Guidance on the Usage and Specifying of Secondary Glazing

This guidance provides useful information on the principles, disadvantages, materials and methods for upgrading the thermal performance of windows with the addition of secondary glazing.

Windows make a major contribution towards the character of historic buildings and every effort ought to be made to retain them. They could also reveal a good deal about the history of a structure; changing architectural taste and style, social hierarchy, building economics, craft skills and industrial advances.

Older windows may well be draughty as eventually they distort as joints become stressed. Although adequate ventilation is essential in older buildings extreme air leakage through windows wastes heat and is miserable for occupants.

Carefully planned and installed secondary glazing allows the original windows to be preserved unchanged, and where necessary repaired, whilst reducing air leaks and conducted heat losses. Consequently there isn’t a damage to historic fabric as well as in most instances the installation is easily reversible.

Recent study has revealed heat losses by conduction and radiation through a window generally speaking can be decreased by over 60% by making use of secondary glazing with a low emissivity (low-E) hard coating facing the exterior. The research has also shown that further savings can be made if the secondary glazing utilizes insulating frames or uses double or vacuum glazed units.

In addition to increasing the thermal performance of windows, secondary glazing can have a number of additional extra benefits including being highly effective at limiting noise transmission.

For listed properties it is important to refer to the Conservation Officer at the local planning authority for guidance ahead of the installation of secondary glazing. Listed Building Authorization may be required in some cases.

What exactly is secondary glazing?

Secondary glazing is nothing new. In the 19th century some buildings were constructed with internal secondary glazing designed as part of the original layout. Often a second double hung sash window or solid panels with counterbalancing weights were fitted in the space below the window. Their function was to cut down the heat loss and provide some measure of sound insulation to the window aperture.

Secondary glazing is a wholly independent window system applied to the room side of pre-existing windows. The original windows remain in place and in their original unaltered form.

Secondary glazing is available as open-able, removable or fixed units. The open-able panels can be either casements or sliding sashes. These types permit access to the external window for cleaning and the opening of both the secondary glazing and external windows for fresh air. New secondary glazing is designed to be detached in warmer months when its thermal extra benefits are not necessary.

Using glazed external protection for windows using either glass or plastic sheet is known as as ‘storm glazing’ rather than just secondary glazing. Doing this can often be used to protect stained glass in churches. Using this type of system it is important to understand the possible circumstances this produces in the airspace between the existing glazing and the extra external glazing where ventilation will be required. Account should also be taken of the environmental conditions inside the structure before designing the installation.

One option is to install storm glazing in the winter months and remove it in the summer months.

Repairing existing window openings

Established hardwood and metal windows can almost always be restored, even when in reasonably poor condition and as a rule at significantly less cost than total replacement. The solid timber used in the past to make windows was of a high quality and very durable.

Lots of Georgian and Victorian windows are still in place today whilst contemporary windows can require replacement after barely 20 years. Reconstructing windows is the best way of keeping the visual character and architectural significance of a building’s elevation and can add to its value.

Before starting any upgrading work such as draught-stripping or the adding of secondary glazing, evaluate what repairs are needed to make the windows fully functional. Windows deteriorate over time so regular renovation, cleaning methods and painting is always a good investment.

For listed buildings, the total replacement of a window is likely to entail Listed Building Consent.

Secondary glazing or double glazing?

Double glazed windows typically have sealed glazing units with 2 panes of glass divided by an air gap (commonly of 12-18 mm) that enhances thermal insulation, particularly if the glass is coated and the air gap is filled with an inert gas. It is an important improvement that has produced sizable energy cost savings and reductions in carbon dioxide emissions, particularly in new buildings. The Building Regulations make double glazing pretty much obligatory in new building.

Often the replacement of current windows using double glazed units can in many situations lead to a change in appearance, particularly the smoothness of new glass and the need for wider hardwood sections and glazing bars.

In historic buildings, there should be a strong preference for renovation rather than replacement as the use of double glazing will inevitably lead to a loss of traditional fabric. Adding secondary glazing would often be the preferred alternative.

