Thermal Performance of Historic Windows

Chris Wood


  Facade of terraced house with sash windows  
  These sash windows (above) were draughtproofed to reduce air infiltration and heat loss. A fan pressurisation test (below right) revealed that significant gaps existed elsewhere in the fabric that were not obvious. The draughtproofing improved the air permeability from over 10m/h @50Pa down to 7.2. A recent BRE study showed that the average for new buildings with double-glazing is 9.8, which means that draughtproofing the windows in this house would make it as efficient as those in the top 20 per cent of new buildings.  

Historic windows are disappearing at an alarming rate. Twenty years ago it was the embryonic double-glazing industry that was driving the market, but nowadays they are being actively supported by energy conservationists who see any tightening of standards as being the way forward. As always, a sensible balance is needed and decisions must be made on the basis of knowledge rather than generic assumptions about the inefficiency of old windows.

There is little dispute as to how important windows are to historic buildings. After all, the front windows of a building are often the first feature to draw the eye. Original survivals from before the 17th century are rare and therefore precious. But it is the ubiquitous double hung sashes which have fared so badly of late, possibly because their continuing use for over 250 years has made them seem commonplace and uninteresting. Although this ingenious invention has given a sense of proportion, symmetry and unity to many a humble terrace, modern replacements have, regrettably, destroyed the appearance and harmony of many of them.


Our government generally recognises the importance of windows to the appearance and character of a historic building, and in this connection the definition is not confined to listed buildings, even if the real protection is. Clear guidance is provided in PPG 15(1) which states that: ‘As a rule, windows in historic buildings should be repaired, or if beyond repair should be replaced “like for like”’. Local authorities usually refuse applications to replace original or appropriate windows from listed buildings, and they are invariably supported by the Secretary of State on appeal.

Windows in non-listed houses, however, even if they form an important feature within a terrace or individual building in a conservation area, are not protected. Replacement does not even require planning consent, unless an Article 4 direction covers the conservation area and windows are specifically protected.

This failure to offer protection to windows in conservation areas is dispiritingly anomalous when one considers that the very raison d’être of their designation is to recognise ‘an area of architectural and historical importance, the character of which it is desirable to preserve or enhance’. Ironically, better protection is offered through the Building Regulations.


Part L of the Building Regulations is concerned with the conservation of fuel and power and therefore covers windows, doors and rooflights, which are known as thermal fittings. Under the Building Act, reasonable provision must be made for the conservation of fuel and power by limiting heat loss through the fabric of the building. Under the 2006 revisions, new Approved Documents (ADs) were produced which gave targets that the government feels constitute reasonable provision. Any proposal to replace a window needs Building Regulations approval under Part L, and a target U-value (measure of heat loss) of 2 for replacements and 1.8 for new windows in extensions has been included. No historic window can reach a U-value of 2, so in recognising their particular needs Part L contains exemptions and requirements to give special consideration to historic buildings.

  Fan pressurisation test equipment

The inclusion of exemptions came as a surprise, being introduced very late in the day by government. Part L is one of the mechanisms which the government is using to comply with the requirements of the European Directive on Energy Performance in Buildings and it allows exemptions where ‘compliance would unacceptably alter their character or appearance’. This applies to listed buildings and buildings in conservation areas. For listed buildings, it does not really mean a significant change because listed building consent would normally be refused in such circumstances. However, the same criterion is applied to unlisted buildings in conservation areas and there is little doubt that replacing windows could certainly ‘unacceptably alter the character and appearance’ of such buildings. An exemption could therefore be claimed, even though such work does not require planning consent. The advice in the ADs is that building control officers should liaise with the conservation officer and, in practice, the latter is usually required to confirm that the building is of historical or architectural importance. The ADs also clearly state that special consideration should be given to ‘work on historic buildings and this should aim to improve energy efficiency where and to the extent that it is practically possible, [but] not prejudice the character of the historic building, or increase the risk of long term deterioration to the building fabric or fittings’. In addition, work specifically permitted for historic buildings includes replacing lost elements such as windows where these are important in maintaining the building’s character.

