and Thermal Improvement of Timber Windows
to conserve the natural environment by reducing heat loss presents
architectural conservationists and home owners with an ethical dilemma
where historic windows are concerned. Jonathan Taylor outlines
the need for sympathetic solutions.
replacement windows in a house in South London. The windows fail
to match the fine details of the originals, or the appearance
of painted timber.
used to heat and construct buildings accounts for over 50 per cent
of all energy consumed in the United Kingdom (transport consumes the
bulk of the remainder). The by-product of producing that energy is
carbon dioxide and sulphur dioxide emissions from power stations,
contributing to global warming and acid rain. As a result, the Government
is now committed to stabilising CO² emissions by the year 2000. The
issues need to be considered by all involved in the design, construction
and maintenance of buildings, not only as a statutory requirement
of the Building Regulations, but also because additional requirements
may be imposed by building owners including housing associations in
particular, and by local authority 'home improvement' grants.
the effect of some improvements can be extremely damaging to an historic
building or place. Double glazing in particular may involve the loss
of the original windows and the introduction of a range of details
which can ruin the appearance of the building.
a building is listed, all alterations which affect its character can
be controlled, and strict controls (Article IV Directions) may also
be imposed by local authorities on specific alterations to buildings
within conservation areas which they might otherwise be unable to
REQUIREMENTS FOR ENERGY CONSERVATION
development is regulated under the Building Regulations. Part L 'The
Conservation of Fuel and Power' (or Part J of the Scottish Building
Regulations) applies strict limitations on the heat loss permissible
from new buildings, from extensions where the floor area proposed
exceeds 10m², and in some cases from existing buildings affected by
alterations. In an existing building it is usually only the components
being altered which are affected by the Regulations. However where
a building is being converted to flats or from non-residential to
residential and for certain other changes of use, they can apply to
the whole building. Residential conversions will be required to achieve
an energy rating which satisfies a 'Standard Assessment Procedure'.
Part L (or J) does not apply to buildings and extensions which are
not habitable. A conservatory for example, may not need to comply
if it is separated from the interior of the building by doors or if
it is under 10m².
insulation standards, which are measured in units of thermal transmittance
or 'U-value', favour double rather than single glazing, but there
is a considerable degree of flexibility, and Part L specifically states
that double glazing 'could be inappropriate in conservation work'.
Acceptable standards are given by the ratio of window opening to floor
area for single as well as double glazing, and variation from standards
are also acceptable 'if compensatory provisions are made'; increases
in insulation to the roof space for example, could be used to off-set
the heat lost through single glazing; solar gain through south-facing
windows may also be taken into account.
is interesting to note that aluminium and PVCu does not perform as
well as timber in terms of their thermal efficiency. In addition both
involve a much greater consumption of energy in their production than
is used to produce timber - over 45 times as much in the case of aluminium
(TRADA figures). Wood, as a natural renewable resource, is clearly
preferable to any man-made alternative.
MAKING SENSITIVE IMPROVEMENTS
older buildings and for listed buildings in particular, where it is
essential to improve the thermal efficiency of the existing windows,
the least obtrusive option is to introduce draught proofing measures.
For casement windows the process is simple, with a wide range of products
available, including durable rubber seals which are discretely rebated
into the window frame. The draught proofing of double hung sash windows
is more complex, requiring the replacement of the parting bead with
a new component incorporating rubber blades to maintain the seal at
the sides, and compression seals to the meeting rail, window head
and sill. These seals may be purchased for installation by a joiner,
or alternately there are companies included in the Building Conservation
Directory which specialise in overhauling, repairing and draught-proofing
sash windows. Thorough treatment using purpose-made fittings has the
added benefit of making the sashes slide more easily and stops them
from rattling with every gust of wind.
glazing also provides an alternative where the exterior of an historic
building needs to be protected, although the double reflection caused
by the second pane of glass can be seen externally. This approach
may be unacceptable internally, as the effect of even the most sensitively
designed secondary glazing on the character of the windows from the
inside can be excessive, and they cannot be used where there are shutters
on the inside.
the 19th century some houses were constructed using secondary glazing
in the form of a second pair of sash windows which drop down together
into a pocket below the window, covered by a hinged extension of the
window cill, and fronted by an attractively moulded panel. Although
rarely used today, this approach provides an interesting solution
where there are no shutters to obstruct. Metal-framed vertical sliding
sashes are commonly used, which fit tight against the window. The
small, simple sections are not too obtrusive during daylight hours,
when they are seen against the light, but at night they can be glaringly
obvious as an entirely alien element in a fine interior.
cheap and simple method of secondary glazing is to fit a single frame
of glass over the whole window within the reveal which can be removed
and stored in the summer. The only limitation on this system is the
size of glass which can be handled easily without breakage, and it
is really only suitable for small windows and windows divided into
smaller lights by mullions and transoms.
glazing allows the originals to be retained and is generally accepted
by planning authorities for use in listed buildings. Double glazing
on the other hand can rarely be added to existing windows without
major modification due to the need for thicker glazing bars to hide
the spacer bars (see diagram), the additional thickness of the sealed
units and the additional weight of glass on fine timber members.
purpose-made casement detail:
The use of a double rebate to prevent draughts makes the casement
sit proud of the frame. The wide glazing bar is necessary to hide
the spacer used to separate the two panes of the sealed unit.