The benefits of double glazing over alternative methods of window improvement are often overestimated. Much of the comfort and energy performance benefits of new double glazing come from the reduction of draughts that will result from well-fitted window frames with integral draught-proofing. These extra benefits are also available through repair and draught-proofing of the existing windows, or from fitted secondary glazing. With regular improvements in the efficiency of secondary glazing it may even be possible for the performance of secondary glazed windows to exceed that of new double glazing.

In terms of noise elimination, double glazed units are no better than single glazed units; and can be to some extent inferior for traffic noise. The important principles for noise reduction are that the windows are well fitted and draught-proofed. Secondary glazing, with its much wider gap between the window panes, is a better sound insulator. Wooden shutters and heavy drapes can also make notable improvements to noise insulation.

Secondary glazing or draught-proofing?

Draught-proofing is more often than not the first alternative to give consideration to for improving the energy efficiency of windows in an older building. As windows are often a foremost source of air infiltration draught-proofing using seals is one of the greatest ways of enhancing comfort and limiting energy use, with little or no change to a building’s look at nominal cost.

However, secondary glazing can give you a noticeably higher thermal overall performance than draught-proofing alone. It may also be chosen where installing draught-proofing seals to the windows is especially difficult.

Similarly many metal-framed windows have gaps that are too big to seal.

Windows with leaded lights may allow air infiltration around the lead which can be addressed with the help of secondary glazing.

Dependant on the building’s use, location and the occupants’ comfort requirements other advantages and benefits of secondary glazing, for instance noise reduction, may have a bearing on the design and style solution.

If secondary glazing is the preferred solution then the outer windows are better left without draught-proofing to make sure that there’s an amount of ventilation to the air space separating the outer windows and the secondary glazing to stop the build up of condensation.

Lead Paint

Lead paints can be harmful to health, especially children.

Lead based paints are often found on older buildings. Sometimes these paints have already been over-painted. If there is any uncertainty about the existence of lead paint on windows that are going to be stripped, it must be assumed that lead paint exists and precautions taken accordingly.

The usage of lead paints already has been generally banned as a consequence of the danger to health. That being said, there is an exclusion to the ban which allows them to be used on Grade I and Grade II* listed buildings. On those structures the traditional appearance of the lead paint, together with its longevity and its fungicidal and insecticidal properties, mean that it is quite often still used. It should only be applied by qualified decorators using proper protective equipment, and it is not recommended for use where it may be in the reach of children.

The benefits of secondary glazing

Although the main purpose of secondary glazing units in older buildings is to improve the thermal capability of windows by draught-proofing as well as reducing the conduction of heat through glass, secondary glazing can provide a number of other benefits.

Thermal Benefits

Heat loss from a room via a window in the heating months are complex as three major elements are in play:

*by convection and conduction, from the warmer room air to the colder of the glass and the frame

*by the cold surface of the window soaking up infra-red radiation from the room

*by uncontrolled air leakage, which can also bring in cold air from the external surfaces or take warm air out from the inside; often called air infiltration, this can occur even when the window is closed.

Heat loss through the glass and frames

It doesn’t matter if it leaves the room by convection, conduction or radiation, the lost heat all goes through the glass and also the frame as conduction. The glass is essentially the most conductive area of the window but heat is typically lost across the frame although at a lesser rate.

Single glazing is a poor energy insulator and readily conducts heat. A typical 4mm thick glass has a archetypal U-value of 5.4W/m2K. The thermal loss through a single glazed window will depend on the overall area of glass, the conductance of the frame material and the quality of the installation of the frame and double glazing materials. A typical value of a hardwood framed single glazed window is 4.8W/m2K.

For thermal performance, the optimum airspace between panes is 16 – 20mm. A better air space lets convection currents to develop within the void and more heat to be shed. The positioning of the secondary unit is typically determined by the window reveal and can often only be fitted at a distance of about 100mm from the prime glazing. Having said that, a significant proportion of the thermal benefit of secondary glazing comes from decoupling the frame from the prime wooden window frame and this can reduce the U-value to approximately 2.5W/m2K.

