Common window problems: fog between the panes, dust and draughts, and water leaks

A window is a system of glass, frame, gaskets, sealant and installation details. A fault in any part shows up as heat, dust or water. For the basics see windows and glazing.

1) Fog between the panes of double glazing

When condensation or fog appears between the two panes (not on the inside or outside face), it means the insulated glass unit (IGU) seal has failed and moisture has got into the space between the panes, giving a milky, foggy look1.

• Effect: insulated glass units work by keeping the insulating air space between the panes, and when the seal fails their insulating effectiveness is compromised1.
• Causes: age, water retention or drainage problems, and direct sun exposure causing the glass to expand and contract1.
• The fix: often you can replace just the glass, not the entire window, reducing cost1, provided the frame is sound.

Condensation on the inner face of the glass in winter or on an air-conditioned room's window is a different matter, usually caused by indoor humidity or a cold surface. See condensation.

2) Dust and draughts

Common causes and practical fixes.

• Worn gaskets or brush seals on sliding windows: replace with matching parts.
• Misadjusted rollers or locks that stop the sash closing tightly.
• A gap between frame and wall: needs filling and a suitable sealant inside and out. See joint sealants.
• Hinged (casement) windows usually seal better than sliding ones.

3) Water leaking around the window

• Blocked drainage holes in the bottom frame channel; clean them so water drains outside.
• Cracked or debonded sealant around the frame outside.
• A sill without an outward slope or without a drip groove under the stone.

4) Annual checklist

1. Look at each double-glazed window from an angle: is there fog between the panes?
2. Clean frame channels and drainage holes.
3. Check the external sealant and gaskets.
4. Test closing and locking on every window.