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Guidance on the Usage and Specifying of Secondary Glazing

Steering on the Utilization and Specifying of Secondary Glazing


This steering gives helpful info on the ideas, disadvantages, supplies and strategies for upgrading the thermal efficiency of home windows with the addition of secondary glazing.

Home windows make a serious contribution in the direction of the character of historic buildings and each effort should be made to retain them. They may additionally reveal an excellent deal in regards to the historical past of a construction; altering architectural style and magnificence, social hierarchy, constructing economics, craft expertise and industrial advances.

Older home windows could be draughty as finally they distort as joints grow to be pressured. Though ample air flow is essential in older buildings excessive air leakage by means of home windows wastes warmth and is depressing for occupants.

Fastidiously deliberate and put in secondary glazing permits the unique home windows to be preserved unchanged, and the place mandatory repaired, while decreasing air leaks and carried out warmth losses. Consequently there is not a injury to historic material in addition to in most cases the set up is definitely reversible.

Latest examine has revealed warmth losses by conduction and radiation by means of a window typically talking may be decreased by over 60% by making use of secondary glazing with a low emissivity (low-E) laborious coating dealing with the outside. The analysis has additionally proven that additional financial savings may be made if the secondary glazing makes use of insulating frames or makes use of double or vacuum glazed models.

Along with rising the thermal efficiency of home windows, secondary glazing can have numerous extra further advantages together with being extremely efficient at limiting noise transmission.

For listed properties it is very important confer with the Conservation Officer on the native planning authority for steering forward of the set up of secondary glazing. Listed Constructing Authorization could also be required in some instances.

What precisely is secondary glazing?

Secondary glazing is nothing new. Within the nineteenth century some buildings have been constructed with inner secondary glazing designed as a part of the unique format. Usually a second double hung sash window or strong panels with counterbalancing weights have been fitted within the house beneath the window. Their perform was to chop down the warmth loss and supply some measure of sound insulation to the window aperture.

Secondary glazing is a completely unbiased window system utilized to the room aspect of pre-existing home windows. The unique home windows stay in place and of their unique unaltered type.

Secondary glazing is on the market as open-able, detachable or fastened models. The open-able panels may be both casements or sliding sashes. These sorts allow entry to the exterior window for cleansing and the opening of each the secondary glazing and exterior home windows for contemporary air. New secondary glazing is designed to be indifferent in hotter months when its thermal further advantages aren’t mandatory.

Utilizing glazed exterior safety for home windows utilizing both glass or plastic sheet is named as ‘storm glazing’ reasonably than simply secondary glazing. Doing this will usually be used to guard stained glass in church buildings. Utilizing such a system it is very important perceive the attainable circumstances this produces within the airspace between the present glazing and the additional exterior glazing the place air flow might be required. Account also needs to be taken of the environmental situations contained in the construction earlier than designing the set up.

One possibility is to put in storm glazing within the winter months and take away it in the summertime months.

Repairing present window openings

Established hardwood and steel home windows can virtually all the time be restored, even when in moderately poor situation and as a rule at considerably much less value than whole alternative. The strong timber used prior to now to make home windows was of a top quality and really sturdy.

Plenty of Georgian and Victorian home windows are nonetheless in place immediately while up to date home windows can require alternative after barely 20 years. Reconstructing home windows is the easiest way of preserving the visible character and architectural significance of a constructing’s elevation and might add to its worth.

Earlier than beginning any upgrading work akin to draught-stripping or the including of secondary glazing, consider what repairs are wanted to make the home windows totally purposeful. Home windows deteriorate over time so common renovation, cleansing strategies and portray is all the time an excellent funding.

For listed buildings, the full alternative of a window is prone to entail Listed Constructing Consent.

Secondary glazing or double glazing?

Double glazed home windows usually have sealed glazing models with 2 panes of glass divided by an air hole (generally of 12-18 mm) that enhances thermal insulation, notably if the glass is coated and the air hole is stuffed with an inert fuel. It is a crucial enchancment that has produced sizable power value financial savings and reductions in carbon dioxide emissions, notably in new buildings. The Constructing Rules make double glazing just about compulsory in new constructing.

Usually the alternative of present home windows utilizing double glazed models can in lots of conditions result in a change in look, notably the smoothness of latest glass and the necessity for wider hardwood sections and glazing bars.

