Previous Webinar on Traditional Windows Care Repair and Improving Energy Efficiency

Speakers: Ian McCaig, David Pickles, John Neale, Alice

Ian: And I'm here, at least in spirit with David Pickles and John Neal. So, David, would you like to say hello?

David: Hello? Hello, everyone.

Ian: John?

John: Hello.

Ian: Very good. Well, we've got a great show for you today, so I'm going to press right on.

The UK has the oldest housing stock in Europe. In England, about 20 percent of homes - that's nearly five million - were built before 1919. Some two million of these are in conservation areas and at least 320,000 are listed buildings. For the most part, older buildings have survived because they're durable and adaptable. And they have an essential role to play in fighting climate change.

Continuing to adapt, upgrade, repair and maintain them, so they go on being useful and viable, makes good social, economic and environmental sense.

When contemplating improvements to save energy in older buildings, the first thing that many owners and residents think about is double glazing and replacement windows. This has been reinforced by highly successful marketing by a well-organised replacement window industry. Many traditional windows have been replaced because they are thought to be burdensome to maintain and not energy efficient. In fact, over half of pre-1919 homes now have plastic windows. But is this where local residents should be investing their hard-earned cash? Might other energy saving improvements represent better value for money in terms of energy savings and carbon reduction? And how do the whole life carbon costs of repair, upgrading and replacement stack up? Now, we've got a little quiz for you now. Alice, where is the quiz?

[they discuss technical issues with the quiz]

Alice:. Right, everybody everybody's voting and I will broadcast the results. There we go, everybody can see.

Ian: So, have we all finished voting? Looks like it. Well, the answer is... In fact, you're all correct. The contribution made by windows depends on the performance of -- Sorry, the contribution made by windows depends on the building and the extent to which windows form part of the external envelope. And as we know, traditional buildings differ considerably. And obviously the amount of energy being lost through the windows on the building on the left is going to be considerably more than on the right. So that would suggest that perhaps, you know, your priorities in terms of energy improvements in those two buildings would be different.

Ian: David.

David: Alright, next slide.

Yes, so this variability really highlights, I think, the importance of considering improvements to windows as part of a whole-building approach to energy efficiency. This is a housing approach that uses an understanding of the building; its context; its significance; and all the factors affecting energy use as a starting point for devising an energy efficiency strategy. The slide here shows a triangle and how all the factors affecting energy use are all interrelated. This approach ensures that energy efficiency measures are suitable, well integrated, properly coordinated and sustainable. In fact, the whole-building approach was the subject of a previous Technical Tuesday webinar, which I'm sure you all attended. If not, it's available for catch up on the Historic England website. It's also described in great detail in our Guidance, which is shown here - "Energy Efficiency and Historic Buildings: How to improve energy efficiency" - which of course, is available for download on our website.

Ian: So, why do we want to try and keep old windows? Well, in purely practical terms, they are durable, functional and repairable. That's why they're still here. And if properly maintained, they will last longer than many types of replacement. They are often made from high-quality materials such as old growth softwood, which are no longer readily available today. Furthermore, research carried out by Historic England and others has shown that, with a little help, traditional windows can meet current thermal performance requirements. Therefore, a repair-not-replace approach makes good economic and environmental sense. But the values of traditional windows go well beyond the merely utilitarian, as John is about to tell us.

John: Historic windows make a powerful contribution to the appeal of England's historic buildings through their design and materials. Think, for example, the play of light on historic glazing. And here is the spectacularly illustrated 16th century Guildhall at Lavenham as an example. When we talk about why historic buildings matter to people, we talk about their heritage values, but we use the language of the National Planning Policy Framework to talk about the significance and the interests which make up that significance: archaeological, historic, architectural. Historic windows tell the viewer about a building's history. They contribute to its character. They're a part of its architecture. They're fundamental to the delight such buildings provoke. Essentially, the interest of historic windows is due to the interplay between the historical development of building trades and industries, and that of architectural taste or fashion, combined with other factors such as historic building regulations.

Historic windows are varied in their development complex and the following images suggests something of that. Here, it's a simple 16th century-stained mullioned window at the end of the Gothic tradition. And on the right, the drawing shows the construction of a typical oak-framed window with leaded lights and then opening light in an iron casement. Windows at this period were constructed of small panes of glass, in lead came, set directly into the windows around on the stone or on opening lights of iron. The 16th century saw the advent of Renaissance classicism. The construction of windows remained the same, while classicism changed first with decorative treatment of windows and then their form. Here, that's Thorington Hall in Suffolk - the 16th and 17th century house much altered. You can see a hybrid window, or an altered window, within the centre large mullions and transoms and high-level lights to either side.

In the same house, this room has what are now classically proportioned windows: taller than they are broad, with mullion and transoms and - this time - square paned, square glazed, small lights. And there's a consonance between the windows and the finishing of the interior. Windows are as much about the architecture of the interior as that of the exterior. Seth Ward's Hospital, 1684, provides a striking illustration of the continuity of window construction while architectural style changed. There is a fully fledged classical building. Window furnishings also add to the interest of historic windows. Here you see elaborate handles and iron sprung, say, a drawing of some other traditional fixings. And these, but also, simply industrially produced furnishings from the early 19th century are distinctive and interesting.

