Windows: the Decision to replace with Double Glazing / Energy Efficiency

Sustainability. There is a general perception that PVCu windows are always better than the original windows that they replace. However, do not automatically assume that damaged windows and doors have to be replaced. One has only to look at the comparable "costs in use" of the old windows relative to their replacements. Modern timber replacements do not use the same quality timber as was used when the original windows were installed. This is particularly true of those windows installed or replaced in the 1960`s - 1970s, when the quality of softwood joinery timber generally used was not particularly high, and therefore far more likely to rot than earlier windows. Even the increased use of preservatives in recent years does not seem to make a significant difference in this regard.

Of course, those Victorian or Georgian windows have already lasted for over 100 years. More often than not, it tends to be the lower sections of the windows that are affected by decay rather than the full window. Such windows therefore tend to be capable of repair and restoration. Consider the current lifespan of plastic windows (Considered by some at 20 to 30 years) and the likely savings in heat loss against the cost of replacing the old windows, including transport costs, and then the costs of disposal: there is little economic argument for replacement.

PVCu windows are made of a less accommodating material than the original timber windows they replace. They can distort if there is any slight structural movement to the building. Adjusting plastic windows to open once movement has occurred is not always possible and is less straightforward than easing a timber window. Also, as emphasized in the small text of many “Guarantees” offered by double glazing companies, the idea that uPVC windows are maintenance free must also be questioned: there are now paint finishes available for plastic, and there are an increasing number of firms who now specialize in their maintenance and repair. How many plastic windows are now beginning to yellow or stain, and the double glazing units or catches fail? There are of course also concerns about the effects of manufacture of PVC on the environment and how this material is disposed of when replacement is necessary. Also, what happens to the PVC during a fire?

Older timber used for joinery was allowed to season naturally and consequently lasted much longer, and the timbers used were also of a more durable species or part of the tree. As a general rule, it is therefore far better to retain original joinery, where possible, rather than to replace with something that has a shorter lifespan.

Many residents express concern about draughts via sash windows. A minimum degree of ventilation is essential to avoid the build up of condensation and decay. The replacement of windows in PVCu does not remove the condensation: it merely displaces it. There is little point sealing up every crack around the windows if you then need to add some other form of permanent ventilation. The Building Regulations of 2006 (now reviewed) even made provision for new windows to have trickle vents designed into them, which allow for air circulation without compromising the security of the window. Interestingly, the amount of ventilation a trickle vent provides is probably equivalent to that which gets in around an old sash window, the only difference being that a trickle vent can be closed, potentially encouraging condensation.

For those windows that are replaced, Building Regulations & NHBC standards now require some additional minimum standards. Any application for Building Regulations for conversion of the loft, or indeed, further replacement of windows, will need to comply with such regulations, and you are therefore be recommended to discuss such alterations with the local Building Inspector before any such works undertaken. There are cost implications. Full compliance with such Building Regulations, will not necessarily be ideal for a building of solid construction, due to the implications, for example, with regard to ventilation, loss of heat through the fabric of the building etc. For this reason, special guidance notes are provided, where properties are Listed Buildings or located within Conservation Areas. Older properties, not regarded as "historic", do not have this protection. However, there is no requirement in Building Regulations to upgrade elements, which do not need replacing.

New uPVC double glazed windows should have the benefit of the residue of 10 Year Warranty. Provision of such Warranty is important since windows of this nature are not fully maintenance free, and do require regular greasing of window catches and hinges, plus regular cleaning of gaskets around the glazing. Any failure of the hinges or catches, without any guarantee, could involve full replacement of the UPVC unit. Equally, many Guarantees only cover the glazing sections for up to 5 years and only then subject to cleaning of the glass every 6 months. It is common for the seals between the two panes of glass in a sealed double-glazing unit to break down, typically after about five to ten years, but almost always within 20 years. When this happens, condensation forms between the panes. Replacement of the sealed unit (but not always of the frame) is then necessary. If there is then a desire for the replacement units to meet current Building Regulations, the cost can be very expensive. Earlier PVC windows have no UV constituent. If no UV constituent exists, such windows will begin to yellow and stain over a period of time. I am now regularly seeing windows from the 1980s that require replacement!

Part L1 of the 2001 Building Regulations makes specific reference to the need to conserve the special character of historic buildings, including buildings in a Conservation Area. There are many companies, who are now able to incorporate draught proofing to original sash windows. No historic window can reach the U values recommended in current Building Regulations.

Appearance, with view to retention of the character of the buildings and their market value, is merely part of the argument. The installation of PVCu windows will not necessarily solve the problems, but could mean removal of an important architectural feature of a house. Any loss of character can adversely impact on Market Value. Retention of original joinery is therefore to be recommended, where possible. ANY replacement will have a shorter total life span than the original. ALL windows require regular maintenance.


English Heritage has now published (2014) new Guidance following Research relating to sash windows and goes a long way towards busting many of the ill-informed myths that surround traditional joinery repairs and, particularly, the oft-claimed merits of double glazing units. As this research shows, much of the energy efficiency achieved by double glazing can be had through simpler and cheaper measures, such as draught-proofing, secondary glazing, thermal blinds and curtains. Please see this link.

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