More detailed guidance on how to apply Part L to historic buildings is set out in English Heritage’s interim guidance note.(2) This is referred to as a second tier document in the ADs and so forms part of them. Building control bodies should refer to it when they are considering applications for historic buildings. English Heritage intends to replace the interim guidance with a new document consisting of over 30 separate leaflets, which is due to be published on its website in Autumn 2008.


There is no conflict between the government and English Heritage in the overall aim to improve energy efficiency in buildings and reduce the amount of carbon dioxide generated by heating, lighting and ventilating buildings. Buildings account for just under half of all carbon generated in the UK and the slow rate of new building means that it is the existing stock which will need to be improved if significant reductions are to be realised. It is the way this is accomplished that is of paramount importance.

Traditional buildings were constructed with solid walls using permeable materials which absorbed a significant amount of moisture that was then disposed of by plentiful ventilation and natural evaporation. Unfortunately, many people equate this with memories of living in draughty, cold houses with sash windows that rattled in the wind and suffered from chronic condensation in the mornings. Such buildings were inherently inefficient and conditions such as these were neither good for the building nor good for its occupants. It is, however, possible to maintain old buildings in a healthy equilibrium provided that excessive air infiltration and heat loss are reduced and reasonable ventilation maintained. Existing windows can be retained and their performance improved: they do not have to be replaced.

One enduring myth is that old windows account for 20 per cent or more of the heat lost from a building. That presumption is also perpetuated by bodies like the Energy Saving Trust, which claims that if you double-glaze your windows you will save 20 per cent of your heat loss (and more if you live in an old building). But every building is different. English Heritage has worked with specialists carrying out fan pressurisation testing of historic buildings (illustrated above) to determine where heat loss is occurring. In all cases, the windows were of historical interest and none had been repaired or particularly well maintained. But when the tests were completed it was apparent that the amount of leakage from the windows was a lot lower than expected; indeed the real problem areas were often not visible to the eye and occurred in quite obscure parts of the building. Unfortunately, applicants for Building Regulations approval do not usually carry out objective tests to determine the condition of the building, relying instead on a computer model which simulates presumed conditions.

The table below is reproduced from the interim guidance note. It shows calculated U-values and the only example that meets the target value of two is the double-glazed window with low emissivity glass and gas fill between both panes. The single-glazed window achieves a U-value of 4.8, but it is interesting to note that significant improvements can be made by benign measures such as closing internal shutters, which brings the U-value down to 3.0. Adding secondary glazing can bring it down to 2.9, depending on its design and quality. Closing curtains at night (to minimise radiant heat loss) also produces a significant improvement.

Thermal improvement table
This table from Building Regulations and Historic Buildings (2) shows that significant improvements to thermal performance can be gained from benign improvements such as using blinds, heavy lined curtains and shutters. Draughtproofing and secondary-glazing can now give U-values which meet the target in the Approved Documents.


Existing windows can be dramatically improved by simple and cost-effective measures which give a far better pay-back than replacement glazing. The first and most obvious course is to repair windows, and this will often mean employing a carpenter or joiner. Simply making sure that gaps between sashes are minimised and run effectively will drastically improve performance. The extent of the work depends on the condition of the windows but could mean new sash cords, re-weighting, replacement of beads and new putty. While this work is ongoing it is worth considering draughtproofing.

  Contractor applying rubber polymer sealant  
  Contractor fitting weather-strip  
  Top: applying rubber polymer sealant, a simple and cost-effective draughtproofing system particularly suitable for metal windows. Although visually obvious, it is totally reversible. Above: Specialist draughtproofing. A channel is routed out to accommodate the polypropylene weather-strip. These systems provide long-lasting improvements to thermal performance.  