And then, of course, there is the glass itself. Here you see crown glass being discoloured. From the 14th to the 19th century, crown glass was widely used. You can see the pattern of cutting and you can see the distinctive reflections on the window on the right. And here is a drawing of the production of cylinder glass - a development of the earliest form of glass manufacturing - which eclipsed crown class in the 19th century when taxation on glass changed. These types of handmade glass - with their distinctive unevenness of surface - create particularly beautiful effects of light and reflection, which modern glass cannot match.

One of the most radical changes in the design of windows was the introduction and rapid adoption of the sash. Introduced in the 1660s; widely used in royal buildings in the 1680s and ubiquitous shortly after that. Research evolved to form the sash we know today - double-hung and counterbalanced. And there's an illustration of an early example on the left, and on the right, a drawing of the construction. The design and construction of sash was changed progressively through the period. Early glazing bars were broad and shallow, as you see on the top left, with ovolo mouldings, and they grew progressively narrower and finer and deeper through the 18th and 19th centuries. Equally, the positioning of the sash box changed, especially in response to the London Building Acts. So in the drawings on the right, you see an early sash box positioned close to the surface of the wall. And then after 1709, when regulations required it to be set back four inches, it became a setback – and then later it becomes concealed by the brick wall. And you can see that illustrated in these photographs here. Of course, the London Building Acts, I should add, also influenced building practice and architectural fashion beyond London. Of course, not all Georgian windows were vertically hung sashes. There were casements on the buildings and sometimes they were horizontally sliding sashes - or Yorkshire sashes - such as these. In this case, with nice, external shutters.

The design of windows, the openings and the windows which close them is a fundamental attribute of every architectural style. As styles changed, so did windows. The late Georgian picturesque movement revived vernacular in Gothic styles. Here is an example: on the left, the late Georgian Gothic Window; and then on the right, a rather more serious Gothic window of the high Victorian Gothic revival from the 1850s or 60s. The most fundamental change of the 19th century was perhaps the industrial production of plate glass, prior to cylinder glass, which led to the introduction of single pane sashes, which were heavier and required horns to support the frames of the upper sash. You see that drawn on the right. Horns were not a feature, of course, of the 18th century Georgian windows. Here too, though, the history of Windows was not linear. The Arts and Crafts movement and the Queen Anne movement both reacted against plate glass. Here, there's a sash window of 1892, which effectively revives the forms of early Georgian or Queen Anne sashes: exposed sash boxes, small panes, multiple panes. And then there are the various phases of modernism. Here, two examples: on the left, one of the famous Hertfordshire County Council school buildings - large panes of glass and very light, aluminium frames. And then, quite different on the right: a flavour of post-modernism in the late 1970s.

The interesting variety of historic windows provides ample reason to conserve them. But not only are historic windows an extraordinary interest in themselves, their commercial replacements tend to be grossly inferior. This is a crude illustration of the point, but contrast the refinement and historic interest of the original with its replacement, and even when an attempt is made to respond to the original, the result is generally disappointing – or often disappointing – whether in plastic or wood, as here.

Historic windows, with the building to which they are part, tell stories about our past. Stories about craft, trade, economy, society, taste. They also delight with their variety of forms and as part of the architecture to which they are integral, and so we should value them.

Ian: So, how can we improve the performance of traditional windows in ways that make economic and environmental sense without diminishing their heritage values and significance? Well, we can start with maintenance and repair to optimise the performance of the window as found. Then we can go further by adding draught proofing and we can exploit traditional features such as awnings, blinds, curtains and shutters. If we need to go further still, we might think about secondary or double glazing. These are the approaches we're going to look at today. And although we'll be focussing mainly on timber sash windows, the principles we'll be discussing apply equally to other types of window. Our advice is based on evidence from research carried out by Historic England, Historic Environment Scotland and others, and you can find these research reports and other references on our website, and I commend them to you.

The first step towards improving the thermal performance of an old window is to make sure that it's in a good state of repair. Relatively simple measures like removing excess paint and mending broken sash cords will allow windows to open and close properly. Traditional windows are seldom truly beyond repair, even when a window is in a sorry state like this one, it can be returned to good working order. There's a popular perception that repairs are short lived and that a repaired window will be inferior to a new one. However, this is not borne out by the fact that there are so many traditional windows continuing to perform satisfactorily, some after many centuries. Provided repairs are done well and subsequent maintenance is not neglected, the service life of a traditional window can be extended pretty much indefinitely.

All the defects we see in this image can be remedied quite easily. Timber-to-timber repairs are the traditional approach to remedying defects. Repair joints should be designed for optimum strength and resistance to water ingress. Stainless steel or brass repair plates can be used for first aid repairs or as part of a permanent repair to reinforce vulnerable joints.

Good repair design: the selection of suitable and durable repair materials and good workmanship are the keys to success.

