Windows and Heat-loss
How to increase thermal efficiency non-destructively
||A typical Georgian sash window awaiting full repair
and restoration. Care is needed to preserve the original
glass where it exists, particularly if joiner repairs are
required to the astragal mullions.
||A typical Georgian shutter assembly with timber
decayed at the joints; painting it has only accelerated
It is often taken for granted that buildings
with double glazing are automatically more
sustainable, and therefore more ecologically
sound, than those without. Questions
remain, however, regarding the balance
between the energy saved by fitting double
glazing and that expended in producing it.
Although rising fuel costs are likely to keep
demand for double glazing buoyant, the
fuel cost saving is typically not very large:
approximately £40 per annum for the average
home. Furthermore, cost savings do not
necessarily equate to an overall carbon saving.
In England, 81.6 per cent of homes are
fitted with double glazing. Research carried out
for Historic Scotland shows that pre-1919 homes
have the lowest proportion of double glazing,
with only 63 per cent having been retro-fitted.
The current Part L1B requirement for double
glazed units is a U-value of 2.2 W/m2K in
England and Wales and 1.8 W/m2K in Scotland.
(U-value is a measure of energy transfer
through a component separating spaces of
different temperature, so the lower the value
the better.) The requirements approximately
halve the heat loss from the equivalent single
glazed window, suggesting a significant saving
in the overall carbon footprint. However,
this apparent energy saving ignores other
elements in the energy equation: the cost
of raw material extraction, unit production,
transport, use of new materials, and the
disposal of the old window. (Approximately
24 per cent of all waste in England is
generated by the construction industry.)
Traditional windows have a bad
reputation with the public: it is often
assumed that they lose heat rapidly, generate
draughts and condensation, and require
complex and expensive maintenance.
This view is supported by an industry
dedicated to retro-fitting double glazing.
In reality, the energy equation is far
more complex than that presented by the
replacement industry. Well-maintained older
windows can last for centuries, far outlasting
the majority of modern replacement windows.
Importantly, areas of damaged or rotten
wood in a timber window frame can usually
be repaired without the need to replace the
entire window. Furthermore, timber itself
can be recycled and reused without the need
for industrial manufacturing processes.
Conversely, modern replacement frames, where they can be recycled as opposed to reclaimed,
require further energy expenditure during
the complex re-manufacturing processes.
Single glazing is a homogenous sheet
with an almost limitless durability. Double
glazing, although more thermally efficient,
has a far shorter expected lifespan because
it relies on other components than the glass
for its efficiency: the seals and the gasses. The
seals break down leading to the formation of
condensation within the unit, or ‘fogging’.
Removal of the condensation requires
replacement of the unit. The most commonly
used argument in favour of double glazing
is that it saves more energy than it takes to
produce. The shorter the lifespan, however,
the less achievable this claim becomes.
PUTTING HISTORIC WINDOWS TO THE TEST
Understandably, double glazing is usually
rejected as an option for upgrading historic
properties on the grounds that it unacceptably
alters the appearance of the building and
displaces historic fabric. The proportions
and setting out of original glazing sections
play an important part in the overall visual
appearance of a building. However, there are
ways of retaining original windows while
improving their thermal performance.
Secondary glazing is usually seen as a
cheap fix but, correctly fitted, it can cut heat
loss by 63 per cent. Maximising the thermal
efficiency is dependent on achieving a close
fit to the opening, with draught proof seals
to the surrounds. Systems fitted close up
against the rear face of the original windows
are still discernible externally, and are
often very visually intrusive on the interior
side, as well as making the operation of
the windows awkward. To their advantage,
secondary glazing systems are usually
reversible, with no loss of historic fabric.
Laboratory testing carried out by Glasgow
Caledonian University for Historic Scotland has
shown how effective the alternatives to double
glazing can be when correctly installed and
used. Unfortunately, however, modern curtains and blinds are generally fitted primarily to
preserve privacy and for solar shading, with
little thought given to potential thermal gains.
Modern curtains are typically manufactured
using lightweight man-made fibres with little insulation value. Fitted with minimal
laps to the window surrounds, they have
little impact in terms of preventing heat loss.
However, traditional brocade curtains using
insulating natural fibres can cut heat loss by
14 per cent. Curtains must be properly lined,
and overlap all sides of the opening so that
they hang against the wall surfaces. The head
detail is also important: if windows are deeply
recessed, curtain heads can sit against the
soffit, otherwise a pelmet housing should be
considered. The performance of curtains can be
further improved by fitting multi-foil or blanket
insulation between the curtain and its lining.
