Tips on the use and specification of secondary glazing


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

Windows make a major contribution to the character of historic buildings. be done to hold them back. They could also reveal a lot about the history of a structure; the evolution of architectural taste and style, social hierarchy, building economics, craft skills and industrial advances.

Old windows may be reduced to water as they eventually become deformed as joints become stressed. Although adequate ventilation is essential in older buildings, excessive air leaks through the windows waste heat and are miserable for the occupants.

Carefully planned and installed secondary glazing keeps the original windows unchanged and, if necessary, repaired. heat loss. Therefore, there is no damage to the historic fabric and in most cases, the installation is easily reversible.

A recent study has shown that heat loss through conduction and radiation through a window can be reduced by more than 60% secondary glazing with a low-e hard coating (low-E) facing the window. outside. Research has also shown that additional savings can be realized if the secondary glazing uses insulating frames or uses double or vacuum glazing.

In addition to increasing the thermal performance of windows, secondary glazing can offer a number of additional benefits. being very effective in limiting noise transmission.

For listed properties, it is important to consult with the local conservation office's conservation officer for advice prior to installation of secondary glazing. In some cases, a building permit may be required

What is a secondary glazing?

Secondary glazing is not new. In the 19th century some buildings were built with internal secondary glazing designed as part of the original layout. Often a second double hung window or full panels with counterweights were installed in the space under the window. Their function was to reduce heat loss and provide some sound insulation at the opening of the window.

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

Secondary glazing is available as removable, removable or stationary units. The panels that can be opened can be frames or sliding frames. These types allow access to the external window for cleaning and opening the secondary glazing and exterior windows for fresh air. The new secondary glazing is designed to be detached during the warmer months when its thermal benefits are not needed.

The use of glazed exterior protection for windows using glass or plastic sheets is called "storm 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 that this produces in the airspace between the existing glazing and the extra exterior glazing where ventilation will be required. Environmental conditions within the structure should also be considered before designing the installation.

One option is to install storm windows in winter and remove them during the summer months.

Repairing existing window openings

Hardwood and metal windows can almost always be restored, even when they are in poor condition and, as a general rule, at a significantly lower cost than replacement total. Solid wood used in the past to make windows was of high quality and very durable.

Many Georgian and Victorian windows are still in place today while contemporary windows may require replacement after just 20 years. Rebuilding the windows is the best way to maintain the visual character and architectural importance of the elevation of a building and can add to its value.

Before undertaking repair work such as stripping or adding secondary glazing, evaluate the repairs needed to make the windows fully functional. Windows deteriorate over time, so regular renovation, cleaning methods and painting are still a good investment

For classified buildings, the total replacement of a window will likely result in the Consent of registered construction.

] Double glazed windows generally have sealed glazings with 2 panes separated by an air gap (usually 12-18 mm) which improves the thermal insulation, especially if the glass is coated and l? gap is filled with an inert gas. This is a significant improvement that has resulted in significant energy savings and reduced carbon dioxide emissions, especially in new buildings. Building Regulations Make Double Glazing Virtually Mandatory in New Buildings

Often, replacing existing windows with double glazed windows can change the appearance, especially the smoothness of new glass and the need for sections wider. and the glazing bars.

In historic buildings, there should be a strong preference for renovation rather than replacement because the use of double glazing will inevitably result in a loss of traditional fabric. The addition of secondary glazing would often be the preferred alternative

The advantages of double glazing over other window enhancement methods are often overestimated. Much of the comfort and energy efficiency benefits of the new double glazing come from the reduction in air currents that will result from well-fitting window frames with built-in anti-fog sealing. These additional benefits are also available through the repair and waterproofing of existing windows or secondary windows. With regular improvements in the efficiency of secondary glazing, it is even possible that the performance of secondary glazing exceeds that of new double glazing.

In terms of noise elimination, double glazing is no better than simple glazing; and may be to some extent inferior for traffic noise. The important principles for noise reduction are that windows are snug and tight. The secondary glazing, with its much wider space between the panes, is a better acoustic insulator. Wooden shutters and heavy drapes can also bring noticeable improvements to sound insulation.

Secondary glazing or fog lights?

The waterproofing is most often the first alternative to take into account to improve the energy efficiency of windows. an old building. Since windows are often the main source of air infiltration, draft is one of the best ways to improve comfort and limit energy consumption, with little or no change in the appearance of the building.

gives you an overall thermal performance well above that of the draw only test. It can also be chosen when it is particularly difficult to install window seals.

Similarly, many metal frame windows have openings that are too large to be sealed.

Windows with lead lights can let air through.