Epoxy resin repair systems also have their place and shouldn't be dismissed out of hand. They allow many defects to be made good with minimal disturbance and loss of fabric and if well designed and skilfully done, can perform very well. They are best used for smaller scale dentistry-type repairs and can be used in conjunction with timber-to-timber repairs. They should not be thought of as a cheap and inferior substitute for traditional joinery repairs, but instead as a method with particular characteristics that can be advantageous in many instances. Think of them as another tool in the toolkit.

When repairing windows, every effort should be made to preserve historic glazing, particularly handmade crown or cylinder glass. These are usually thin and can be easily broken when removing defective putty. However, the risk of damage can be reduced by using an Infra-Red putty lamp. Radiant heat warms and softens the putty, but passes through the glass without greatly heating it, thereby reducing the risk of cracking.

David: Right, yeah. Having got your window into good repair and working order, its thermal performance can then be further enhanced by draught proofing. Inserting draught seals gives the greatest benefit in energy saving for the lowest environmental and economic costs. A wide range of seals and gaskets are available and our guidance, shown here, goes into some detail with this. So, this drawing here shows brush seals put into parts of the window, routed out and then inserted. Next slide. So, some of these are intended for DIY applications by the more sophisticated system, such as the brush seals, require professional help.

This picture on the left shows the brush seal put into part of the sash. And then the middle photo shows the sort of DIY arrangement with use of a sealant which is pressed in and then allowed to set into the frame. Measurements carried out by Historic England and Historic Environment Scotland and others show that uncontrolled air infiltration through a typical sash window can be reduced significantly when draught seals are installed. The chart on the right here compares air infiltration rates through an unimproved window and a standard trickle vent, and then a professionally draught-proofed window. It shows very considerable improvement.

Ian: We should also consider the benefits of traditional window features or accessories that were provided to control heat gain and losses through windows. These include external blinds and awnings. When you start looking out for the evidence of these in the form of blind boxes, it's striking how common a feature they are on south and west facing windows. Internally, heavy curtains and blinds were used to reduce heat losses and increase comfort. In the research mentioned earlier, the U-value of a repaired sash window was measured in a climate chamber. U-values were then remeasured when the window was covered by heavy curtains and different types of insulated blind. Shutters provide thermal benefits too. U-values were measured in the same experimental set-up as the curtains and blinds. And the results of these experiments are summarised in this chart. As you can see, these features yield significantly good thermal improvements. But aside from the U-value reductions, they also bring substantial comfort to the occupants of the building. I think sometimes we get a bit too hung up about U-values.

Shutters, awnings, blinds and curtains all provide a measure of control over draughts, heat loss and overheating from solar gain. Historic diaries and housekeeping manuals document the care with which these controls were used, especially to control light levels to prevent fading of furniture and fabrics. Although shutters, curtains and blinds only improve the thermal performance of windows at night, this is a time when the rate of heat loss is likely to be at its greatest. In a well-maintained sliding sash window, each sash could be placed at any height within the opening. This offers extremely fine control over the level of ventilation given by an open window, from the tiniest crack up to half the area of the whole window opening. A particular benefit is that when both sashes are open, ventilation occurs through the bottom and top of the window. And in some of this can help to set up a natural convection pattern as warm air at the top of the room is drawn out and cool air is drawn in at the bottom. So coupled with shutters, awnings and blinds, we have here a highly sophisticated environmental control system.

David: So where a window is in good condition but needs further improvement - that's after the whole-building approach to reducing heat loss - the next thing to consider is the glass.

Researchers say that the U-value of a traditional single glazed sash window is around 4.8 to 5.1. This could be reduced to something in the region of 4.2 by applying a Low-E film to the existing glass to reduce radiant heat loss from the interior. Low-E films are unobtrusive and can be applied and removed relatively easily. However, they need to be selected carefully as they vary in the amount of radiant heat they block from the outside. This is important as solar gain is wanted as part of a building's heating strategy in winter.

Greater reductions in heat loss can be obtained by replacing single glazing with insulating glass units, or IGUs. Double-glazed IGUs consist of two layers of glass separated by a sealed or gas-fill cavity. Triple glazing has three layers of glass and two cavities. Conventional double-glazed IGUs are around 22 to 28 millimetres thick overall.

A slim profile, double glazing has been developed in response to perceived market opportunities in the conservation sector and now widely available. They come in at thicknesses of around 10 to 16 millimetres with various types of glass and different types of gas filling. Some use handmade glass for the outer pane, giving the characteristic distorted reflections of historic glass. Low-E glass is also used for the inner pane - pretty much always used, actually. Inert gases such as Argon, Krypton, Xenon or mixtures are used to fill the void. These have a much lower thermal conductivity than air. Xenon gives the best performance but is at least double the price of Krypton. Most recently, vacuum double-glazed IGUs have become available.

These are typically about seven to eight millimetres thick and have a cavity of about 0.2, from which the air has been sucked out in a similar principle to the vacuum flask. The performance of all types of IGU depend on the integrity of the perimeter seals. And although the technology has improved over the years, the lifespan is still limited to about 15 to 25 years.