Another traditional approach is to use Victorian
blinds, which can reduce the heat loss further
to 28 per cent. In other words, blinds are
twice as effective as curtains. Roller blinds
are an easily-sourced modern equivalent.
These are easily fitted close up against the
window frame, and when retracted do not
show to the exterior. Most modern blinds
have a small cross section of less than 35mm.
Domestic installations usually leave the blind
exposed below the window soffit internally.
||This replacement uPVC window is visually intrusive in a
historic setting, particularly when set against original
windows. The bulk of the sections, opening pattern and
mullion setting out are all different from those of the
original Victorian sash.
||Secondary glazing set inside the original sash is less
visually intrusive externally although it is more visually
intrusive internally, and does affect the use of the
original window. Some installations are fully reversible.
||This secondary external glazing prevents the use of the
Blind housings hidden above the soffit
require disturbance and modification of the
linings, and they necessitate some loss of
historic fabric. The advantage is that they are
visually very discreet and operate well with
shutters because the housings are above the
top shutter rail. This may not be an issue where
the original shutters and linings have been
lost, or have already been heavily modified.
When fitted with a low emissivity film on
the window-facing side of the blind, the heat
retention of roller blinds alone can improve to
45 per cent. Unfortunately, this does increase
the bulk of the blind, making it less discreet or
resulting in greater disturbance to the historic
fabric. The film also alters the appearance of
the building when the blind is drawn as its
reflective surface is very apparent externally.
The honeycomb blind is a more recent
double-layer system which can improve the
base figure to 36 per cent. While neither of these
modern solutions have a traditional appearance
internally, fitting is reversible and can minimise
alteration to the window surrounds.
Convenient automated blind or
shutter systems are becoming increasingly
popular. They can be linked to daylight
sensors and central switching so that they
minimise heat loss when the building is
unoccupied and at night. However, even
small motors can be visually intrusive,
and hiding them usually entails some
disturbance and loss of historic fabric.
Traditional shutters on their own can reduce
heat loss by 51 per cent when in use. Yet
in many buildings where they are fitted
they have fallen into disuse despite their
thermal retention and security benefits. Their
heat loss reduction approaches the energy
savings achieved by standard double glazing
units with no modifications required.
Modifying the shutters by adding
insulation can increase their performance to the equivalent of low emissivity double glazing.
Results from a Lister Housing Co-operative
property show an improvement in the U-value
from 5.5 to 2.2 W/m2K. However, the thickness
of the insulation if retrofitted to historic
shutters would usually affect its operation,
requiring modifications to its hinges and
housing. If the original shutters are replaced
with modern insulated shutters, an important
(and increasingly rare) part of the historic fabric
would be lost, but where the original shutters
are missing, the introduction of modern
insulated designs may well be more appropriate,
as it is arguably more ‘honest’ to fit a new
design than a facsimile of a traditional one.
Where shutters exist and blinds can be
fitted, the U-value can actually be improved
beyond the levels achieved by double
glazing, although issues can arise in locating
the blind housing so as to avoid fouling
the shutters. The combination of shutters
with blinds and curtains can achieve the
same U-value as modern insulated shutters
with minimal impact on historic fabric.
Alternatives to double glazing systems that have
minimal impact on the appearance of historic
buildings do exist. Shutters and blinds are
simple and robust, easily repaired and, if used
correctly, their efficiency is comparable with
that of double glazing. Shutters also have the
added benefit of improving home security. All
systems, both historic and modern, need to be
correctly managed to ensure they are used to
maximum efficiency, but they can reduce heat
loss more effectively than double glazing alone.
Arguably the best starting point for all
energy efficiency measures is to go back to the
source and review the heating installation and
maximise its efficiency. In the case of historic
buildings, the occupants also need to develop
an understanding of how their buildings
Old buildings with a high thermal mass
behave differently to modern lightweight
buildings: applying heating at maximum
output over a short period is inefficient.
Old buildings which were built and
used historically in ways that were highly
sustainable, are often forced to adapt to modern
lifestyles and levels of comfort which are
inherently unsustainable. Clearly, adapting
old buildings to 21st century requirements
is often vital to their survival. As English
Heritage has acknowledged, the vast majority
of historic buildings need a beneficial use
to justify their continued maintenance
costs. But perhaps those who use historic
buildings need to adapt too, understanding
and working with their buildings to reduce
energy consumption and improve efficiency.
The Building Conservation Directory, 2009
BRUCE CLARK BSc BArch RIBA qualified
as an architect in Dundee and works for
Nash Partnership in Bath. He
leads the listed buildings team and heads a
sustainable design research group within
the practice. He has a long-standing interest
in reducing the environmental impacts
of buildings by learning from history.
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