Secondary Glazing for French Doors

Sliding is the usual choice on doors in daily use — panels park to one side and stay there through the summer. Double-leaf hinged units seal better and suit doors that are mainly used in warmer months, or where a slider track at threshold level would be a trip hazard.

Getting the bottom detail right matters both acoustically and practically. We use a low-profile track that sits under 12mm proud of finished floor level, with a brush-and-compression combination seal so the sound path under the panel is closed without creating a lip you catch a toe on.

Full-height panels are visually significant, so sightlines are kept to 25mm and frames are colour-matched to the doors. Where the original doors have glazing bars, panel divisions are set to align with them so the two layers read as one.

The whole design problem with French doors is that a secondary layer must not stop the doors opening. We solve it with full-height hinged panels that swing clear of the door leaves, hung from a sub-frame fitted to the reveal or architrave. You open the secondary panel, then the doors — the same sequence as a sash window, just at floor height.

Where the doors are used constantly in summer, we can hang the secondary panels to fold back against the return wall so they sit out of the traffic route rather than standing proud into the room. On garden doors in London terraces this is normally the detail that decides whether the installation gets used or resented.

At floor level the panel has to deal with a threshold that is often uneven and a floor finish that may have changed since the doors were fitted. We take a levelled measurement across the opening and set the sub-frame cill to bridge it, with a brush or gasket seal that closes the gap without becoming a trip edge.

Glass at this height is a safety matter as well as an acoustic one. Full-height panels are glazed with laminate, which is a safety glass under critical-location rules and also happens to be the acoustically better choice — one specification satisfies both requirements.

French doors are usually the worst-performing element in a London rear elevation: large glazed area, a long perimeter seal and often an original single-glazed unit. Adding a sealed secondary layer with 6.4mm or 10.8mm acoustic laminate and a 100mm cavity gives the same order of improvement as on a window, and removes the draught that runs across the floor from the threshold.

Hinged secondary glazing is the delivery route for this specification, and it carries the 10-year standard warranty or 25 years on Premium and Heritage builds.