In historic buildings, there must be a powerful choice for renovation reasonably than alternative as the usage of double glazing will inevitably result in a lack of conventional material. Including secondary glazing would usually be the popular various.

The advantages of double glazing over various strategies of window enchancment are sometimes overestimated. A lot of the consolation and power efficiency advantages of latest double glazing come from the discount of draughts that may consequence from well-fitted window frames with integral draught-proofing. These further advantages are additionally out there by means of restore and draught-proofing of the present home windows, or from fitted secondary glazing. With common enhancements within the effectivity of secondary glazing it could even be attainable for the efficiency of secondary glazed home windows to exceed that of latest double glazing.

By way of noise elimination, double glazed models aren’t any higher than single glazed models; and may be to some extent inferior for site visitors noise. The essential ideas for noise discount are that the home windows are properly fitted and draught-proofed. Secondary glazing, with its a lot wider hole between the window panes, is a greater sound insulator. Picket shutters and heavy drapes may also make notable enhancements to noise insulation.

Secondary glazing or draught-proofing?

Draught-proofing is as a rule the primary various to offer consideration to for bettering the power effectivity of home windows in an older constructing. As home windows are sometimes a foremost supply of air infiltration draught-proofing utilizing seals is among the best methods of enhancing consolation and limiting power use, with little or no change to a constructing’s take a look at nominal value.

Nonetheless, secondary glazing can provide you a noticeably greater thermal general efficiency than draught-proofing alone. It could even be chosen the place putting in draught-proofing seals to the home windows is particularly troublesome.

Equally many metal-framed home windows have gaps which are too large to seal.

Home windows with leaded lights might permit air infiltration across the lead which may be addressed with the assistance of secondary glazing.

Dependant on the constructing’s use, location and the occupants’ consolation necessities different benefits and advantages of secondary glazing, for example noise discount, might have a bearing on the design and magnificence answer.

If secondary glazing is the popular answer then the outer home windows are higher left with out draught-proofing to make it possible for there’s an quantity of air flow to the air house separating the outer home windows and the secondary glazing to cease the construct up of condensation.

Lead Paint

Lead paints may be dangerous to well being, particularly kids.

Lead primarily based paints are sometimes discovered on older buildings. Typically these paints have already been over-painted. If there may be any uncertainty in regards to the existence of lead paint on home windows which are going to be stripped, it have to be assumed that lead paint exists and precautions taken accordingly.

The utilization of lead paints already has been typically banned as a consequence of the hazard to well being. That being mentioned, there may be an exclusion to the ban which permits them for use on Grade I and Grade II* listed buildings. On these constructions the normal look of the lead paint, along with its longevity and its fungicidal and insecticidal properties, imply that it’s very often nonetheless used. It ought to solely be utilized by certified decorators utilizing correct protecting gear, and it’s not beneficial to be used the place it could be within the attain of youngsters.

The advantages of secondary glazing

Though the primary function of secondary glazing models in older buildings is to enhance the thermal functionality of home windows by draught-proofing in addition to decreasing the conduction of warmth by means of glass, secondary glazing can present numerous different advantages.

Thermal Advantages

Warmth loss from a room by way of a window within the heating months are advanced as three main components are in play:

*by convection and conduction, from the hotter room air to the colder of the glass and the body

*by the chilly floor of the window absorbing infra-red radiation from the room

*by uncontrolled air leakage, which might additionally usher in chilly air from the exterior surfaces or take heat air out from the within; usually known as air infiltration, this will happen even when the window is closed.

Warmth loss by means of the glass and frames

It would not matter if it leaves the room by convection, conduction or radiation, the misplaced warmth all goes by means of the glass and in addition the body as conduction. The glass is basically probably the most conductive space of the window however warmth is usually misplaced throughout the body though at a lesser fee.

Single glazing is a poor power insulator and readily conducts warmth. A typical 4mm thick glass has a archetypal U-value of 5.4W/m2K. The thermal loss by means of a single glazed window will depend upon the general space of glass, the conductance of the body materials and the standard of the set up of the body and double glazing supplies. A typical worth of a hardwood framed single glazed window is 4.8W/m2K.

