Black edges on a bathroom mirror? What causes them and how to slow the spread

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Black spots creeping in from the edges of a bathroom mirror look like dirt at first. They are not. The dark patches are the mirror's reflective backing breaking down, and once it starts, no cleaning product will bring the silver back. What you can do is understand why it started and stop it from spreading fast. What the black edge actually is A mirror is glass with a thin layer of silver on the back, protected by paint. Where moisture, cleaning chemicals, or salty air reach that backing, the silver oxidises and turns black. Because the backing is behind the glass, the damage shows through as dark spots or a dark border you cannot wipe away from the front. People call it desilvering or mirror rot. Why bathrooms speed it up Three things attack the backing. Steam and condensation keep the mirror damp for hours, and water wicks into the edges where the backing is most exposed. Ammonia-based glass cleaners, sprayed directly on the mirror, run down and pool at the bottom edge, eatin...

Window condensation: find the moisture source before replacing the glass

Quick answer: Water droplets on the room-side glass often mean moist indoor air is meeting a cold surface. Start by locating the moisture and reducing its source, not by ordering replacement windows.

What is the problem? First locate the water

Check which surface is wet. Water on the inside face of the glass is different from fog trapped between the panes of a sealed glazing unit. Persistent damp on nearby walls may have another cause, such as rain or a plumbing leak.

  • Room-side droplets: review everyday moisture and ventilation.
  • Fog between sealed panes: ask a glazing professional to assess the unit and seals. Cleaning the exposed glass will not reach it.
  • Wet wall, stained plaster or recurring musty smell: investigate the wider damp problem rather than assuming the window is to blame.

Why condensation happens

Cooking, showering, drying clothes and breathing add moisture to indoor air. When that air meets colder glass, water can condense into droplets. Low airflow and cold areas can make damp more persistent. A brief patch that clears is not the same as recurring wet surfaces or mould.

These principles apply to aluminium-framed windows too. The frame material alone does not tell you why a particular room is damp. Installation, glazing, weather, moisture sources and ventilation all matter.

A safe routine, step by step

  1. Notice the pattern. Record the room, surface and timing. Is it mainly after cooking, bathing or indoor laundry? Is it worse after rain?
  2. Dry accessible droplets. Use a clean cloth on the glass and sill. Follow the window maker's care instructions; do not dismantle hardware or lean out to reach high glazing.
  3. Reduce moisture at its source. Cover cooking pots where appropriate. Dry laundry outside when conditions allow, or use a suitable ventilated space.
  4. Use existing ventilation. Run bathroom and kitchen extractors as intended. Keep provided vents clear. Do not seal ventilation openings as a shortcut to warmth.
  5. Watch what changes. If droplets keep returning, or damp spreads, arrange an assessment. A dehumidifier may help manage moisture but does not repair a leak or failed glazing seal.

What you need

A clean soft cloth and a short written record are enough for the first checks. You do not need a new chemical cleaner to diagnose condensation. For ordinary streaks after the glass is dry, see our safe glass-cleaning routine.

When to get professional help

Get help for persistent damp, mould, leaks, damaged window fittings or moisture between sealed panes. If you rent, document the problem and contact your landlord or property manager. Do not attempt structural, electrical or elevated repairs yourself. Damp and mould deserve prompt attention, especially where vulnerable people live.

Questions readers ask

Does condensation prove my windows need replacing?

No. First establish where the moisture is and whether ventilation, indoor moisture or a leak is involved. A glazing assessment is useful for fog within a sealed unit.

Will wiping the glass stop it returning?

Wiping removes the water you can reach. It does not remove the source of moisture or solve a ventilation or leak problem.

Sources and limits

This is general home-maintenance information, not an inspection of your property. Sources: Energy Saving Trust: damp and condensation, Energy Saving Trust: ventilation, and Bob Vila: condensation on windows. Advice should be adapted to local weather and the manufacturer's instructions. Prepared 4 October 2026.



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