There are a number of firms specialising in installing different systems. The most simple is a rubber polymer sealant, which on curing forms a flexible engineering gasket. All the surfaces need to be carefully cleaned and the seal is applied to one of the surfaces, with a release agent applied to the opposite surface. The window, which can be timber, steel or aluminium, casement or sash is then closed gently while the seal cures. The finished seal is then fettled and cleaned with the whole job taking little more than two hours. It is a relatively cheap process and, importantly for conservationists, it can easily be reversed. Testing at the Building Research Establishment (BRE) has shown that air leakage is reduced by over 50 per cent and energy savings improved by over 25 per cent.

The most effective draughtproofing is carried out by specialist firms who repair the windows before routing out channels to install appropriately sized draughtproof strips of polypropylene yarn (left), which is inconspicuous on completion. Depending on the size of the windows this can take a couple of days, especially if they are being re-puttied and painted. There is no doubt that this system of draughtproofing has a dramatic effect on reducing heat loss and cold air infiltration. Indeed, this system was dramatically and successfully used on test windows at Cowes Castle (discussed below).

Work already done by BRE in the early 1990s showed that even net curtains can mitigate and reduce the impact of cold draughts. Certainly, heavy lined curtains reduce the loss of heat, particularly at night when warm air is easily radiated outwards into the colder night sky. Existing shutters can again be beneficial in this regard, particularly if they are relatively tight-fitting. They were of course regularly and effectively used during the 18th century when a more strict ritual of opening and closing them was employed. External shutters added to the thermal benefits as well as providing effective security and improved noise-reduction.

The most dramatic improvements in thermal performance, however, are provided by effective secondary glazing systems. Those at the top of the market are now double-glazed and include low emissivity glass which results in U-values of 2 and better. In other words, they outperform most double-glazing systems and, when combined with the draughtproofing already described, achieve values of 1.5. Secondary glazing also makes much more dramatic improvements to noise insulation than can be achieved by double-glazing. Visually, however, secondary glazing systems are not to everyone’s taste and, from the outside, they can also give the double reflection for which doubleglazing is often criticised. There are other more lightweight systems available which are fitted using magnetic strips, allowing them to be easily removed in the summer months when they are not needed. Although they improve energy efficiency dramatically, these do not achieve the U-value of 2.


Old glass is particularly precious and should be kept. However, a large proportion of glazing will have been replaced over time, raising the question of whether the more recent glass could be replaced with double-glazing. The problem is that two sheets of glass are far heavier than the original timber frame was expected to bear, even presuming it could accommodate the greater depth. Windows divided by glazing bars almost inevitably have to be sacrificed if doubleglazing is to be inserted. Even if the glazing bars could provide adequate depth, it seldom makes practical or financial sense to try to fit small double-glazed panes into existing windows.

However, double-glazing can be inserted into existing metal frames where there are few or no divisions within the window and there is adequate available depth. This has been done successfully in some standard steel windows and indeed original fittings have also been incorporated.


Disseminating this information is one of the biggest problems today: indeed few of the benign systems described have been empirically tested. As a result, English Heritage has joined forces with Historic Scotland to carry out fullsize tests of the performance of existing timber windows when the various improvements have been carried out. Heat-flow sensors are being applied to windows in occupied buildings and a whole series of tests are taking place using the newly installed environmental chamber at Glasgow Caledonian University. The set up is very straightforward (see illustrations below) with the external side pressurised and set to specific temperatures and the other side simulating the building interior. As well as actual U-values, this set up enables assessments to be made of heat loss and air infiltration improvements as well as energy savings.

When this work is completed it is planned to publicise it widely, together with the findings of other relevant studies. The English Heritage campaign of the 1990s, Framing Opinions, produced a great deal of information on the costs of different methods of improving energy efficiency. Double-glazing was found to be the most expensive with the longest payback period. Far more effective were the benign improvements outlined above. All the figures are being revised and brought up to date.