This needs to be taken into account when thinking about the financial and carbon costs of retrofits, which we'll cover later on in the talk.

However, there are practical limitations on the use of slim profile double glazing in many older historic windows, not least the loss of surviving historic glass. Old handmade crown and cylinder glasses can be as thin as two millimetres. In glazing rebates in the slender styles, rails and glazing bars are correspondingly very small. And this drawing here on the slide actually is a really useful one, I think, because it illustrates very clearly the problem of fitting in different thicknesses of slim profile IGUs that are available. So we took three different types and show the corresponding cover required for the edge seals and the different depths that they have. So, in accommodating the increased glazing thickness, the rebates have to be deep enough to cover and protect the perimeter seals. Failure to cover these seals can shorten the life of them and as well, it doesn't look too good because they're exposed on the outside.

Ian: Where replacement of single glazing with double glazed IGUs is justified, this can be either done within the existing sashes, enlarging the glazing rebates as required, or by replacing the sashes within the existing frame. Either way, in sliding sash windows, the counterbalancing weights will need to be adjusted to compensate for the heavier glazing. Later, Victorian and Edwardian windows like the one on the left have thicker styles and rails that can be fairly readily adapted to accommodate double glazed IGUs. Windows with slender styles and rails are trickier, but multipaned windows with very slender glazing bars are, for the reasons that David has explained, pretty much impossible to glaze in that way.

So where double glazed IGUs are installed in multipaned sashes, applied glazing bars may be the only way of reproducing the original appearance of the window, where the glazing bar is very slender. I can hear sharp intakes of breath coming from you all now. But if it's done well with spaces provided between the inner and outer panes in line with the glazing bars, the result can be visually satisfactory, although the glazing will tend to have a flatter appearance than multiple panes. It will also perform better thermally than individual pains, as heat is not lost through the glazing bars. In addition, it reduces the length of the perimeter seals by more than 40 percent. Although there may be situations where this approach is acceptable, where the pattern of glazing is important to the significance of the building, it will be preferable, as John has intimated earlier, to use secondary glazing with a new single glazed window.

David: So, many of the problems of replacement glazing can be avoided by installing secondary glazing, as Ian has just been discussing. This is installed internally, leaving the existing windows unaltered and without draught proofing to prevent condensation occurring within the gap between the two windows. Secondary glazing can increase the thermal performance of a window to levels similar to replacement double glazing. And it can actually surpass the performance of replacement glazing as it covers both the frame and the glass of the existing window.

Thermal performance can be increased still further if Low-E glass, IGUs, or vacuum glazing is used within the secondary glazing system itself. Secondary glazing is available in a wide range of configurations to suit various arrangements of existing windows, allowing for ventilation and providing access for cleaning. Most types of secondary glazing are designed to be removed during the winter months when its thermal benefits are not required. Secondary glazing, though, is not entirely without its problems. It can sometimes be troublesome to open and close, particularly if you're leaning across in front of a sink or something like that. If not properly draught sealed, it can lead to condensation and mould on the windows - so, moist air getting in from the room into the cavity. Therefore, the appropriate product selection and good quality installation is really important. It's important to note again that the original window should not be draught-proof, where the secondary glazing is installed to minimise the risk of condensation.

Also, some find the appearance of the double reflection, shown here on the right, troubling. But we sort of think this is pretty much a small price to pay when the upgrading of historic windows is justified as part of a whole-building energy strategy. Given that it allows the window to be retained and altered, it's also very easily reversible.

Ian: Well, to conclude this brief review of upgrading measures, this chart compares the whole window U-values of double and secondary glazing, and it shows how effective secondary glazing can be, especially when Low-E glass or vacuum double glazing is used. John.

John: Our principal message is that historic windows are important to historic buildings, that they can be repaired; that with repair and other measures their conservation is compatible with the imperative of saving energy, there will be times when it's appropriate to replace them. Sometimes, inevitably, historic windows will be beyond repair. Equally, sometimes historic buildings will have modern windows which detract from their character and significance. This replacement offers the opportunity to enhance a building's significance, or perhaps its thermal performance.

The question of replacement should be considered in light of the National Planning Policy Framework's policies for heritage assets and particularly of the desirability of sustaining and enhancing the significance of heritage assets and of the great weight to be accorded with conservation of designated heritage assets. We suggest the following approach when dealing with listed buildings. Where historic windows, whether original or later insertions, make a positive contribution to the significance of a listed building, they should be retained and repaired where possible. If beyond repair, they should be replaced with accurate copies. Where historic windows have been replaced with windows whose design follows historic patterns, these usually make a positive contribution to the significance of listed buildings. When they do, they should therefore be retained and repaired where possible and if beyond repair, replaced by accurate copies. If the windows being replaced to multipaned sashes or casements, it will probably not be possible to fit slim profiled insulated glazed units in the new windows, as we've heard. If the pattern of glazing is important to the significance of the building, it would be preferable to use secondary glazing with the new windows rather than lose the pattern. Where historic windows have been replaced, on the other hand, with ones whose design doesn't follow historic patterns - either original or later - these are unlikely to contribute to the significance of listed buildings, and replacing sash windows with new windows of a sympathetic historic pattern is likely to enhance or reinforce the significance of the building. And you can see that it has been done here in this Georgian townhouse. On the left, you see the modern windows which are not of special interest. And on the right you see how the reinstatement of the historic pattern has enhanced the character of the building.