For thermal efficiency, the optimum airspace between panes is 16 – 20mm. A greater air house lets convection currents to develop inside the void and extra warmth to be shed. The positioning of the secondary unit is usually decided by the window reveal and might usually solely be fitted at a distance of about 100mm from the prime glazing. Having mentioned that, a big proportion of the thermal good thing about secondary glazing comes from decoupling the body from the prime picket window body and this will cut back the U-value to roughly 2.5W/m2K.

The usage of low emissivity glass for the secondary glazing can additional enhance the thermal effectivity to lower than 2.0/m2K. To take care of this determine it is very important maintain the end clear – the usual is ‘visually’ clear.

Warmth loss by means of air leakage

Warmth losses from a typical typical window are primarily by means of breaks across the window. With bigger home windows the proportion of warmth shed by conduction by means of the glass tends to be bigger. Since draughts, attributable to convection and air infiltration make individuals really feel chillier, the individuals within the room might flip up the heating, and in addition function it for longer.

Bespoke secondary home windows, with environment friendly perimeter sealing and brush or compression seals on the opening panels, generate an efficient seal over the entire of the mainframe of the primary window and might considerably average extreme draughts.

Previous to embarking on a strategy of draught-proofing, take into consideration a fan pressurisation evaluation to search out out the actual sources of air penetration and decide which home windows require consideration as they will distinction considerably within the quantity of draughts they let in.

Quantifying Draughtiness

The draughtiness of a constructing will rely on the quantity of air that may go by means of its exterior envelope – partitions, flooring and roof masking. That is alluded to because the air permeability. The trade customary is to articulate the permeability of a wall, roof, or full construction envelope accepting a strain distinction of fifty Pascals throughout the wall. The permeability is then measured as the quantity of air (in cubic metres) that may go in an hour by means of a sq. metre of wall (or roof, or flooring) and expressed as m³/h/m² (m³/hm² or m/h) at a strain distinction of fifty Pascals (50 Pa).

Whereas leaks within the construction is what creates draughtiness and air flow, what’s extra essential for the construction and its occupants is the velocity at which air proceeds by means of the construction. That is most easily measured because the variety of instances that the air within the constructing alters every hour (proven as ac/h). It will rely on the strain distinction between the skin and the within of the constructing, and once more the trade norm is to imagine a strain distinction of fifty Pa.

The connection inside these two measures is given by the next formulation:

Air adjustments per hour = Permeability x exterior floor space of constructing x Inside quantity of construction

The conversion from air adjustments per hour at 50 Pa to air adjustments per hours below regular situations (round 4 Pa) is simply too elaborate to clarify now. In a single constructing an actual 0.8 ac/r was equal to 14 ac/h (50Pa).

Noise Insulation

Home windows are one of the vital prone elements of a constructing to noise transmission because of their pretty light-weight building. Relying on the variety of openings and the standard of the seals between the openings, a single glazed window with no seals might solely obtain a noise discount of 18 – 25dBA. When closed, sealed double glazed models carry out barely higher than single glazing as a result of the two panes of glass are rigidly linked with a nominal cavity so the two panes resonate as one.

A secondary window with an air house of 10cm or extra de-couples the motion of the 2 panes of glass and reduces the resonance between the 2. Sound insulation of as much as 45dBA can usually be obtained.

Elevated ranges of sound insulation are attained because the hole grows notably if the window reveals occur to be lined with an acoustic materials, though nominal enhancements occur with cavities over 200mm. The usage of thicker or acoustic laminate glass inside the secondary window additionally improves the acoustic capabilities of the set up.

Safety from UV light

Extremely Violet (UV) light from the solar may trigger in depth injury to work, materials, furnishing and different objects. The usage of a movie, both in laminated glass within the secondary glazing unit or placed on as a movie to the first window, will take in UV light and cut back this danger of injury.

Photo voltaic achieve

Home windows can admit massive quantities of photo voltaic power resulting in overheating. Secondary glazing could make this extra critical in the event that they decrease summer season venting. Although, mid-pane blinds, glare coatings and summer time air flow of the air house can be utilized to assist make the room much less sizzling.

Some secondary glazing methods may be taken down in the summertime months.


All air contains some water vapour, however heat air can maintain extra vapour than chilly air. As quickly as heat, moistened air is cooled it’ll attain a temperature at which it can not maintain all of the vapour, and the water will condense out. This temperature known as the dew level.

