Windows Doors and Porches
Traditional windows are an intrinsic part of the character of our historic and vernacular buildings. In Ireland, most surviving traditional windows are timber- framed, vertically sliding sash windows with single glazing. Other traditional windows include casements or fixed lights of timber or cast iron, leaded lights and twentieth-century metal framed windows. The quality of the timber and workmanship found in older windows is generally far superior to that found in modern ones and, when properly repaired and maintained, traditional windows will commonly outlast modern replacements. For further information, see Windows: a guide to the repair of historic windows in this Advice Series.
10-15% of the heat lost is through its windows
Between 10-15% of the heat lost from a building can be through its windows, by a combination of radiant heat loss through the glass, conductive heat loss though the glass and frame and ventilation heat loss through gaps in the window construction. This is low compared with the estimated average 25% heat loss through the roof and 35% through external walls. Yet windows are most often the first target of energy efficiency works.
In terms of heat retention within a building, older windows may appear to perform poorly when compared to some modern windows. It is, however, possible to repair and upgrade traditional windows to bring them up to a similar, if not higher, standard than modern double-glazed windows and to improve the comfort of occupants without damaging the character of the building. Prior to considering works, the actual heat loss through the windows should be considered. In buildings where windows are small compared to the overall wall area, upgrading the windows may not result in a significant improvement in comfort levels or in energy savings.
Factors to consider when replacing windows
When considering the replacement of windows, a number of factors should be taken into consideration. First and foremost is the potential effect on the character of the building and the architectural heritage value of the existing windows. Also to be considered are the financial cost, the energy required to produce a new window, its embodied energy, and the environmental cost related to disposal of waste. Modern double-glazed window units are expensive and high in embodied energy. The initial financial cost and embodied energy consumption may never be recouped by cost and energy savings on heating bills within the serviceable life of such windows. Instead, simple upgrading of existing historic windows can eliminate draughts and reduce heat loss. This costs less and is kinder to the environment than fitting new replacement windows.
uPVC or Not?
The use of uPVC in traditional buildings should generally be avoided. uPVC is a material with very high embodied energy which has a short lifespan as it is difficult, if not impossible, to repair. Simple wear-and- tear often results in whole units requiring replacement after relatively short periods of time. The manufacture of uPVC also results in many toxic and environmentally damaging by-products. In addition, uPVC is generally not recycled or compressed and must be disposed of in landfill sites as the burning of uPVC can result in the emission of toxic fumes.
The installation of uPVC windows in a protected structure or within an architectural conservation area is generally not considered acceptable as such windows would be inconsistent with the character of historic buildings.
Draughts may result in heat loss and are also uncomfortable, resulting in a perception that a room is cooler than it actually is. Draught proofing of a window will not improve its U-value but stopping draughts will reduce heat loss and improve the thermal comfort of the occupants. The overall aim should be to gain control of the rate of ventilation in the room concerned.
The first step in reducing draughts is to overhaul the windows by carrying out any necessary repairs and ensuring that the sashes or opening lights operate properly within the frame. A window that is in good working order can be fitted with draught-proofing strips. However, with some particular old, delicate or valuable window frames, cutting grooves to insert draught proofing will not be appropriate and expert advice should be sought on alternative methods of upgrading.
Typically gaps up to 6 mm can be filled with any one of a variety of available strips including nylon brushes, pile (dense fibre), polypropylene with foam filler and silicone rubber tubes. The fitting of strips varies with some fixed to the surface of the frame and others fitted into the frame by cutting grooves into it. When fitting a product that requires grooves in the frame care should be taken to ensure that the joints are not damaged in cutting the grooves: these are best fitted by a specialist joiner. Care should also be taken to ensure that existing ironmongery such as handles, catches and hinges will continue to function correctly following draught-stripping and that the colour of the product is appropriate to the window. Dimensions of draught strips should be appropriate to the gap to be filled as larger strips will put pressure on the window itself and smaller ones will not adequately seal the gap. Strips should have some flexibility in them to ensure they will work with the expansion and contraction of timber between summer and winter months. Metal and timber casement windows can be upgraded with similar type draught strips. Casement windows can also have mastic sealants applied to form a moulded profile when the opening section is closed over the mastic to shape the sealant to the gap. Care should be taken to use a barrier to prevent the opening window from sticking to the silicone and the window frame when fitting the seal.
There is a wide range of quality in available draught- proofing products and assurances should be sought as to the lifespan of a product prior to fitting. In addition, it is important that the product can be removed easily without causing damage to the historic window frame to ensure that when it reaches the end of its life it can be replaced. It should also be noted that flexible draught-proofing strips such as brushes and rubber will cease to operate correctly if painted as part of redecoration works.