Depending on the use, location and occupant comfort requirements, the other advantages and benefits of secondary glazing, such as noise reduction, may affect the design and construction of the building. the style.

If secondary glazing is the preferred solution, it is better to leave the exterior windows without waterproofing to provide some ventilation of the air space separating the exterior windows and the secondary glazing to prevent it from occurring. condensation buildup.

Lead Paint

Lead paints can be harmful to health, especially children.

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

The use of lead paint has already been banned. the danger to health. That being said, there is an exclusion to prohibition that allows them to be used in Grade I and Grade II * buildings. On these structures, the traditional appearance of lead paint, as well as its longevity and its fungicidal and insecticidal properties, make it still used often. It should only be applied by qualified decorators who use proper protective equipment and it is not advisable to use it within the reach of children.

Advantages of Secondary Glazing

Older buildings aim to improve the thermal capacity of windows by protecting them from drafts and by reducing heat conduction through the glass, secondary glazing can offer a number of other advantages

window in the heating months are complex because three major elements are involved:

* by convection and conduction, from the warm air of the room to the cold glass and frame

* by the cold surface of the window soaked infrared radiation from the room

* by uncontrolled air leaks, which can also bring cold air external surfaces or evacuate the warm air from the inside; often called infiltration of air, this can happen even when the window is closed.

Loss of heat through glass and frames

It does not matter if it comes out of the room by convection, conduction or radiation, the lost heat disappears through the glass and also the frame as conduction. Glass is essentially the most conductive area of ​​the window, but heat is usually lost through the frame, albeit at a lower rate.

Single glazing is a low energy insulator and easily conducts heat. A typical 4 mm thick glass has an archetypal U value of 5.4 W / m2K. The heat loss through a single glazed window will depend on the overall glass surface, the conductance of the frame material and the quality of the frame installation and the double glazed materials. The typical value of a framed single-pane hardwood window is 4.8 W / m2 K.

For thermal performance, the optimal airspace between the panes is 16 to 20 mm . Better air space allows convection currents to develop in a vacuum and evacuate more heat. The positioning of the secondary unit is typically determined by the window and can often be installed only at a distance of about 100 mm from the main glazing. That said, a significant portion of the thermal advantage of the secondary glazing comes from the decoupling of the frame of the first choice wooden window frame, which can reduce the U value to about 2.5 W / m2K.

The use of low-emissivity glass 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 due to air leakage

Heat loss from a typical classic window is mainly due to breaks around the window. With larger windows, the proportion of heat lost through conduction through glass tends to be greater. Since air currents, caused by convection and air infiltration, make people feel less hot, people in the room can increase the heating and use it more. long time.

Custom-made secondary windows with perimeter sealing and brush or compression seals. the opening panels generate an effective seal on the entire mainframe of the main window and can significantly moderate excessive water struts.

Before undertaking a sealing process, think of a pressurization analysis of fans the sources of air penetration and determine the windows that require attention because they can contrast significantly in the amount of air currents that they allow to penetrate.

Quantifying Clearances

The size of a building depends on the amount of air that can pass envelope – walls, floor and roof cover. This is what is called air permeability. The industry standard is to articulate the permeability of a wall, roof or entire structure envelope by accepting a pressure difference of 50 Pascals across the wall. The permeability is then measured as the amount of air (in cubic meters) that will pass in one hour through one square meter of wall (or roof, or floor) and expressed in m³ / h / m² (m³ / hm² or m / h The pressure difference is 50 Pascals (1965 Pa)

Although the leaks in the structure are what creates the ventilation and ventilation, what is more important for the structure and its occupants is the speed to This is most simply measured as the number of times the air in the building changes every hour (shown as ac / h) .This will depend on the pressure difference between the air and the air. Outside and inside the building, and the standard of the industry is to assume a pressure difference of 50 Pa.

The relationship between these two measures is given by the following wording: [19659002] Changes in air per hour = Permeability x external building surface x V Inner olume of structure

The conversion of air changes per hour to 50 Pa to changes in air per hour under normal conditions (about 4 Pa) is too complex to explain now. In a building, a true value of 0.8 ac / r was equivalent to 14 ac / h (50Pa).

Sound Insulation

Windows are one of the most sensitive parts of a building with noise transmission. Depending on the number of openings and the quality of the seals between the openings, a single pane without joints can achieve only a reduction in noise of 18 to 25 dBA. Once sealed, sealed double glazing is barely more efficient than single glazing because both panes are rigidly connected to a nominal cavity, so that both panes sound as one.

A secondary window with a space of 10cm or more. couple the movement of the two panes and reduces the resonance between the two. Sound insulation up to 45 dBA can usually be achieved.