Heat moist air passing over a chilly floor could also be cooled domestically beneath the dew level by which case moisture build-up or condensation will happen. This final result causes the acquainted condensation on the within of chilly home windows. Condensation on the exterior window can come up if the secondary system is opened up for air flow in chilly climate particularly the place rooms are considerably humid.

These condensation hazards might be minimised the place the secondary glazing is both:

*capable of be stored closed in chilly climate, as a result of there are various technique of ventilation- older properties generally have sufficient air flow from different elements

*discovered the place the abnormal path of air stream is from exterior to inside, for instance on the windward aspect of a construction, on the decrease flooring or the place a delegated pure or mechanical extraction product helps to make sure inward airflow

*fitted with gadgets which keep away from reverse air stream in undesirable situations

*the place the first and secondary assemblies incorporate some possibility technique of ventilating between the skin and the room inside however bypassing the cavity between the prime and secondary glazing

Historic buildings safety

A secondary window affords an additional barrier to entry and subsequently can current improved safety. This may be notably appropriate when the usage of an historic constructing is being modified and a sophisticated diploma of security and safety is required. The secondary glazing can present that extra safety while preserving the present home windows.

Secondary glazing and the Constructing Rules

There aren’t particular necessities for secondary glazing for present buildings inside the Constructing Rules. The Half L Permitted Paperwork set U-value requirements for home windows however these will solely embody present buildings:

*if the home windows are previous restore and there is not various however to exchange them


* if the constructing is present process a ‘change of use’

The Half L Permitted Paperwork customary for home windows is 2.2/m2K. This efficiency determine may be achieved when secondary glazing with low E glass is utilized in mixture with the first window. Therefore secondary glazing affords the chance to enhance the power effectivity of an older constructing while preserving its historic look and worth.

Including secondary glazing to the prime home windows may be helpful in helping a concession to be arrived at when attempting to improve the thermal efficiency of a big construction.

Secondary glazing methods – supplies

When choosing secondary glazing models for a constructing it is very important use a system in step with the design and supplies of the house. There are a number of proprietary secondary glazing methods out there which give setups which are configured to swimsuit the actual circumstances of the construction.

Proprietary methods usually have coated aluminium frames. This permits the design of slim-line methods that may match inside the depth of the employees bead of a typical sash window so shutters and window sills may be saved.

Methods with extra vital framing sections are stronger and might allow seals, fixings and counterbalancing. The methods might use an aluminium outer body fitted to a gentle wooden floor or seasoned timber surrounds relying on the design and fixing components. The provider of those methods present, manufacture and set up providers.

Alternatively a bespoke system may be manufactured composed of a sub-frame, generally of strong wooden, into which opening casements or transferring sashes are fastened. Particular person glazed home windows may be hinged in order that they fold up like shutters or function much like sash home windows.

The significance of conventional ‘respiration’ efficiency

Most conventional properties are manufactured from porous supplies which don’t incorporate the obstructions to exterior moisture (cavities, rain-screens, damp-proof programs, vapour obstacles and membranes) that are customary in trendy constructing.

In consequence, the porous material in early homes tends to take in extra moisture, which is then launched by inner and exterior evaporation. When conventional buildings are working as they have been designed to, the evaporation will maintain dampness ranges within the constructing material beneath the phases at which decay can begin to develop. That is also known as a ‘respiration’ constructing.

If appropriately preserved a ‘respiration’ construction has particular benefits over a contemporary watertight constructing. Porous supplies akin to lime and/or earth primarily based mortars, renders, plasters and limewash act as a protect for environmental dampness, absorbing it from the air when humidity is excessive, and releasing it when the air is dry. Current day constructing depends on mechanised eradication to take away water vapour fashioned by the actions of people.

As old style buildings must ‘breathe’ the usage of vapour obstacles and lots of substances incessantly present in trendy buildings have to be prevented when making renovations to enhance power effectivity, as these supplies can entice and retailer moisture and create issues for the constructing.

The use any modern-day substances must be primarily based upon an knowledgeable analysis the place the importance of their inclusion and the chance of issues are totally acknowledged. It’s also essential that constructions are properly taken care of, or else enhancements made in power effectivity might be negated by the issues related to water ingress and/or elevated draughts.

Sliding methods

Horizontal sliding methods comprise two or extra panels sliding on glide pads or wheels for bigger home windows, with the panels sliding inside the body. Most panels may be simply taken out by lifting into the top body and swinging them out.