Equally, replacing unsympathetic windows with more sympathetic windows incorporating slim profile insulated glazed units may have no adverse effect on the building's significance while improving its performance. As to the appearance of the glass itself, the reflective properties of secondary and double glazing, as compared to modern, polished single glazing, are unlikely to harm the significance of the building to any degree. But that said, a loss of historic glass would affect the building's significance.

It is, though, generally desirable to reproduce the broken reflections of multipaned windows by individually glazing each pane. This is an approach. Decisions in particular cases will need to be made in the light of the particular circumstances; in the light of the archaeological, historical or architectural values touched upon and the effect of proposals on significance. And they should be proportionate to that significance. This approach is relevant when dealing with windows and buildings in conservation areas and, indeed, to historic buildings more generally. We'll turn to approval and consent later.

Ian: Well, we've been looking at ways of improving the thermal performance of windows, but how do they stack up in financial and environmental terms? The financial benefits of energy saving improvements to windows depend on several factors. They include the thermal performance and cost of improvements, the area of windows in relation to the building envelope and the amount and cost of energy used for heating. In homes of traditional construction, payback periods for replacement windows and double glazing are measured typically in decades. The energy performance certificate for my own home, an extract of which you see here, recommends replacing single glazed windows with Low-E double glazing. If the annual fuel bill savings quoted are correct, the payback period will be between 75 and 150 years. Given the 20-year service life for IGUs and a similar service life for PVCU window frames, this doesn't look like a particularly good deal.

For the window improvements or replacements to be worthwhile in carbon impact terms, the savings in operational carbon emissions have to exceed, or at the very least balance, the carbon emitted during their production, transportation and installation and eventual disposal.

But the life cycle assessment of windows is complicated and hampered by the lack of comparable data from environmental product declarations or other sources on window assemblies and their components. That said, the available data suggest that timber windows perform well in terms of carbon impact. The carbon emitted during production is more than offset by the carbon sequestered in the timber. In fact, most historic single glazed windows will be carbon negative: more carbon is sequestered than emitted during their production.

But if the window frame is disposed of - either to landfill or incineration - the sequestered carbon will be released back into the environment. And if materials that don't sequester carbon are then used for replacement frames, the carbon impact will be greater still. The service life of a timber frame window is at least twice that of a PVCU frame. And the carbon emitted in production is roughly half. However, that doesn't include the sequestered carbon. When this is taken into account, the life cycle carbon impact of timber window frames is carbon negative.

The choice of glazing used in a window also has a significant impact on its carbon performance. Although the thermal performance of gas-filled insulated glazed units is greater than air-filled ones, their carbon impacts are greater and they vary considerably depending on the choice of gas. Vacuum double glazing has the lowest embodied energy of all the retrofit glazing choices. Compared with double glazing and replacement windows, secondary glazing represents an optimal solution in many cases. Single, Low-E secondary glazing is relatively easy to install, less expensive and lower in embodied carbon. Also, it has less impact on heritage values and is easily reversible. If windows need to be renewed, timber replacements of suitable design are preferable as they offer good thermal performance, superior durability and significantly lower carbon impact.

Well, we're nearing the end of our time together today - we're approaching it, anyway - so let's end by considering where the householder stands on permissions, approvals and consent. John, what is the position with development management?

John: If historic windows are beyond repair, or if windows detract from the character of a building and its proposed to replace them, approval may be required before replacement is undertaken.

[they discuss a technical issue with the recording]

Ian:  David is going to say something about building regulations.

David: Yeah, okay.

Alice: Okay.

David: So, if an existing window is replaced, then it becomes a controlled fitting under the building regulations. This requires compliance with standards covering heat loss, which is under Part L; glazing safety - Part N; ventilation - Part F; and fire safety Part B. But not all these regulations will necessarily apply as they depend on the particular context of which the windows is positioned. Part L requires that the new window complies with the current U-value, which is 1.6 at the moment. Part L can also be triggered by the building undergoing a change of use. Most window installers are registered with a competent person scheme and are able to issue a Certificate of Compliance, so that there is no requirement for a formal building regs. application. But crucially, to help reconcile thermal performance and building conservation -- for designated buildings -- with those listed in conservation areas and those which are scheduled monuments, these are exempt where compliance would unacceptably alter their character and appearance. They're also what's called "special considerations", which can apply to locally listed buildings, those in National Parks and traditionally constructed buildings. However, these exemptions and special considerations are down to the specific context and the interpretation by building control or the improved inspector. It would be interesting to hear stories of how these particular exemptions are actually carried out in practice, because I think it's an area that we would like to know more about. So I think that's me on building regs.

