How Secondary Glazing Works

Thermal insulation

The additional pane of glass traps an insulating layer of air and cuts down heat loss by 40-50%. This improves to more than 60% if a low-e glass is used in the secondary window and provides a u-value that is close to Part L standards. Sealed units can also be fitted to provide triple glazing and lower u-values.

guidance

Condensation

When warm moist air meets a cold surface condensation can form and this is most evident in winter months when windows are closed. Well sealed Secondary Glazing presents a warmer surface to the room and effective seals will minimise the amount of moist air reaching the cold outer glass. This will reduce but not eliminate condensation. It is strongly recommended that the outer window is not fully sealed to allow ventilation of the cavity.
Low-e glass is a better insulator than standard glass and hence less heat will be conducted from the warm interior. This results in the outside glass being colder and more prone to condensation in certain conditions. It is an indication that the insulating glass is performing correctly.
 

Draught proofing

The secondary window frame is bedded in acrylic mastic and all opening panels incorporate high performance seals. These measures ensure that air infiltration through the whole of the existing window and the associated ingress of dust particles is significantly reduced.

Comfort

The reduction in draughts and establishment of a more even temperature around the room will noticeably improve comfort levels.

Low-e glass

A hard metallic coating on the surface facing the cavity helps significantly with heat containment. There is a small reduction in light transmittance and a slight roughness on the coated surface can lead to light scatter which may be visible as a haze at times when the sun is strong and low. This is not a defect in the glass. The coated surface should be cleaned occasionally with a lint free cloth and clean soapy water. Abrasive materials should not be used.

How secondary glazing can improve building energy performance