Secondary Glazing

Secondary glazing leaves the original window untouched and fits a second, independent window inside it. The air gap between the two works with the glass to reduce noise and heat loss. In most cases the installation can be taken out later without damaging the historic window or reveal.

For noise reduction, the size of the air gap between the original window and the secondary unit matters a great deal, and a wider cavity generally performs better. Acoustic laminated glass and well-sealed frames improve results further, especially against traffic, rail and aircraft noise.

Condensation in the gap between the two windows is reduced by making the secondary unit the more airtight of the two. The original window should not be fully sealed, so any moisture that gets into the cavity can escape outside rather than being trapped against the glass.

Owners of listed buildings should seek listed building consent before installing secondary glazing, as the requirements depend on the building and the council. Secondary glazing is often seen as a minimal, reversible change, but carrying out works that need consent without it is a criminal offence.

Opening styles include vertical sliders that line up with sash window meeting rails, horizontal sliders for wide openings, hinged casements, lift-out panels for windows that are rarely opened, and fixed panes. The choice depends on how you use the window, cleaning access and fire escape needs.

Low-level secondary glazing is still glazing, so the safety glazing rules for critical locations apply. In England, Approved Document K treats glass within 800mm of the floor as a critical location, so low-level secondary panes should be toughened or laminated safety glass.

If an existing window is relied on for escape in a fire, the secondary glazing in front of it must open easily and give enough clear opening. The surveyor checks which windows are escape windows, so the opening style and catches do not block a way out.

Slim aluminium frames are designed to sit within the window reveal and line up with the original glazing bars or meeting rails. Matching sightlines keeps the secondary unit discreet from outside and inside, which matters for period homes and conservation areas.

A low-emissivity coating on the secondary glass reflects heat back into the room and improves the insulation of the combined window. It is a useful choice for single-glazed sash and casement windows where the originals must be kept for character or planning reasons.

The price of secondary glazing depends on the size and number of windows, the opening style, the glass specification, any shaped or arched heads, reveal depth and condition, and access. Acoustic laminated glass and shaped frames cost more than standard clear glass in rectangular frames.

Secondary glazing can also add security, because an intruder would have to break through two separate windows. Laminated glass and locking catches on the secondary unit add further resistance, which is useful for ground-floor and basement windows in period properties.

Glass for secondary glazing is heavy, so the frames need firm fixings into the reveal or a supporting sub-frame. The surveyor checks that plaster, timber linings and window boards are sound enough to carry the weight, and allows for any packing needed in uneven reveals.