High levels of sound insulation are obtained when the space increases, especially if the window is covered with an acoustic material, although nominal improvements occur with cavities greater than 200mm . The use of thicker or acoustic laminate glass in the secondary window also enhances the acoustic capabilities of the installation.

Protection Against Ultraviolet Rays

The ultraviolet (UV) light of the sun can cause significant damage to paints, fabrics and furnishings and other objects. The use of a film, either laminated glass in the secondary glazing, or film on the primary window, absorbs UV light and reduces this risk of damage.

Solar Gain

Windows can receive large amounts of solar energy causing overheating. Secondary windows can aggravate the situation if they minimize ventilation in the summer. However, mid-glazed blinds, anti-glare coatings and a summer air space vent can be used to make the room less warm.

Some secondary glazing systems can be dismantled during the summer months

All air includes steam, but warm air can retain more steam that cold air. As soon as the warm, moist air is cooled, it reaches a temperature at which it can not retain all the steam and the water condenses. This temperature is called the dew point.

Hot and humid air that passes on a cold surface can be cooled locally below the dew point, in which case there will be accumulation of moisture or condensation. This result causes familiar condensation inside cold windows. Condensation on the outer window may occur if the secondary system is open for ventilation in cold weather, especially when the rooms are somewhat damp.

These risks of condensation will be minimized when the secondary glazing is either:

* closed in cold weather, because there are other means of ventilation – old properties usually have sufficient ventilation of the other parts [19659002] * where the ordinary direction of the airflow is from the outside to the inside, for example on the windward side of a structure, on the lower floors or when a designated natural or mechanical extraction product helps to ensure the incoming airflow

* equipped with devices that avoid reverse airflow under unwanted conditions

* where assemblies primary and secondary incorporate the exterior and interior of the room but bypassing the cavity between the main glazing and the secondary glazing

Security of Historic Buildings

A secondary window provides an additional barrier at the entrance and can therefore offer improved security. This can be particularly appropriate when the use of a historic building is being modified and a high degree of safety and security is required. Secondary glazing can provide this extra security while preserving existing windows.

Secondary glazing and building regulations

There are no specific requirements for secondary glazing of existing buildings in building regulations. Approved documents in Part L establish U-value standards for windows, but these include only existing buildings:

* if windows are repaired and there is no windows alternative to replacing them


* the building undergoes a "change of use"

The Part L approved documents for windows standard is 2, 2 / m2K. This performance figure can be achieved when the low glass secondary glazing E is used in combination with the main window. Secondary glazing therefore offers the opportunity to improve the energy efficiency of an old building while preserving its historical appearance and value.

The addition of secondary glazing to the main windows can be helpful in helping to obtain a performance concession of an important structure.

Secondary Glazing Systems – Materials

When selecting secondary glazing for a building, it is important to use a system consistent with the design and materials of the space. There are several exclusive secondary glazing systems that offer configured configurations based on the particular circumstances of the structure.

Exclusive systems usually have coated aluminum frames. This allows the design of thin-line systems that can insert into the pearl depth of a typical sash window, thereby saving flaps and window sills.

and counterweight. The systems may use an outer aluminum frame mounted on a softwood floor or wooden surrounds arranged according to the design and fastening elements. The supplier of these systems provides manufacturing and installation services.

It is also possible to manufacture a custom-made system consisting of a sub-frame, generally made of solid wood, in which are fixed doors or movable frames. The individual glazed windows can be hinged so that they can fold like shutters or function as sash windows.

The Importance of Traditional Respiratory Performance

Most traditional properties are made of porous materials that do not incorporate obstacles to the outdoors. Moisture (cavities, rain shields, waterproof layers, vapor barriers and membranes) that are standard in modern buildings.

Therefore, the porous fabric of the first houses tends to absorb more moisture, which is then released by internal and external evaporation. When traditional buildings work as expected, evaporation will maintain moisture levels in the building fabric below the stages at which decomposition can begin to develop. This is what is often called a "breathing" building.

If a "breathable" structure is properly preserved, it has certain advantages over a modern watertight building. Porous materials such as lime and / or earth based mortars, plaster, plaster and lime wash act as a shield against the ambient humidity, absorb it from the air when l & # 39; The humidity is high and release it when the air is dry. The current building relies on mechanized eradication to remove the water vapor formed by the actions of individuals.

As old buildings must 'breathe', the use of vapor barrier and many substances frequently found in modern buildings should be avoided to improve energy efficiency, as these materials can trap and store Moisture and create problems for the building.

