Secondary Glazing specialists in London.
Heat loss from a room through a window during the heating season is complex as three main mechanisms are in play:
- by convection and conduction, from the warm room air to the colder surfaces of the glass and the frame
- by the colder surface of the window absorbing infra-red radiation from the room
- by uncontrolled air leakage, which can either bring in cold air from the exterior or take warm
air out from the interior; often called air infiltration, this can occur even when the window is closed.
HEAT LOSS THROUGH THE GLASS AND FRAMES
Whether it leaves the room by convection, conduction or radiation, the lost heat all passes through the glass and the frame as conduction. The glass is the most conductive part of the window but heat is also lost through the frame albeit at a lower rate.
Single glazing is a poor thermal insulator and readily conducts heat. A typical 4mm thick glass has a typical U-value of 5.4W/ m2K. The thermal loss through a single glazed window will depend on the total area of glass, the conductance of the frame material and the quality of the fit of the framing and glazing materials. A typical value of a timber framed single glazed window is 4.8W/ m2K.
For thermal performance, the optimum airspace between panes is 16 – 20mm. A larger air space allows convection currents to develop within the cavity and more heat to be lost. The positioning of the secondary unit is usually dictated by the window reveal and can often only be fitted at
a distance of about 100mm from the primary glazing. However, a significant proportion of the thermal benefit of secondary glazing comes from decoupling the frame from the primary timber moving parts and 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 performance to less than 2.0/ m2K. To maintain this figure it is important to keep the coating clean – the standard is ‘visually’ clean.
HEAT LOSS THROUGH AIR LEAKAGE
Heat losses from a typical traditional window are predominantly through gaps around the window. With larger windows the proportion of heat lost by conduction through the glass tends to be greater.
Since draughts, caused by convection and air infiltration make people feel colder, the occupants may turn up the heating, and also run it for longer. Purpose-made secondary windows, with efficient perimeter sealing (to prevent condensation on the primary window) and brush or compression seals on the opening panels, form an effective seal over the whole of the frame of the original window and can significantly reduce excessive draughts.
Windows are one of the most vulnerable parts of a building to noise transmission due to their relatively lightweight construction. Depending on the number of openings and the quality of the seals between the openings, a single glazed window without seals may only achieve a noise reduction of 18 – 25dBA. When closed, sealed double glazed units perform little better than single glazing because the two panes of glass are rigidly connected with a minimal cavity so the two panes resonate together.
A secondary window with an air space of 100mm 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 achieved. Higher levels of sound insulation are obtained as the gap increases particularly if the reveals are lined with an acoustic material, though minimal improvements occur with cavities beyond 200mm. The use of thicker or acoustic laminate glass within the secondary window also improves the acoustic performance of the installation
A second window provides an additional barrier to entry and therefore can provide improved security. This can be particularly appropriate when the use of an historic building is being changed and a higher degree of security is required. The secondary glazing can provide that additional security whilst retaining the existing windows.
All air contains some water vapour, but warm air can hold more vapour than cold air. When warm, damp air is cooled it will reach a temperature at which is cannot hold all the vapour, and the water will condense out. This temperature is called the dew point.
Warm damp air passing over a cold surface may be cooled locally below the dew point in which case condensation will take place. This effect causes the familiar condensation on the inside of cold windows.
Condensation on the outer window may arise if the secondary system is opened for ventilation in cold weather particularly where rooms are relatively humid.
These condensation risks 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 buildings commonly have adequate ventilation from other parts located where the normal direction of air flow is from outside to inside, for example on the windward side of a building, on the lower floors or where a designated natural or mechanical extraction system helps to ensure in ward airflow fitted with devices which avoid reverse air flow in adverse circumstances where the primary and secondary assemblies incorporate some alternative means of ventilating between the exterior and the room interior but bypassing the cavity between the primary and secondary glazing
Secondary glazing is nothing new. In the 19th century, some buildings were constructed with internal secondary windows incorporated as part of the original design. When you use secondary double glazing on windows, you stand to generate plenty of advantages. The rewards contain significant ranges of sounds reduction, draft proofing, thermal insulation, enhanced safety and safety and reduction of airborne dust.
Our secondary double glazing is lightweight, durable and requires very little maintenance. All fixing and screws are hidden by a specialist trim to give a seamless look. We supply and fit curved secondary glazing for arched windows. Every installation comes with a 10-year product guarantee.
GUARANTEED TO BEAT ANY QUOTE ON THE MARKET