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...

Kitchen windows wet after cooking? Check ventilation before replacing glass

Quick answer: If cooking leaves your kitchen windows wet, check how moisture leaves the room before replacing the glass. A cooker hood, a window and built-in vents do different jobs. Start with the equipment you have and ask a professional before altering ducts, wiring or gas-appliance ventilation.

What is the problem, and why does it happen?

Cooking releases water vapour, odours and particles. Moist air meeting colder glass can form droplets. A room that looks clean can still have an air-quality problem: some pollutants cannot be seen or smelled. Condensation alone does not identify the cause or prove that the aluminium frame or glass has failed.

A window and a cooker hood are not interchangeable

A window allows natural airflow, but how much air moves depends on conditions and the layout. An extractor positioned above the cooking area is intended to capture cooking by-products near their source. Some hoods discharge outside; others filter and recirculate air. Check the actual model and its instructions rather than assuming every hood removes moisture in the same way.

Provided trickle vents, wall vents and whole-home systems are also part of the ventilation plan. Do not block them to stop a draught. Draught-proofing and controlled ventilation need to work together.

Safe first checks, step by step

  1. Locate the moisture. Note whether droplets appear on the room-side glass during cooking or whether fog is trapped between sealed panes. Our window-condensation guide explains that distinction.
  2. Read the hood's instructions. Identify whether it extracts outdoors or recirculates, and follow the recommended operating and filter-care routine.
  3. Use existing extraction while cooking. Keep accessible filters and vents maintained as the manufacturer specifies. Do not remove parts you are not instructed to service.
  4. Reduce avoidable moisture. Use lids on pans where appropriate and avoid combining indoor laundry drying with a poorly ventilated cooking area.
  5. Use safe natural airflow when appropriate. A window may help, but consider security, outdoor pollution, weather and appliance instructions. An open window is not a substitute for a required ventilation system.
  6. Record what remains. Persistent damp, mould or odours justify an assessment. Do not buy new glazing or a larger fan before checking the cause.

Planning a kitchen layout before buying

Mark the hob, existing window, hood and possible external duct route on a simple plan. Ask an installer to assess the feasible route, equipment requirements and permissions. A visual room planner can help organise ideas, but it cannot specify airflow, prove building-code compliance or approve a gas or electrical installation.

For the cost worksheet, you can use our free kitchen budget planner to organise your own prices before buying. It is a planning aid, not a ventilation design or contractor quote. Its shopping-list Find links may be affiliate links; the site owner may earn from qualifying purchases.

What you need for the first checks

The appliance manual, a clean cloth for accessible droplets and a few notes are enough to begin. Avoid a purchase-first approach. If you rent, ask the property manager before changing fixed equipment or cutting a wall.

When to get professional help

Get help for ongoing damp or mould, a hood that does not appear to work, damaged fittings or moisture between sealed glazing panes. Leave duct changes, wiring, elevated work and gas-appliance ventilation to qualified people. If a carbon-monoxide alarm sounds or an appliance may be unsafe, stop using it and follow your local emergency guidance. Do not diagnose safety by smell.

Common questions

Will a bigger window solve kitchen condensation?

Not necessarily. Window size is one part of the room; the moisture source, extraction route and overall ventilation also matter.

Can I seal every vent to keep the kitchen warm?

No. Blocking planned ventilation can trap moisture and pollutants. Ask for advice on balancing warmth and ventilation rather than sealing openings blindly.

Sources and limits

General planning information, not an installation specification. Sources: Energy Saving Trust: home ventilation, Energy Saving Trust: damp and condensation, and Which?: cooker hoods and windows. Local rules and the actual manufacturer's instructions take precedence. Prepared 5 October 2026.


 

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