The use of modern substances should be based on an informed assessment where the importance of their inclusion and the risk of problems are fully recognized. It is also important that structures are well maintained, otherwise energy efficiency improvements will be offset by problems with water infiltration and / or more air currents. important.

Sliding Systems

Sliding on skates or wheels for larger windows, with panels sliding in the frame. Most panels can be easily removed by lifting them in the frame of the head and swaying them.

On vertical sliding systems, both panels slide in the frame. Some work on spring loaded rockers that fully support the weight of the chassis and those that are not can be difficult to use and are only suitable for extremely small windows. For sash windows, they usually have the most inner upper sash, to improve operability and make it easier to secure the upper belt lock. Up and down tilt sliding systems allow the frames to be tilted inwards for cleaning. Fenders and braking systems to prevent slippage in the open position can be fitted.

These variants are suitable when conventional ventilation is required.

Hinged Systems

Hinged doors are available in single or double version depending on the size of the window. Partitions are often provided with restraint

. This type of system is frequently used when the entire window needs to be covered to avoid any line of sight on the secondary product. These work well for larger windows, where high compression seals are needed to optimize sound insulation or to reduce airflow, or where full access is required for cleaning / repair or to provide a means of evacuation. multipoint locking can be used.

These designs are suitable when regular ventilation is needed. Since the leaf insinuates into the room when it is open for ventilation, this can create a safety hazard. A restrictor can be installed to keep the window open to a predefined minimum.


A lightweight frame with the panel raised from the bottom to remove it. These are best used for windows that are fixed or rarely open and where availability is only occasionally needed for cleaning. They are also useful for windows with unusual configurations.

When it is suggested to remove the secondary glazing, it is advisable to have a dedicated storage space.


A lightweight and easy to remove system is a secondary glazing magnetic system. Multipolar magnetic tapes mounted on UV stable transparent cast acrylic combine with inverted magnetic tapes on the edge of the window frame to hold the panel in place.


Permanent signs are useful where no access is required with other opening signs. Special attention should be paid to how to access the glass and cavity space for cleaning and maintenance.

Panels fully secured to prevent the risk of condensation inside the cavity can be mitigated. 19659002] Form

There are limitations, but it is possible to shape all secondary glazing designs according to the profile and style of the exterior window

Secondary Glazing Installation

Secondary glazing may have a negligible visual result. The layout should seek to be as judicious as possible with undersized frames hidden on the outside and unpretentious from the inside.

There are many different ways to install secondary glazing on a window opening. Consider styles and specifications as soon as possible to ensure a successful end result

A good way to start is to contact one of the specialized companies that offer help and advice on Planned installation.

While the framing material can be light, the glass is deceptively heavy – 10 kg / m² for windows of 4 mm in thickness and 15 kg / m² for 6 mm. Manufacturers will provide recommendations for size and weight restrictions for the safe use of scheduled secondary glazing.

In the progression of design, the following are an indication of the type of factors to be considered: damage minimization and useful ease. The purpose of the installation will dictate the location of fixation and the type of glazing chosen.


La conception de la fenêtre d'origine peut être utilisée pour décider du style du vitrage secondaire à installer. Les dimensions de la fenêtre principale sont fixes, mais le vitrage secondaire peut être conçu en unités de taille gérable


Les fenêtres secondaires sont généralement situées immédiatement à l'intérieur des châssis ou à un endroit approprié dans la profondeur de la fenêtre. . Une évaluation de l'ouverture de fenêtre existante par la société de consultants permettra d'identifier les limites, par exemple s'il y a une profondeur satisfaisante dans le puits pour localiser le vitrage secondaire.


Lorsque des volets ou autres menuiseries sont présents, un examen attentif

Parfois, un vitrage secondaire peut être placé entre la fenêtre principale et les volets de sorte que les volets en bois fonctionnent toujours. Si les volets en bois sont logés à l'intérieur de la fenêtre, il peut être possible d'installer un vitrage secondaire sur le côté des volets des fenêtres

Si le vitrage secondaire ne peut pas être inséré sans rendre les volets inopérables, les volets pourraient être fermés. IMPACT VISUEL

Les vitrages secondaires peuvent être visuellement intrusifs vers l'extérieur et vers l'intérieur s'ils sont mal conçus. Pour minimiser l'effet visuel des fenêtres secondaires à l'extérieur, essayez de vous assurer que le vitrage secondaire n'est pas plus petit que la surface vitrée de la fenêtre existante. Essayez de placer toutes les divisions dans le vitrage derrière les rails de réunion de la fenêtre ou les barres de vitrage. Les reflets plats du verre moderne dans les vitrages secondaires peuvent être réduits en utilisant un verre antireflet.


Source by Ian M Hall

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