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Can I get secondary glazing for windows like mine?

Can you get secondary glazing for windows like sash, casement, bay and leaded ones? Yes you can. It’s cheaper than replacing windows with new double glazing units, and can, when installed properly, bring almost equivalent energy savings, as well as improving comfort and noise insulation.

What is secondary glazing?

Secondary glazing is an independent system fitted on the inside of existing windows. Units hold glazing in frames of timber, aluminium or plastic; either glass or flexible acrylic glazing may be used.

Units are removable or openable, in order to clean or open the window itself, and may be removed in warm periods when not required.

Why fit secondary glazing?

Secondary glazing fitted over sash windows in a bay window

Secondary glazing fitted over sash windows in a bay window

  • Heat losses from conduction and radiation through the window as a whole can be reduced by over 60%, especially with the help of a low emissivity (low-e) coating on the surface of the inner pane that is facing the outside. This provides a U-value that is close to Part L standards.
  • Secondary glazing allows the embedded energy (and carbon) of existing windows to be retained.
  • The outside appearance of the window is preserved.
  • It removes the need to dispose of the old windows.
Another solution for easy removal of the secondary glazing

Another solution for easy removal of the secondary glazing

  • Economical and permanent, it also provides effective sound insulation if the panes are 10 cm or more apart.
  • It is possible to minimise condensation by draught-proofing the inner pane.
  • The frames are typically compression-fitted for easy removal to clean the window or for escape in case of fire.
  • Secondary glazing is not subject to building control regulation, apart from with Listed Building Consents.

Always repair existing windows before fitting secondary glazing.

Secondary glazing for listed buildings 

Secondary glazing for windows, in this case fitted over leaded windows in a listed property

Secondary glazing fitted over leaded windows in a listed property

There is, in principle, no conflict between energy efficiency requirements in Part L of the Building Regulations and the conservation of historic and traditionally constructed buildings.

In this case, it is recommended by heritage bodies and approved under PPS5 Planning for the Historic Environment. In these cases, it is important to consult with your local planning department.

Draught proofing

Both secondary glazing and double glazing achieve their thermal benefits partly by reducing draughts, or unwanted air infiltration. In the case of secondary glazing, this is because the units fit over gaps between the casement and the window frame.

It is often easier to install secondary glazing than successfully draught-proof some types of windows, such as sash windows and leaded windows.

Noise reduction

Usually there is a larger gap between the pane of glass in the secondary glazing and that in the original window, than is the case between both panes in double glazing. This results in improved noise reduction compared to double glazing. The larger the gap, the more noise reduction.

Condensation

Small trays of desiccant left in between the secondary glazing and the window to absorb moisture and prevent condensation

Small trays of desiccant left in between the secondary glazing and the window to absorb moisture and prevent condensation

Condensation can occur in the gaps between the panes. Effective seals will minimise the amount of warmer, moist air reaching the cold outer glass. This will reduce but not eliminate condensation.

This is the case especially if the secondary glazing is opened. Placing in the gap open containers of hygroscopic or desiccant materials, such as silica gel or other chlorides, available from a hardware store, can help by absorbing moisture. These must be removed and dried on a heater when they become saturated, then replaced.

Fresh air coming from outside through leaks in the outer window can help to reduce condensation within the cavity.

Secondary glazing insulation value

Insulation and windows - U values comparedU-values indicate the rate of heat flow across a material: the lower the value, the better the insulation. Adding any kind of secondary glazing will improve the thermal performance of single glazed windows:

  • A single-glazed metal-framed window will have a U-value of around 5.7W/m²K, whereas a timber-famed one will be around 4.7/m²K.
  • Adding secondary glazing with standard glass will bring this down to between 2.6 and 2.9/m²K.
  • If low-e glass is used, this will bring it down further to between 1.8 and 1.95/m²K.
  • Providing sealed units will reduce it even more to between 1.6 and 1.7/m²K.