[They discus a technical hitch with the participants and John rejoins]

John: So, to start again with consent, listed building consent would be necessary to replace windows in a listed building. Approval is not usually necessary in conservation areas, but in many such areas, additional control in the form of articles or directions will be operable, requiring approval of replacement windows. Local authorities’ websites should provide information about the level of control which applies. Secondly then, some more complicated points relating to works to windows in listed buildings. Repairs shouldn't require listed building consent, unless entailing substantial replacement. Replacement of early blown glass, crown glass or painted, etched or decorative glass will require consent. Reinstatement of lost features such as glazing bars is likely to require consent. Alteration or removal or reinstatement of blind boxes is also likely to require consent. The installation of draught-proofing and secondary glazing is unlikely to require consent unless it would damage the fabric of the building or, for example, prevent the working of historic shutters. Consent may be required for the removal of window furniture, depending on its contribution to significance. For many of these works, judgement will be needed, and we would encourage owners to discuss works with planning authorities, if in doubt. Certificates of lawful works can be obtained for works which do not require listed building consent to avoid ambiguity. Back to you, Ian.

Ian: Okay, well, that actually just about winds up the presentation. We hope it's been interesting and helpful. To conclude, our message overall is repair, not replace, as you all have gathered. But here are a few more takeaways that you can read at your leisure. Meanwhile, we're here to take some questions.

Alice: OK, so I have taken some questions from the chat box, but I will also put this box in here if there's anybody who'd like to add more questions while we take the ones that were originally asked in the chat. Matt, if I could just get you to start the Q&A box, because I don't seem to be able to do that.

And the first question I have for you gentlemen is "I've got condensation issues with leaded light secondary", but I'm not sure if it's part of a question, it seems to have disappeared. I don't know if you have anything to say about condensation issues with leaded light or secondary windows.

Ian: I mean, the question regarding "Have we done any research?" or "Do we have any guidance?", some research work has been done in connection with stained glass windows and the use of the environmentally protective glazing. And if you look on our website, I think you will find that there is a research report, and the Guidance document is certainly in the process of being produced. I'm not quite sure where that is in the publication schedule, but it's on its way. But I mean, that relates primarily to stained glass windows in leaded light in ecclesiastical building rather than a domestic context, although there might be something there that is useful.

Alice: Another question we have is, "Recent LBC guidance suggests inserted brush seal should not be approved."

Ian: John, do you want to say something about that?

John: I'm not quite sure which guidance that is. At the moment, we've got out consultation, guidance about what does and doesn't need listed building consent. And that says that that sort of repair might need consent. It doesn't say it shouldn't get consent. I'm not quite sure that's where the question arises from. If not, I don't know which is which guidance it does come from. It's certainly not our approach.

Alice: Okay, and then there's the next question: "U-values are overplayed."

Ian: Oh, that was my off-the-cuff remark. There are a lot of things that influence the thermal comfort of people in buildings and U-values are a measure of the rate at which heat is transmitted through the building envelope. It doesn't always reflect comfort conditions. So, for example, some things that you could do to increase thermal comfort, you know, like having finishes that remain closer to air temperature. I mean, in medieval buildings, people had sort of wall hangings and things like that. That will actually reduce the amount of radiant heat that you lose from your body and people are very, very sensitive to radiant heat loss. So, a feature like that would make you feel more comfortable. It would probably allow you to feel comfortable to lower air temperature. But it wouldn't actually contribute very much to a U-value.

Alice: OK, we have another question from earlier: "What are your thoughts on vacuum insulated glass?"

David: Ah. Well, we actually use this in one of our research projects, didn't we, Ian, at Bolsover? We used it in the secondary glazing. It's incredibly expensive, and when we used it, I know that it came all the way from Japan, it had to be especially imported.

Ian: Well, I think there is a product now produced in Belgium, which --

David: Yeah, it's slightly different -- It's still from Pilkington, isn't it?

Ian: No, it's a Belgian firm, and in some ways it appears -- well, visually, at any rate, superior to the Pilkington product. The building in Pilkington product had little sort of spacer dots which were visible, and they also had a little sort of black button where the air had been sucked out of the cavity between the sheets of glass. So the Belgian product doesn't have any of those sort of visual disadvantages. But it is I think in many respects -- this is a personal opinion, preferable to gas-filled double glazing.

David: It certainly gives a very good performance, doesn't it?

Ian: And it gives a very, very good performance. Yes, yes. But as David said, it is expensive.

David: And I mean, we showed it actually in one of those drawings that we put up, which showed the comparative types of IGUs.

Ian: Yeah, I mean, even though it's thinner than the gas-filled IGUs --

David: It's still got the cover requirement, hasn't it?

Ian: You've still got the cover requirements at the edge. Yes, that's right. So it won't work any better in practical terms with very thin glazing bars than with any of the other types of slim profile IGU. But I think it has potential value when used in secondary glazing and perhaps in windows with, you know, one-over-one configuration windows.

Alice: OK, another earlier question, which I think has been repeated was: "Where do you stand on Histoglass, RT+? Higher thermal performance? Single glazing? Any thoughts?"