As discussed in the section on ventilation above, windows in rooms with no alternative means of ventilation such as wall vents or open flues should never be fully draught proofed.
External doors in an older building may have become ill-fitting over the years and so are often poor at keeping in heat. Traditional doors can be draught proofed in the same way as windows with various draught-proofing strips widely available. The bottom
of external doors can also be fitted with a weatherboard providing this can be achieved without damage to a historic door. Letterbox brushes or flaps can be fitted to reduce draughts. For historically important buildings, discreet draught proofing should be used. In some buildings it may be possible to provide a draught lobby to the interior of the external doors. For a draught lobby to be successful there must be adequate space to close the external door prior to opening the internal door. Installing a draught lobby in a protected structure may require planning permission and the architectural conservation officer in the local authority should be consulted when considering works.
A single sheet of glass will transfer heat quicker than a double-glazed unit. People feel colder sitting close to single-glazed windows as they lose heat by radiation to the cold inner surface of the glass. Tall windows can result in what is known as ‘cold dumping’, where the temperature of the air next to window is considerably colder than the rest of the room, as the cold air is denser and heavier it falls, or dumps. This is one of the primary reasons for placing radiators below windows. There are simple solutions to keeping heat in a room with single glazing that are more effective than fitting double- glazed units and more appropriate for use in a historic building and several of these are discussed below.
Many Georgian and Victorian buildings were originally constructed with internal timber shutters to the windows. During the Edwardian period, shutters began to fall out of fashion and were supplanted by heavy curtains. The best way to reduce heat loss in the evenings and at night is to use such shutters. If they are no longer operational they should be repaired and put back in working order. Blinds or heavy curtains, which could include an insulated inter-lining, when used with the shutters will further improve heat retention; there are specially designed thermal blinds available which can improve on this again. There may be some scope for upgrading shutters using a thermal lining applied to the rear of the shutter panels; for the shutters to continue working it is important that the overall thickness of the shutter is not increased. The feasibility of upgrading will depend on the available depth between the shutter panel and shutter frame.
It is likely that the available space will only allow for a lining depth of approximately 5 to 10 mm. High- performance, super-insulating linings should be considered for thin spaces of this type.
Where the original timber window shutters have previously been removed from a building, or from parts of a building, consideration should be given to reinstalling shutters of an appropriate design accurately based on evidence, for example, from shutters on an adjoining contemporary building or from evidence within the building itself.
The window aprons (the area of wall between the window sill and the floor) can be an area of increased heat loss as the wall thickness was often reduced at windows to provide a recessed opening. Where the window apron is timber panelled, the panels can be carefully removed and the void behind filled with insulation. The depth of insulation possible will depend on the available space between the timber panelling and the external wall. A specialist joiner should be consulted and appointed to undertake works to the shutters and window apron.
Check the shutter box
The shutter box, into which the shutters fold when not in use, is often a source of draughts that is overlooked. To reduce or eliminate air movement in and around the edge of the shutter box, the exterior should be pointed up with an appropriate material which remains flexible following hardening and provides a long-lasting unobtrusive seal between the window frame and surrounding masonry. From the inside, the junction of the interior of the shutter box and the wall should be caulked with environmentally friendly hemp/lime products or other suitable materials. When sealing the interior of the shutter box, it is important to ensure that the caulking does not interfere with the operation of the sash weights or the shutters, such as may occur if using expanding foam, which is not easily controlled.
For buildings that are primarily used during the day it may be appropriate to consider secondary glazing. Secondary glazing is a full-sized window panel fitted directly inside the existing window, which acts in a similar way to double glazing. It can be temporary or permanent and should be fitted to slide or open inwards in such a way as to allow for easy opening of the original windows for ventilation purposes, cleaning and emergency escape. The style and manner in which the unit opens should be visually appropriate for the window to which it is being fitted and easy for the end-user to operate. Any division in the panel should be located to match the frame of the existing window, such as at the meeting rail of a sash window. Duplication of individual panels looks unsightly from the exterior and should be avoided. Secondary glazing should be sealed to the interior but the original windows should be ventilated to the exterior to prevent condensation forming between the two windows, which is not only unsightly but is potentially damaging to the historic building fabric. Therefore, if secondary glazing is to be fitted, the original windows should not also be draught proofed.
While secondary glazing is effective it is only appropriate if it does not affect the character of the windows and room in which it is fitted. Formal rooms or rooms with high quality decorative finishes may be compromised by the fitting of secondary glazing. The use of the room is also important. If rooms are plain and used as, for example, offices or kitchens, the fitting of secondary glazing may be appropriate. If rooms are not often used during the day it would be more appropriate to leave the windows as they are and use any existing shutters.