Some secondary glazing units have received performance labels using Window Energy Ratings, the system run by the British Fenestration Rating Council (BFRC), an independent government-supported body, administered by the Glass & Glazing Federation (GGF). This allows them to be easily compared.

The rating includes the frame as well as the glazing.

The most thermally efficient windows have insulated frames. Metal frames are the least thermally efficient.

Secondary glazing for windows like mine

DIY secondary glazing fitted to a sash window. Note the knob and handles, and the fastenings to aid removal and replacement

DIY secondary glazing fitted to a sash window. Note the knob and handles, and the fastenings to aid removal and replacement

Secondary glazing can either be made:

  • With plastic glazing
  • With standard glass
  • With low-e (emissivity) coated glass
  • As sealed units.

They are either supplied as standard units, or can be made specially for windows of unusual sizes. It is possible to make frames yourself.

Vertical sliding secondary glazing over sash windows in a bay window

Vertical sliding secondary glazing over sash windows in a bay window

Systems can either be:

  • Hinged
  • Lift outs
  • Sliding
  • Magnetic, removable
  • Fixed
  • Shaped.

As such, they can therefore fit any of these kinds of windows:

  • Casement
  • Sash
  • Sash with shutters
  • Crittall
  • Stone-mullioned
  • Circular
  • Arched
  • Bay
  • Conservatory
  • Leaded (stained glass).

All systems should be fitted with effective seals of the brush or compression varieties to eliminate draughts.

Vertical sliding secondary glazing over sash windows in a bay window

Vertical sliding secondary glazing over sash windows in a bay window

Sliding systems can be horizontal or vertical. Vertical types are typically deployed for sash windows. Tilt-in vertical sliding systems are easy to clean. Sliding systems are not draught proof.

Hinged systems can be single or double leaf dependent upon the window size.

Lift outs are used for windows that fixed or seldom opened, where access is only required occasionally. They are also used when the window is an unusual shape.

Secondary glazing units separately fitted for a transom and main window. The clips allow easy removal for cleaning and opening

Secondary glazing units separately fitted for a transom and main window. The clips allow easy removal for cleaning and opening

Removable windows often use a magnetic secondary glazing system. These employ multi-polarised magnetic strips that are fitted to sheets of clear cast acrylic, because it is flexible. Similar magnetic strips are glued to the edge of the window frame to hold the panel in place. Acrylic is not as durable as glass, however.

Fixed panels are used where no access is required, or side-by-side with other panels which can open. These sealed units completely avoid the risk of condensation.

Shaped units are custom made to fit, for example, curved headed windows or bowed windows.

Installation

A solution found to fixing secondary glazing to a window where there is little space for securing it

A solution found to fixing secondary glazing to a window where there is little space for securing it

Secondary glazing units are most commonly fitted onto the frame within the window reveal, or at a suitable position within the reveal if this is not possible. Where shutters are present, they usually can fit between the shutter and the window, or on the inside of the shutters.

Maintenance

Cleaning: the inside surface can be cleaned as normal. Any low-e coated surface should be cleaned occasionally with a lint free cloth and clean soapy water. Abrasive materials should not be used.

Protection of the materials. If the frame or the glass is made of acrylic, this can be damaged by high temperatures, over 50°C, which can be achieved in the cavity in hot weather. In this case it is advised to remove the units during these periods and store them safely.

The frames are usually supplied with ironmongery such as fastenings, hinges or locks which may need the occasional lubrication.

eco renovation - talk to homeowners

© David Thorpe Mar 2013. David is Manager of Green Deal Advice and author of Sustainable Home Refurbishment: The Earthscan Expert Guide to Retrofitting Homes for Efficiency

Also see:
Secondary glazing at free open house events in September
Magnetic strip secondary glazing
English Heritage guidance on secondary glazing
Historic Scotland – Thermal performance of Traditional windows (see p13-14)
Warm Bath – guide to improving energy efficiency (see p28)
Plastic window film option – Enerlogic
Sash window replacement
Slimlite double glazing
Encapsulated stained glass/leaded light windows
Energy efficient windows