These are commonly up to 44mm thick
mid-19th century casement detail: The single rebate construction and the narrow glazing bar produce
finer details. Sills were usually of stone or render, as the
near horizontal surface inevitably collects water and decays
a building is listed, planning authorities usually require existing
windows to be retained and repaired where possible, unless they are
clearly falling to bits and cannot be repaired, or are later additions
which are obviously the wrong design for the building. A fine antique
piece of furniture is prized not only for its completeness but also
for its imperfections, and so may a window; early glass may display
the radiating ripples and tiny air bubbles indicating that it was
hand blown; and sash windows in particular are highly intricate and
sophisticated pieces of furniture, superbly constructed to last, and
deserving careful restoration.
protection applies to every part of a building, and consent is required
for the alteration as well as the removal of its windows. Their replacement
by new, double glazed windows will usually be resisted for two reasons;
the loss of original fabric and the change in character and detail.
is more flexibility for introducing double glazing in unlisted buildings
within a conservation area, as the emphasis of control is on protecting
the external appearance of a building and the affect of alterations
on the character of the area. However, here too the need to retain
glazing bars presents almost insurmountable problems, as the 20mm
width of a typical glazing bar is too narrow to hide the spacing bar
of a sealed unit. One solution is to incorporate dummy timber glazing
bars which follow the original profile and pattern exactly, but are
'planted' onto the inner and outer faces and do not run through.
glazed windows have also been produced with smaller spacer bars and
the glazing bars have been whittled down to the bare minimum. In this
way it is possible to produce a 25mm wide glazing bar which works.
In a large window with a single glazing bar, the difference will not
be as noticeable as in a small-paned sash.
conservation areas alterations to windows cannot be controlled. The
use of replacement PVCu and aluminium is on the decline in many areas
as people become increasingly aware of their disadvantages in design
and performance, and their negative effect on house value. More sensitive
solutions in timber, including double glazed examples, undoubtedly
provide a less damaging solution which are broadly welcomed.
new thermally efficient windows, secondary glazing or draught-proof
seals are introduced, the loss of ventilation around the old frames
may promote condensation problems and damp within the building. Sources
of damp include kitchens and bathrooms, breath, building materials
such as drying plaster, and solid external walls and floors. In a
properly ventilated building, damp may never be seen as a problem.
Seal the windows and doors however, and the humidity level can rapidly
escalate to the point where painted finishes on external walls begin
to flake, mildew appears in bathrooms and kitchens in particular,
and at around 20 to 30 per cent the risk of timber decay increases
substantially. The controlled ventilation of the whole building must
be considered, taking into account the supply of fresh air from outside,
at a low level, passing through the building to exit ideally through
the chimney stacks and under the eaves without the need for visible
ventilators and avoiding draughts. Particular attention should be
paid to under-floor spaces, ground floor rooms and bathrooms. Heat
exchangers can also be used to recover much of the heat lost in this
manner as part of a comprehensive approach to thermal efficiency.
carried out with care and consideration, it is possible to improve
the thermal efficiency of buildings with minimal impact on their appearance
and historic interest through the use of a wide variety of measures.
Although there will be exceptional cases where it is possible to introduce
double glazing into historic buildings, and greater opportunity in
conservation areas, it should be remembered that this is just one
of a number of methods of improving the thermal efficiency of buildings.
- David Wrightson,
'Mandatory Double Glazing: Heat Loss Versus Heritage',
Article in Context, issues 45 and 46, Association of Conservation
Officers, (www.ihbc.org.uk) London, 1995
Supplement to Conservation Bulletin, English Heritage, London, 1991
Trust for Historic Preservation, Repairing Old and Historic Windows.
The Preservation Press, Washington DC, 1992
- Harry Munn, Joinery for Repair and Restoration Contracts, Attic
Books, Builth Wells, Powys, 1989
This article is reproduced from The Building Conservation Directory, 1996
JONATHAN TAYLOR is the editor of The Building Conservation Directory and a co-founder of Cathedral Communications Limited. He studied architectural conservation at Heriot-Watt University, Edinburgh and has a background in architectural design, conservation and urban regeneration.
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