Secondary Glazing for Bay Windows

Splayed bays (angled returns) are the most common on Victorian terraces and are glazed with mitred corner posts. Square bays project at 90 degrees and are simpler, but the deep returns often need narrower side panels. Curved and bow bays are handled as a faceted series of narrow panels that follows the arc closely enough to be visually seamless.

A bay treated as three independent rectangles leaves a vertical gap at each internal corner. Two gaps running the full height of a bay can represent a larger open area than every other leak in the room combined, and will cap the system at 40-42dB regardless of how good the glass is.

Mitred posts with continuous gasketing close those corners completely, which is the difference between a bay that performs and one that merely looks glazed.

Bays usually contain a radiator, a window seat or both. Frames are set forward of the radiator so convection is not trapped against the glass, and cill-level detailing is designed around seat depth so panels still slide freely.

A bay is not one window. A canted Victorian bay is three separate openings meeting at two angled mullions; a square Edwardian bay is three at right angles; a curved bay may have five or six narrow lights on a shallow radius. Each light gets its own measurement, its own frame and its own opening style, because the centre light is usually the one you open and the returns are usually the ones you do not.

The mullion angles matter more than the widths. We take the internal angle at each junction rather than assuming 45 degrees, because settlement in London clay leaves plenty of Victorian bays a couple of degrees out. Angled corner posts are then made to the measured angle so the frames meet cleanly with no packing gap for sound to travel through.

The mix we specify most often is a hinged or sliding centre light for ventilation with fixed or lift-out returns. That keeps the acoustic performance of the returns as high as possible while leaving one easy ventilation route, and it is usually cheaper than making every light openable.

Where the bay has a window seat, radiator or fitted shutters, the frame is set to clear them — a seat cushion that fouls a hinged panel is the most common complaint we get called to on installations done by others. Hinged secondary glazing is the delivery route for most bays because a hinged panel copes with an angled reveal better than a track does.

Bays lose heat disproportionately: three or more glazed faces, a large exposed perimeter and often an uninsulated void below the cill. Sealing the glazed faces removes the draught and the radiant cold that make a bay unusable in winter, and rooms with bays typically show the biggest subjective improvement of anywhere in the house.

What secondary glazing does not fix is the floor void under the bay. If your bay is cold from below as well as from the glass, we will say so and recommend the insulation work alongside — it is the difference between a bay that is tolerable and one you actually sit in.