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12 October, 2008
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Surviving the test of time
Winter 2007
Published:  23 February, 2008

There are companies specialising in the repair of historic windows

With proper repair and maintenance historic wood windows can have a long life. TRADA consultant architect Patrick Hislop RIBA explains

A great many of the softwood windows installed into buildings in the UK in the past 200 years are still in use. There are also many examples of hardwood windows dating from much earlier, although the opening lights might be in wrought iron rather than wood.The natural durability of the oak from which they were made is the main reason for their durability.
In the case of the softwood windows, their longevity is down to a number of factors, including careful selection of the wood, the lead-based paints with which they were historically finished, and probably the '‘draughtiness'’ of the buildings in which they were installed.
Lead-based paint was traditionally used to protect both the wood and the linseed oil-based putty used for glazing. This formed an effective barrier to moisture when used with an appropriate primer, but also had the advantage of some flexibility in taking up any moisture-induced movement. Modern oil-based paints were substituted when lead-based paints were banned, and have often proved brittle, with little flexibility. Cracking of the paint film can occur as a result and the consequent penetration of moisture in many cases led to the decay of wood windows that had survived many decades previously.
The third element, the draughtiness of historic buildings, meant that there was little risk of condensation forming on the single glass panes of the windows. Adding insulation, blocking off natural ventilation such as chimneys, and the introduction of more heat-using fuel such as coal-gas or paraffin, which generate a considerable amount of water vapour, all substantially increased the risk of condensation on the inside of windows. Part of the eventual failure of old softwood windows through decay was the result of internally generated condensation rather than external weather.

Traditional paint finishes must be removed before modern vapour permeable finishes are applied over 

The more recent introduction of flexible vapour permeable paints, the requirements for some provision for permanent ventilation to interiors of buildings and the dry heat of central heating systems will do much to extend the ‘life’ of old windows. However, the downside is that, with improved insulation and more airtight buildings, the risk of condensation on single glass will increase. This makes it particularly important to ensure that the inside of window frames and glazing bars is finished with a fully vapour resistant finish, even when a vapour permeable finish is applied to the outside of the frames. The same should apply when maintaining older windows, particularly if, for conservation reasons, single glass is to be used. Note, however, that vapour permeable finishes cannot be applied over traditional paints, which must be removed first.
Another factor, particularly applicable to traditional putty-glazed windows, is that modern vapour permeable paints will not protect the putty in the same way as high-film forming oil- or lead-based paints, leading to premature drying out and cracking of the putty. Where possible, and if not constrained by conservation officers, it is advisable to substitute bead glazing for the putty to overcome this problem when modern vapour permeable paints are to be used.
If localised decay has occurred in softwood members, it may be possible to cut out the damaged parts and fill with epoxy resin. This will, when fully bonded, restore the structural strength of the section, but it is important that any surface coating in the vicinity of the repair is removed before the filler is inserted, to ensure a good bond. The wood must also be at a moisture content suitable for a filler. Long-term protection to otherwise untreated wood can be provided by inserting boron rods into critical areas. In the event of a localised high moisture content the boron salts will then leach through the wood to provide protection against fungal attack.
It is not generally possible (and usually not permitted on conservation projects) to substitute insulated glass units for single glass panes into traditional windows, because of the small size of the frames and glazing bars. If improved thermal insulation is required, the simpler solution is usually to add secondary glazing internally.
Older oak windows can also be repaired, but if the oak is to be left unfinished it may be necessary to patch in sections of oak rather than merely fill with epoxy resin, as this will be very visible. A combination of epoxy filler concealed behind oak patches may be one solution. If oak windows are being generally renovated, any remaining tannin in the wood may corrode mild steel or iron fittings when exposed to wetting. It is therefore always advisable to use stainless steel fixings with oak. A possible traditional alternative is to use oak dowels, driven or glued in position, rather than any metal fixings.

The eventual failure of softwood windows can be the result of internally generated condensation 

There are now a number of commercial window joinery companies that specialise in the repair of historic windows, whether sash or casement windows, and are usually able to do this within limits set by conservation officers. For more extensive repairs, particularly on historic buildings, it may be advisable to use their specialist services.
TRADA too can provide background information on such matters as the choice of species, suitable paints, preventative treatment and structural fillers. For further information visit www.trada.co.uk.

Keywords: Window wood
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