Individual case studies are to be included as well. A few years ago, independent testing was carried out to compare the performance of timber windows with PVCu double-glazing. This resulted from an impasse after the Isle of Wight Council served an enforcement notice against the Royal Yacht Squadron for replacing timber windows with PVCu without consent in the Grade II* Cowes Castle. One single-glazed timber casement window and one timber double-glazed sash (the council had previously approved double-glazing) were compared to two top-of-the-range PVCu windows. The tests were for weather tightness, which included air permeability, wind and water tests. The timber windows performed best in all the tests: indeed it was the single-glazed window that performed the best in the air permeability test. The problem for the PVCu windows was that they distorted by a good 5mm under pressure and, unlike the timber, did not return to the same profile. This meant that they leaked straight away.

Thermal performance test graphic
Above and below left: the test set up at Glasgow Caledonian University. One side of the chamber represents outside conditions where pressures and temperatures can be varied. The inside face of the window will be subject to various ‘improvements’ and these are measured by heat-flow meters on the glass. Below right: a single glazed timber window subjected to weather testing in an independent test-house; it out-performed double glazed PVCu in all of the tests.


There is a definite need to improve the thermal performance of existing buildings in order to reduce carbon emissions and slow climate change. However, a sensible balance is needed. Unpublished data from the BRE has shown that if the half million or so listed buildings in England had all their windows replaced by double-glazing, it would save between 0.16 and 0.36 per cent of total energy used in this country. It is surely not worth sacrificing our architectural heritage for so small a gain, particularly if substantial improvements can be made in more benign ways.

From the conservation standpoint, the overriding aim is to ensure that existing fabric is retained. Part L includes exemptions and special considerations that should provide the flexibility to ensure that windows are not unnecessarily replaced. Real improvements can be made in the performance of existing windows and real reductions can be made in energy wastage. This is, after all, the underlying intention of the revisions to Part L.

Before contemplating making changes to existing windows, it is wise to try to discern just how the building is working: where heat is being lost and cold air entering. Do not rely on computer models or generic assumptions, they sometimes bear little resemblance to reality.

Retaining original windows certainly has to be more sustainable than replacement. The type and quality of softwood that was used for most 18th and 19th century windows cannot be sourced today and throwing it away makes little sense when the whole emphasis of the 21st century must be on making maximum use of the resources we already have. Ultimately, the Building Regulations per se will not lead to reductions in the use of energy in buildings; that will be down to the occupiers and the way that they choose to live.



(1) Planning Policy Guidance 15: Planning and the historic environment 1994 – HMSO, London, 1994
(2) Building Regulations and Historic Buildings – Balancing the needs for energy conservation with those of building conservation: an Interim Guidance Note on the application of Part L, English Heritage, London, 2004


This article is reproduced from The Building Conservation Directory, 2008

Update, September 2012
Recently there have been several significant changes in UK government planning guidance and policy.

In England Planning Policy Guidance Note 15: Conservation of the Historic Environment (PPG15, 1994) and Planning Policy Guidance Note 16: Archaeology and Planning (PPG16, 1990) have been cancelled by the Government. Initially replaced by Planning Policy Statement 5 (PPS5) in March 2010, current policy guidance for England is now given in the National Planning Policy Framework (NPPF) issued in March 2012. Further guidance is proposed, but in the meantime the guide which originally accompanied PPS5 remains in force - see PPS5 Historic Environment Planning Practice Guide.

In Scotland the principal statutory guidance on policy is now Scottish historic environment policy (SHEP), which was published in December 2011, with subsidiary guidance given in Historic Scotland’s Managing Change leaflets. These documents together replace the Memorandum of Guidance on Listed Buildings and Conservation Areas published in 1998.


CHRIS WOOD is the head of the Building Conservation and Research team at English Heritage and a member of the IHBC technical panel.

Further information




Secondary glazing

Timber windows

Window glass

Windows: leaded lights

Windows: metal
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