the indications are that
these standards will provide more
stringent tests for double glazing
units.
Maintenance
Careful maintenance is needed to
preserve the appearance and
integrity of timber windows,
particularly those installed in
heritage buildings. If poorly
maintained, timber windows are
more disposed to movement,
shrinkage, cracking and distortion;
they are also more likely to be at risk
from wet rot, particularly in the case
of untreated softwood windows.
Steel windows corrode as a result
of the failure of protective zinc or
paint systems. Aluminium or PVC-U
windows may have deteriorated in
use. Painting of installed PVC-U
windows is not normally
recommended; in particular, white
windows should not be painted in
dark colours. All windows will have
associated hardware which may be
worn after many years of use or may
have corroded or seized.
A number of window designs rely
on gaskets of various kinds to ensure
adequate weathertightness, and these
will need to be inspected and
replaced if they lose resiliency or are
damaged in use (Figure 4.18).
Many traditional windows retain
the glass panes with linseed oil and
whiting putty which if inadequately
protected dries, cracks and
eventually falls out. Replacement of
the 'back putty' will generally
require removal of the glass. Beaded
glazing or gasket glazing may be
Case study
Dewpoint test on sealed double glazed units
It was clear that all the small double glazing
units in the timber windows on a housing
estate were showing condensation within the
units in a comparatively short period of time,
and further failures were daily being observed.
In particular, some small double glazing units
exposed to large amounts of driving rain were
observed to have a high rate of failure. Since
moisture was present between the panes, the
desiccant in these units was obviously
exhausted and further failures could be
anticipated.
The units were externally glazed with beads
and butyl glazing compound in a fully bedded
system. Gaps between the ends of the beads
and the frame of 2-4 mm were found on a
number of window panes and gaps had also
been left where there should have been a full
bed of butyl, in turn allowing water to permeate
through to the rebate area in contact with the
edge seal of the double glazing units.
Protrusion of the spacer bar above the
sightline of the glazing bar frame was
observed for many units.
A dewpoint test was carried out on sample
units, recording information on the room
orientation, window, double glazing unit, time
to dew formation, and dewpoint temperature.
Moisture was present within all the double
glazing units. The range of dewpoint
temperatures found for the large double
glazing units was from -28
C,
much lower than the dewpoint temperatures
for the smaller units. The average time to
failure of these larger units was estimated to
be between 2 and 5 years. The remaining
small double glazing units all had high
dewpoints, indicating that they had a high
probability of early failure.
It was concluded from the site
inspections and the tests that the earlier
deterioration of the small units as compared
with the larger units was a function of a
number of factors.
●
°
C to -45
°
The west facing small units were subject
to more driving rain than the east facing
larger units
The early failure and high dewpoints of
the small double glazing units was a
function of poor glazing technique and
inappropriate materials and methods
●
The protrusion of the units into the sight
line had allowed UV light and heat to
affect the edge seal
●