Secondary glazing should always be fitted in such a way that it is still possible to use existing shutters. Slim-line secondary glazing is available which can be fitted in place of the staff bead between the bottom sash and the shutters. This allows the shutters and curtains to be used at night when outside temperatures are lower. The combination of secondary glazing, shutters and curtains has the potential to match the insulation properties of triple-glazed windows. Secondary glazing alone can result in better overall thermal performance than a standard double- glazed window. The fitting of secondary glazing should be reversible and carried out with minimal interference to the existing window, shutters and linings. The fitting of secondary glazing in windows which retain no historic linings to the interior allows for more flexibility in the type and size of secondary glazing frame which may be fitted.
Secondary glazing has the added advantage that it can be removed and safely stored during warmer months to maximise solar gain (the heat gained from the sun through the windows). When sunlight passes through a pane of glass, its light and heat are absorbed or reflected; the greater the number of panes of glass the smaller the amount of heat and light passing to the interior of the room. It is therefore advantageous to be able to remove the secondary glazing during the summer and benefit from the maximum light and heat from the sun. During the winter, when the sun is not as hot, the amount of heat lost from the interior, if not secondary glazed, will outweigh the amount of heat to be gained from the sun. Secondary glazing has an additional benefit in that it reduces the amount of noise which passes through a window.
Original or early-replacement windows in a traditional building should generally not be replaced with double-glazed windows. Replacing windows in a protected structure requires planning permission and this is unlikely to be granted as double glazing will rarely be in keeping with the character of traditional buildings; modern double-glazed windows are made with chunky sections of framing which are necessary to hold the double-glazed units in place. These proportions are very different to those of traditional windows which are generally made of fine timber sections.
The fitting of double-glazed units into existing timber frames is rarely appropriate or achievable; in order for the glazing units to be effective at reducing heat-loss the gap between the glass panes in the unit should be a minimum of 12 mm, resulting in a total unit depth of approximately 20 mm including the two pieces of glass. Historic sash frames are generally finely crafted from slim sections of timber, the depth and strength of which would not be adequate to support double- glazed units. The existing windows would be both visually and physically compromised as a result. In addition the aesthetic appearance of the black or silver edging to the double-glazed units is unsightly.
Double-glazing technology is constantly improving and research is currently being undertaken to reduce the depth of double-glazed units, while maintaining effective U-values. The use of slim-line double-glazed units may be appropriate in situations where one- over-one pane sash windows require replacement but not where the existing historic glass survives or where the new units would be too heavy for the historic window frames. As with all double-glazed units, the cost of these high-tech components is unlikely ever to be recouped over their lifespan, while the gases used to fill the cavity can have a high embodied energy.
Upgrading traditional rooflights generally involves some loss of historic character. Older rooflights should be maintained in good working order. If a rooflight has reached the end of its working life it may be replaced with a modern rooflight that matches the existing, probably timber or steel, with similar profiles. As rooflights differ from windows in detailing and design, it will often be possible to incorporate a double-glazed unit. New, small double-glazed rooflights are available off the shelf. Light shafts leading to a rooflight should be insulated in the course of providing roof insulation.
Anybody who has been unfortunate enough to have lived in a house with single glazed windows will remember all too clear those damp days with mist covered windows. The most basic form of double glazed window, on the other hand, is a far superior relation. This was the first generation of energy efficient windows. The development of thermal resistant windows continue to evolve with triple glazed gas filled windows becoming the new norm.
Double-glazed windows consist of two panes of glass separated by a gap of about 16mm. This void between inner and outer glass creates an insulation barrier thus retaining the heat inside the home. The best energy performing windows are ones that use low emissivity glass. (Low-E). the Low-E glass contain a radiation barrier coating. These coating reflect radiant heat thus adding to the heat gain performance of your home. Further improvements can be made by ensuring that the window frame and surrounding areas are installed with airtight materials. It is important, therefore, to use contractors that will deliver very high standard of installation.
All windows should display its u-value. The u-value is the rate at which thermal energy is conducted. The lower the u-value the better the energy savings.
Windows that have an energy rating will have the u-value of the window displayed on the energy label. A u-value is a measure of how easily heat can pass through a material. Materials that let out more heat have higher u-values whereas materials that let less heat pass through them have lower u-values.
In some cases, windows with a higher energy performance rating might have a higher u-value than windows with a better energy efficiency rating. This might seem the wrong way round as lower u-values indicate better insulation levels. However, in these cases it will be that there are other aspects of the window that make them better overall such as coating used on the glass and the gap between the glass panes.