David: Well, I mean, that is just one of the types of slim profile glazing, which I think we showed -- I think one of those probably was a Histoglass configuration. I mean, they're all very similar, I think, apart from the vacuum which is rather different. They're all based on the gas infill. I mean, there is a huge number of variants in terms of the types of glass that --

Ian: I think Histoglass do a single glazing, which I think has a low-emissivity film applied to it. So, yes, you will gain some advantage in it will reduce radiant heat loss to some extent. But if you want to look back through the slides of this presentation at the chart that we showed earlier, that shows we're a Low-E film sits in relation to other forms of insulated glass. I think the Histoglass material will be sort of similar to that.

Alice: OK, we had another earlier question: "If you use applied glazing bars and you get a broken window, don't you need to replace the whole window?"

Ian: Yes!

Alice: OK, I think that was also answered a little bit in the chat.

Ian: I think the same person asked whether the applied glazing bars would fall off, and that's a very good question. I actually have, where I live, a window that was here when I got here, which is double glazed with applied glazing bars inside and outside and white spacers between the two sheets of glass which correspond to the addition of the glazing bars. I've been here 12 years now and the window has been redecorated externally once and it's not actually showing any signs of falling off. I think there are double sided adhesive tapes which are used in the automotive industry, which are incredibly durable. I think if a good adhesive is used, then they don't fall off... quickly.

Alice: Another earlier question is: "Can you say something about the ventilation implications for reducing the ventilation from windows by improving them?" I'm not sure I understand that question.

Ian: Yeah, well, I understand what you're saying, and, yes, you must ventilate. I mean, I think, you know, the mantra is 'no insulation without ventilation'. If you're going to reduce -- Or if you're going to increase air tightness, for example, you know, you must provide ways of removing surplus moisture from the air within the building. So, that might mean opening the window, for example.

Alice: Another question is: "You mentioned new timber won't last as long as old timber, but what about Accoya for example?"

David: Ah, yes. I'm not an expert on Accoya, but it's certainly incredibly more durable, isn't it, than quickly grown softwood? I think, Ian, you've sort of looked at that in some detail, haven't you?

Ian: Yeah. Accoya is a chemically modified timber. It's actually Radiata pine, which is acetylised. Basically the sapwood is acetylised, so that means that water can't attach itself chemically to the cellulose in the timber and thereby making it susceptible to insect damage and rot. So it doesn't actually make for a very durable timber and it works pretty much like a soft wood, you know. I mean, it machines well. You can glue it. You can paint it. I think it's a very interesting product. Its main disadvantage is its costs. It is, again, very expensive.

Alice: Okay, and now we're moving --

Ian: Sorry, no, it's alright, go on.

Alice: We can now move on to the questions in the question box. We'll start at the top. "Can you expand on the whole elevation approach where some windows are historical and others compromised?" I think that was the question.

Ian: That sounds like one for you, John.

John: Yes. Well, I think that the approach I described in the talk can be applied to elevations like that. If windows are compromised and detract from the significance, or detract from the value of the building, there's a potential to replace them with something more appropriate. And that something more appropriate might match the other windows... if it's a unified elevation of the Georgian house, for example - that's obviously desirable. Or it might simply improve on what's there now while also performing better from a thermal point of view. So, I think you can apply the approach which I've described, which is set out in our windows guidance.

Alice: Okay, and then the next question from James: "Would you consider a frame too rotten to be repaired? Is this just down to an individual joiner's skill?"

David: Ah, now that's a good question, because I think this whole issue of 'when is the window beyond repair?' is quite a tricky one. I think a lot of people quickly come to the conclusion that a window is beyond repair when actually, most bits can actually be really easily repaired, particularly sills. What do you think, Ian? There's no, sort of, hard and fast rules, is there, but --

Ian: I suppose one of the measures you could apply, I suppose, is when, you know, the cost of repair starts to exceed the cost of replacement.

David: And also there's the problem of actually finding joiners to do the repairs, and I think it's probably less of an issue than it used to be, in that a lot of the firms that do draught-proofing will actually do repairs before they do the draught-proofing so they can actually get the window in good shape. But I think it's still I think it's still quite tricky to get a good joiner to do window repairs.

John: Can I just add to that? Another consideration apart from the cost is the significance of the window. So, yeah, logically, the more interesting, the more important it is to repair it, the more effort it merits.

Alice: We've had an additional question on timbers - if there was any advice on paint for outside timber windows? Just to add to the timber conversation.

Ian: Well, there are -- I mean, there have been quite considerable developments, I think, you know, in paint technology and paint products over the last 20 years. So, if one is sort of starting with bare timber, you know, there's quite a wide range of materials that may well be more durable than the kind of B&Q materials that we've been sort of accustomed to in the past. And I mean, some of the linseed oil paints have looked very promising in that respect.

Ian: But if you're painting previously painted surfaces, then I think, you know, again -- you need to choose good quality materials. And there are sorts of materials -- paint materials that have, you know, pretty good durability now. But if you're over-painting, it depends on just on how good your substrate is. So, you know, if you're putting new paint on old paint that's not adhering very well, then it's not going to last very long. Again, you know, it's quality of workmanship as well as the quality of the material that you're using... as the coating.

