Leaded Light Problems: Bowing, Draughts and Cold
Leaded lights are a fundamentally different construction to a glazed sash: dozens of small panes held in soft lead cames, bedded in cement putty, with a saddle bar or two carrying the weight. Everything that goes wrong with them follows from that.
Why they bow
Lead creeps under its own weight over decades. Once a panel starts to bow, the cames on the convex face open slightly at every joint, which lets in air and water, accelerates the corrosion of the ferrous saddle bars, and increases the bow. Bowing over about 15 mm across a light generally means the panel needs releading rather than patching.
You cannot double glaze a leaded light — and encapsulation is a compromise
Encapsulating an original light between two panes of float glass is offered widely, but it changes the light quality, kills the depth and texture of hand-drawn glass, and is not reversible. Most conservation officers in Sheffield and Chesterfield treat it as a loss of historic fabric where the glass is original.
The room-side answer
A slim secondary panel set behind the light does three things at once: it stops the air leaking through every came reaching the room, it warms the room-side surface so the condensation that corrodes the cames stops forming, and it shields the panel from impact and from the thermal shock that cracks old glass. Nothing is done to the original.
- No alteration to lead, glass or putty
- Removes the condensation that corrodes cames
- Impact protection for irreplaceable glass
- Fully reversible
Frequently asked questions
If it is bowing badly, yes — do the leadwork first, then glaze behind it. Fitting a panel in front of a failing light just hides the deterioration.
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