The use of low emissivity glass for the secondary glazing can further improve the thermal efficiency to less than 2.0/m2K. To maintain this figure it is important to keep the finish clean – the standard is ‘visually’ clean.

Heat loss through air leakage

Heat losses from a typical conventional window are primarily through breaks around the window. With larger windows the proportion of heat shed by conduction through the glass tends to be larger. Since draughts, caused by convection and air infiltration make people feel less warm, the people in the room may turn up the heating, and also operate it for longer.

Bespoke secondary windows, with efficient perimeter sealing and brush or compression seals on the opening panels, generate an effective seal over the whole of the mainframe of the main window and can substantially moderate excessive draughts.

Prior to embarking on a process of draught-proofing, think about a fan pressurisation analysis to find out the particular sources of air penetration and determine which windows require attention as they can contrast substantially in the amount of draughts they let in.

Quantifying Draughtiness

The draughtiness of a building will depend upon the amount of air that can pass through its exterior envelope – walls, floor and roof covering. This is alluded to as the air permeability. The industry standard is to articulate the permeability of a wall, roof, or complete structure envelope accepting a pressure difference of 50 Pascals across the wall. The permeability is then measured as the amount of air (in cubic metres) that will pass in an hour through a square metre of wall (or roof, or floor) and expressed as m³/h/m² (m³/hm² or m/h) at a pressure difference of 50 Pascals (50 Pa).

While leaks in the structure is what creates draughtiness and ventilation, what is more important for the structure and its occupants is the speed at which air proceeds through the structure. This is most simply measured as the number of times that the air in the building alters each hour (shown as ac/h). This will depend upon the pressure difference between the outside and the inside of the building, and again the industry norm is to assume a pressure difference of 50 Pa.

The relationship within these two measures is given by the following formulation:

Air changes per hour = Permeability x external surface area of building x Interior volume of structure

The conversion from air changes per hour at 50 Pa to air changes per hours under normal conditions (around 4 Pa) is too elaborate to explain now. In one building a real 0.8 ac/r was equivalent to 14 ac/h (50Pa).

Noise Insulation

Windows are one of the most susceptible parts of a building to noise transmission due to their fairly lightweight construction. Depending on the number of openings and the quality of the seals between the openings, a single glazed window with no seals may only achieve a noise reduction of 18 – 25dBA. When closed, sealed double glazed units perform barely better than single glazing because the 2 panes of glass are rigidly connected with a nominal cavity so the 2 panes resonate as one.

A secondary window with an air space of 10cm or more de-couples the movement of the two panes of glass and reduces the resonance between the two. Sound insulation of up to 45dBA can typically be obtained.

Elevated levels of sound insulation are attained as the gap grows particularly if the window reveals happen to be lined with an acoustic material, although nominal enhancements happen with cavities over 200mm. The use of thicker or acoustic laminate glass within the secondary window also improves the acoustic capabilities of the installation.

Protection from UV light

Ultra Violet (UV) light from the sun might cause extensive damage to paintings, fabrics, furnishing and other objects. The use of a film, either in laminated glass in the secondary glazing unit or put on as a film to the primary window, will absorb UV light and reduce this risk of damage.

Solar gain

Windows can admit large amounts of solar energy leading to overheating. Secondary glazing can make this more serious if they minimize summer time venting. Though, mid-pane blinds, glare coatings and summer ventilation of the air space can be used to help make the room less hot.

Some secondary glazing systems can be taken down in the summer months.

Condensation

All air includes some water vapour, but warm air can hold more vapour than cold air. As soon as warm, moistened air is cooled it will reach a temperature at which it cannot hold all the vapour, and the water will condense out. This temperature is called the dew point.

Warm moist air passing over a cold surface may be cooled locally below the dew point in which case moisture build-up or condensation will take place. This outcome causes the familiar condensation on the inside of cold windows. Condensation on the external window can arise if the secondary system is opened up for ventilation in cold weather especially where rooms are somewhat humid.