Alice: Another question regarding Part B: "When undertaking repairs to severely damaged sash windows, which sit low to the internal floor, is there a safe option to retain historic glass which is not safety glass? Secondary glazing is not an option due to dimensions of the window and internal shutters.

David: Well, I think that's sort of down to the building control to really work out, I think. I mean, these things really need discussion, I think. I don't think there's hard and fast rules, really. I mean, if the window is sort of historically significant, there's obviously the issue of exemptions. I think it really needs to be looked at in its particular context.

Alice: We have a question from Guy on the chat: "Can the perimeters of IGUs be bedded into standard putty?"

Ian: I think they can be, but that's generally not recommended, I think, by the producers.

David: I think it does vary, it seems. Some are okay with and some are not. I think it probably depends on the exact seal being used. I think it does vary.

David: Yes, follow the manufacturer's instructions. Yes.

Alice: The next question from Mary is: "Most manufacturers will only guarantee small panes of IGU for five years. Where does the 15 to 20 years come from?"

Ian: Well, I think the 15 -- Is that right? I've not seen such short period guarantees as that. That's interesting to hear. I mean, 15 to 20 years -- 15 years is not an uncommon period for the guarantee. But generally they will, you know, they will last longer than that. But I think basically what this is reflecting is the problems that we were talking about earlier about using individual IGUs in multipaned windows. And it's clear that the manufacturers are a bit nervous about that.

David: Yeah, I think that's what's given rise to the applied glazing bar --

Ian: The applied glazing bar approach, yes.

David: Which an awful lot of window manufacturers who have constructed new windows with multiple small panes, a lot of them now are not doing individual IGUs for those. They're only using one sheet. And I think that's borne out probably because of the risk of seals breaking down on, you know, multiple panes.

Alice: The question following that is: "Some joiners have told me that they cannot guarantee slimline double glazing. What do you think of this?"

Ian: I think the slim profile double glazing possibly has improved over the years. When it started, it was a bit of a cottage industry. And I think the quality of the units was quite sort of variable. So you never quite knew how much gas they had in or whether the edges were properly sealed and things like that. So I think they possibly acquired, you know, maybe a bit of a bad reputation at the outset. But I think, certainly, things have improved. The quality of them has improved over the years.

Alice: The question following this is asking to explain the issue of compensation in this regard. I think it's in regard of slimline double glazing. I'm not sure, there's no extra...

David: Is that probably when the seal has failed and condensation has got in between the two sheets of glass?

Ian: Unless it's a question relating to secondary glazing, because you can get condensation if the outer window is too well-sealed and the inner window isn't sealed enough.

Alice: Okay. "FINEO slim vacuum sealed, double glazing is often not allowed in grade two listed buildings. What is the reason behind this?"

John: I don't know if the question is about a particular type of --

Ian: The FINEO glass, John, that's the vacuum double glazing. So it would be, you know, sort of about seven millimetres thick.

John: Yeah, I mean, I think there are a number of things to say in response to that. Firstly, attitudes to this whole question have been shifting slowly over recent years. So, the position now is probably more liberal than it was a few years ago. Nonetheless, if the units couldn't be fitted without adjusting the glazing bars, that might be a reason for not allowing it. It would rather depend on a case-by-case basis looking at the particular windows and the effect of installing the units on the glazing bars and the character of the windows concerned, I would've thought.

Alice: "Are there any recommendations for the secondary glazing installation on the exterior versus interior of a window?"

Ian: That's an interesting question. I'd be interested to know where the questioner is located in the world because, certainly, it's not something you see in the UK at all often. I mean, I've seen it in the United States --

David: I have seen it in the UK, but very, very rarely. It does look very strange, I think. So it's not something I think we would be--

Ian: I mean, I think it's quite common practice in North America where you have storm windows which are fitted sort of externally rather than -- So it's sort of secondary glazing on the outside rather than on the inside.

John: I think this guy's from Canada.

Alice: And then I have another question: "In terms of replacing historic timber windows in an unlisted building in a conservation area, do you consider that replacement UPVC would be similar in appearance to a timber window? As if it's not similar in appearance, planning permission is required under the Householder Technical Guidance document.

John: I think that's a rather vexed subject and it will depend on the nature of the UPVC replacement. Clearly, very often they don't resemble timber windows very closely at all. Some other types may do so, again, there's a judgement to be made in each case, I suspect.

Ian: Yeah, and if we care about the life cycle, you know, the whole life carbon costs of these things, then that is a very strong argument against PVCU.

Alice: And I think that is where we will draw these questions to an end and thank you very much, David, Ian and John. There are a few more questions, but I will send these to you, Ian, and we will look at them.

Ian: Okay.

Alice: We don't have time to finish them, but we will send them in a report to you so that we can get them through. So thank you very much, everybody, for attending. I'm just going to close down the chat.

Ian: Thank you for the interesting questions.

Alice: Yes, thank you.