On vertical sliding methods, the 2 panels slide inside the body. Some function on spring balances which totally help the burden of the sash and people that don’t may be troublesome to function and are solely appropriate for very small home windows. For sash home windows they often have the highest sash innermost, to enhance operability and permit the latch on the prime sash to be acquired to extra simply. Tilt-in up and down sliding methods permit the sashes to hinge inwards for cleansing. Restrictor stays and braking methods to stop sliding within the open place may be fitted.

These variations are appropriate the place typical air flow is required.

Hinged methods

Hinged casements can be found as single or double leaf with the body measurements depending on the window dimension. Casements are sometimes fitted with restrictor stays.

Any such system is incessantly used the place the entire window is to be coated to keep away from any sightlines on the secondary product. These work properly for bigger panes, the place excessive compression seals are required to optimise sound insulation or to cut back air stream, or the place full entry is required for cleansing/restore or to supply a method of egress.

In conditions with safety calls for, multi-point locking can be utilized.

These designs are appropriate the place common air flow is critical. Because the sash initiatives into the room when opened for air flow this will likely create a security hazard. A restrictor may be fitted to carry the window open at a pre-specified minimal.

Elevate outs

A light-weight body with the panel lifted from the underside to take away. These are greatest used for home windows which are fastened or seldom opened and the place availability is barely occasional mandatory for cleansing. They’re additionally helpful for home windows of surprising configurations.

The place it’s prompt to take away secondary glazing it’s advisable to have devoted space for storing out there.


A light-weight and simple to take away system is a magnetic secondary glazing system. Multi-polarised magnetic strips fitted to UV secure clear solid acrylic mix with reverse magnetic strips on the sting of the window body to carry the panel in place.


Everlasting panels are helpful the place no entry is required or together with different opening panels. Cautious consideration must be given to the way to entry the glass and the cavity house to scrub and keep.

With wholly fastened panels to keep away from the chance of condensation inside the cavity may be lessened by offering vents within the secondary glazing unit.


There are limitations however it’s attainable to form all designs of secondary glazed models to the profile and magnificence of the exterior window.

Putting in secondary glazing

Secondary glazing can have negligible visible final result if rigorously deliberate. The format ought to search to be as considered as attainable with undersized frames hid from view from the skin and unassuming from inside.

There are quite a few other ways of putting in secondary glazing to a window opening. Think about the kinds and specs as early as attainable within the scheme to make sure a profitable ultimate consequence.

A great way to begin is to contact one of many specialist corporations who supply assist and recommendation on the deliberate set up.

While the framing materials could also be light-weight glass is deceptively heavy – 10Kg/m² for 4mm thick panes and 15Kg/m² for 6mm. Producers will present suggestions for dimension and weight restrictions for the protected utilization of the deliberate secondary glazing unit.

Within the design development the next is a sign of the kind of elements to be thought of with key points being the minimisation of injury and ease of use. The aim of the set up will dictate the fixing location and kind of glass chosen.


The design of the unique window can be utilized to resolve the type of the secondary glazing to be put in. The scale of the first window are fastened however the secondary glazing may be designed into manageable sized models.


Secondary home windows are usually located instantly contained in the established sashes or at an acceptable location inside the depth of the window reveal. An evaluation of the present window opening by the advisor firm will establish the constraints, for instance whether or not there may be passable depth within the divulge to find the secondary glazing.


The place shutters or different joinery are current, cautious consideration might be required.

Typically secondary glazing may be located between the first window and the shutters in order that the picket shutters nonetheless function. If the picket shutters are housed inside the window reveal it could be attainable to put in secondary glazing on the room aspect of the window shutters.

If the secondary glazing can’t be inserted with out making the shutters inoperable the shutters may very well be fastened closed however not modified in order that they are often introduced again into use at a later date.


Secondary glazing may be visually intrusive outwardly and inside if badly designed. To minimise the visible impact of secondary home windows externally, attempt to ensure the secondary glazing isn’t smaller than the glazed space of the present window. Attempt to place any divisions within the glazing behind the window assembly rails or glazing bars. The flat reflections of contemporary glass inside secondary glazing may be lowered through the use of anti-reflective glass.


Supply by Ian M Corridor

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