These condensation hazards will be minimised where the secondary glazing is either:

*able to be kept closed in cold weather, because there are alternative means of ventilation- older properties commonly have enough ventilation from other parts

*found where the ordinary direction of air flow is from outside to inside, for example on the windward side of a structure, on the lower floors or where a designated natural or mechanical extraction product helps to ensure inward airflow

*fitted with devices which avoid reverse air flow in unwanted conditions

*where the primary and secondary assemblies incorporate some option means of ventilating between the outside and the room interior but bypassing the cavity between the prime and secondary glazing

Historic buildings security

A secondary window offers an extra barrier to entry and therefore can present improved security. This can be particularly suitable when the use of an historic building is being modified and an advanced degree of safety and security is required. The secondary glazing can provide that additional security whilst preserving the existing windows.

Secondary glazing and the Building Regulations

There aren’t specific requirements for secondary glazing for existing buildings within the Building Regulations. The Part L Approved Documents set U-value standards for windows but these will only include existing buildings:

*if the windows are past repair and there isn’t alternative but to replace them

or

* if the building is undergoing a ‘change of use’

The Part L Approved Documents standard for windows is 2.2/m2K. This performance figure can be achieved when secondary glazing with low E glass is used in combination with the primary window. Hence secondary glazing offers the opportunity to improve the energy efficiency of an older building whilst preserving its historic appearance and value.

Adding secondary glazing to the prime windows can be useful in assisting a concession to be arrived at when trying to upgrade the thermal performance of a significant structure.

Secondary glazing systems – materials

When selecting secondary glazing units for a building it is important to use a system in keeping with the design and materials of the space. There are several proprietary secondary glazing systems available which provide setups that are configured to suit the particular circumstances of the structure.

Proprietary systems normally have coated aluminium frames. This allows the design of slim-line systems that can fit within the depth of the staff bead of a typical sash window so shutters and window sills can be saved.

Systems with more significant framing sections are stronger and can permit seals, fixings and counterbalancing. The systems may use an aluminium outer frame fitted to a soft wood ground or seasoned timber surrounds depending on the design and fixing elements. The supplier of these systems provide, manufacture and installation services.

Alternatively a bespoke system can be manufactured composed of a sub-frame, commonly of solid wood, into which opening casements or moving sashes are fixed. Individual glazed windows can be hinged so that they fold up like shutters or operate similar to sash windows.

The importance of traditional ‘breathing’ performance

Most traditional properties are made of porous materials which do not incorporate the obstructions to outside moisture (cavities, rain-screens, damp-proof courses, vapour barriers and membranes) which are standard in modern building.

As a result, the porous fabric in early houses tends to soak up more moisture, which is then released by internal and external evaporation. When traditional buildings are working as they were designed to, the evaporation will keep dampness levels in the building fabric below the stages at which decay can start to develop. This is often referred to as a ‘breathing’ building.

If appropriately preserved a ‘breathing’ structure has definite advantages over a modern watertight building. Porous materials such as lime and/or earth based mortars, renders, plasters and limewash act as a shield for environmental dampness, absorbing it from the air when humidity is high, and releasing it when the air is dry. Present day building relies on mechanised eradication to remove water vapour formed by the actions of individuals.

As old-fashioned buildings need to ‘breathe’ the use of vapour barriers and many substances frequently found in modern buildings must be avoided when making renovations to improve energy efficiency, as these materials can trap and store moisture and create problems for the building.

The use any modern day substances need to be based upon an informed evaluation where the significance of their inclusion and the risk of problems are fully recognized. It is also important that structures are well looked after, or else improvements made in energy efficiency will be negated by the problems associated with water ingress and/or increased draughts.

Sliding systems

Horizontal sliding systems comprise two or more panels sliding on glide pads or wheels for larger windows, with the panels sliding within the frame. Most panels can be easily taken out by lifting into the head frame and swinging them out.

