Secondary Glazing for Crittall Windows

The short answer

Keep the steel window, add a slim aluminium secondary unit behind it, usually hinged to mirror the original casement and finished in black to echo the frame. You keep the look that makes the building, and gain the performance the single glazed steel section cannot give.

Why Crittall needs a room-side answer

Original Crittall frames are a 20 mm steel section holding a single 4 mm pane. There is no cavity to fill and no meaningful thermal break, so replacement double-glazed units either look wrong (heavier sightline) or lose the character entirely. Room-side secondary glazing leaves the steel frame visually unchanged from the pavement and outside.

Our Crittall specification

Slim-line aluminium hinged casement or horizontal slider set behind the original, with a 100 mm cavity for acoustic work or 50–75 mm for thermal-only. Powder-coated to match the interior reveal; most owners choose matte black to echo the Crittall frame.

• Hinged casement mirrors the original opening
• 6.4 mm laminated acoustic glass standard
• 50–200 mm cavity depending on target
• Matte-black RAL 9005 to match Crittall

Hinged or sliding on a steel window

Most Crittall casements open outwards on a side hinge with a projecting handle and peg stay. A hinged secondary leaf opens inwards, clearing the handle completely, and its meeting line can sit behind the original steel mullion. On wide multi-light bays, or where a radiator or worktop blocks a swinging leaf, a horizontal slider does the same job sideways.

Rust, condensation and the steel frame

Cold steel attracts condensation, and standing water is what starts the rust along the bottom rail. A sealed room-side unit keeps moist room air away from the cold frame, which slows that cycle. Frames that are already corroded should be treated and painted before the secondary unit goes in; our rusting steel frames guide explains what to look for.

What the survey checks on a steel window

Steel windows vary more than timber ones, so the survey looks at how far the handles and peg stays project, whether the reveal is plastered, tiled or timber lined, and whether the frame sits in a steel sub-frame or directly in the masonry. It also notes where the original mullions and transoms fall, so the secondary frame lines can be set directly behind them. On larger mid-century openings with many small panes, the aim is to add as few new lines as possible, keeping the view through the original grid clear and the building recognisable from the street.

Listed and conservation Crittall

Post-war Crittall installations in mid-century houses are increasingly locally listed. Room-side secondary glazing typically clears without formal consent because nothing changes on the elevation. We supply the survey pack a heritage officer will want to see.

Where it works

Mid-century semis and detached, converted warehouses and mills, 1920s–50s commercial fronts, garden-room extensions, plus later steel replacements in the same style. We have installed on all of the above across Sheffield, Leeds and further afield.