Secondary Glazing And Films

What this is

Where a window is single glazed but sound, or where its frames are original and worth keeping, a second layer can be added on the room side instead of replacing the whole unit. That layer runs from a shrink-fit film on a temporary frame, through magnetic or clip-fixed acrylic panels, to a permanent slim aluminium system with its own opening lights.

Telling whether your home needs it

Hold the back of a hand near the glass on a cold evening. If the air there is markedly colder than the middle of the room, or if condensation forms on the inside of the glass overnight, the window is a weak point. Curtains that move when the wind gets up indicate air passing the frame rather than heat passing the glass, which is a different problem worth solving at the same time.

What the job involves

The trapped layer of still air is what does the work, so the new layer has to seal around its whole perimeter. Film systems are taped to the frame or to a light timber surround and tightened with gentle heat. Panel systems need accurate measurement of each opening, since older openings are rarely square. Permanent systems are set into the reveal and need the reveal itself checked for square and for a sound fixing surface.

Things to watch

Seal the frame draughts first; a second layer over a leaking frame traps moving air rather than still air. Keep at least one opening light usable in each habitable room for ventilation and for escape. Where the original window has trickle vents, do not block them behind the new layer without providing ventilation another way.