On vertical sliding systems, the two panels slide within the frame. Some operate on spring balances which fully support the weight of the sash and those that do not can be difficult to operate and are only suitable for extremely small windows. For sash windows they usually have the top sash innermost, to improve operability and allow the latch on the prime sash to be got to more easily. Tilt-in up and down sliding systems allow the sashes to hinge inwards for cleaning. Restrictor stays and braking systems to prevent sliding in the open position can be fitted.

These variations are suitable where conventional ventilation is required.

Hinged systems

Hinged casements are available as single or double leaf with the frame measurements dependent on the window size. Casements are often fitted with restrictor stays.

This type of system is frequently used where the whole window is to be covered to avoid any sightlines on the secondary product. These work well for larger panes, where high compression seals are required to optimise sound insulation or to reduce air flow, or where full access is required for cleaning/repair or to provide a means of egress.

In situations with security demands, multi-point locking can be used.

These designs are suitable where regular ventilation is necessary. Since the sash projects into the room when opened for ventilation this may create a safety hazard. A restrictor can be fitted to hold the window open at a pre-specified minimum.

Lift outs

A lightweight frame with the panel lifted from the bottom to remove. These are best used for windows that are fixed or seldom opened and where availability is only occasional necessary for cleaning. They are also useful for windows of unusual configurations.

Where it is suggested to remove secondary glazing it is advisable to have dedicated storage space available.

Removable

A lightweight and easy to remove system is a magnetic secondary glazing system. Multi-polarised magnetic strips fitted to UV stable clear cast acrylic combine with reverse magnetic strips on the edge of the window frame to hold the panel in place.

Fixed

Permanent panels are useful where no access is required or in combination with other opening panels. Careful consideration needs to be given to how to access the glass and the cavity space to clean and maintain.

With wholly fixed panels to avoid the risk of condensation within the cavity can be lessened by providing vents in the secondary glazing unit.

Shaped

There are limitations but it is possible to shape all designs of secondary glazed units to the profile and style of the external window.

Installing secondary glazing

Secondary glazing can have negligible visual outcome if carefully planned. The layout should seek to be as judicious as possible with undersized frames concealed from view from the outside and unassuming from within.

There are numerous different ways of installing secondary glazing to a window opening. Take into consideration the styles and specifications as early as possible in the scheme to ensure a successful final result.

A good way to start is to contact one of the specialist companies who offer help and advice on the planned installation.

Whilst the framing material may be lightweight glass is deceptively heavy – 10Kg/m² for 4mm thick panes and 15Kg/m² for 6mm. Manufacturers will provide recommendations for size and weight restrictions for the safe usage of the planned secondary glazing unit.

In the design progression the following is an indication of the type of factors to be considered with key issues being the minimisation of damage and ease of use. The purpose of the installation will dictate the fixing location and type of glass chosen.

EXISTING WINDOW DESIGN

The design of the original window can be used to decide the style of the secondary glazing to be installed. The dimensions of the primary window are fixed but the secondary glazing can be designed into manageable sized units.

PHYSICAL CONSTRAINTS

Secondary windows are typically situated immediately inside the established sashes or at a suitable location within the depth of the window reveal. An assessment of the existing window opening by the consultant company will identify the limitations, for example whether there is satisfactory depth in the reveal to locate the secondary glazing.

SHUTTERS

Where shutters or other joinery are present, careful consideration will be required.

Sometimes secondary glazing can be situated between the primary window and the shutters so that the wooden shutters still operate. If the wooden shutters are housed within the window reveal it may be possible to install secondary glazing on the room side of the window shutters.

If the secondary glazing cannot be inserted without making the shutters inoperable the shutters could be fixed closed but not modified so that they can be brought back into use at a later date.

VISUAL IMPACT

Secondary glazing can be visually intrusive outwardly and inside if badly designed. To minimise the visual effect of secondary windows externally, try to make sure the secondary glazing is not smaller than the glazed area of the existing window. Try to position any divisions in the glazing behind the window meeting rails or glazing bars. The flat reflections of modern glass within secondary glazing can be reduced by using anti-reflective glass.