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...
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Foggy Double-Pane Windows: Why Condensation Builds Up Between the Glass and What You Can Do

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That milky haze sitting between the two panes of your double-glazed window is not dirt you can wipe away. It is moisture trapped inside the sealed unit, and it is telling you something specific: the seal on your insulated glass unit (IGU) has failed. The good news is you have several options, and not all of them involve buying a new window. This guide walks through what is happening, why it happens, and every realistic fix, from the cheapest to the most thorough.

What the problem actually is

Modern double-pane windows are two sheets of glass separated by a spacer bar and sealed around the edge. The gap between the panes is filled with dry air or an insulating gas like argon, and the spacer usually holds a desiccant material that absorbs any trace moisture left inside at the factory. When the edge seal cracks or ages, outside humid air slowly works its way in. The desiccant saturates, and from that point on, every temperature swing pumps a little more moisture into the gap. The result is fog, water droplets, or a cloudy film that never wipes off, no matter how hard you clean both sides of the glass.

Why it happens

  • Age: Most IGUs last 10 to 25 years. Sealants break down with UV exposure and daily expansion and contraction.
  • Heat and sun exposure: South- and west-facing windows fail sooner because the glass heats up and the trapped air expands, pushing against the seal every afternoon.
  • Water pooling at the frame: Clogged drainage holes or a sill that stays wet keeps the bottom edge of the seal damp, which speeds up failure.
  • Frame movement: A house settling, or an aluminium or uPVC frame that was slightly out of square at installation, puts constant stress on the glass edge.
  • Pressure changes: Windows shipped or installed at very different altitudes can lose their seal early.

How to confirm it is seal failure (and not surface condensation)

Before you plan a repair, check where the moisture is:

  • Moisture on the room-side surface that wipes away with a cloth is an indoor humidity issue, not a failed window. Lower indoor humidity and the problem goes away.
  • Moisture on the outdoor surface on cool mornings is actually a sign your window is insulating well. It is harmless.
  • Moisture between the panes that you cannot touch from either side is a failed seal. That is the case this guide covers.

Your options, cheapest first

1. Leave it and monitor (free)

A foggy IGU still works. It has lost some insulating gas and a little efficiency, but it is not an emergency. If the fog is mild and the window is in a low-visibility spot, you can simply note it and check it every few months. If the fog grows or water starts pooling at the bottom of the unit, move to the next options.

2. Defogging service (roughly $70 to $150 per window)

A technician drills tiny holes in the glass, flushes the inside with cleaning and drying solutions, then fits one-way vents so the unit can breathe. This clears the view and is the cheapest professional fix. The trade-off: the insulating gas is gone for good, and the seal is not restored, so the energy performance stays slightly reduced. For a window you mainly want to look through, it is a reasonable middle path.

3. Replace just the glass unit (roughly $150 to $400 per window)

In most aluminium and uPVC frames, the IGU itself is a separate component held in by glazing beads. A glazier can pop the beads out, measure the unit, and fit a brand-new sealed IGU into your existing frame, often within an hour per window. You get a fresh seal, fresh gas fill, and usually a new warranty of 5 to 10 years, at a fraction of the cost of a whole new window. This is the best-value repair for most homeowners.

4. Sash or full window replacement ($400 and up)

If the frame is warped, corroded, or more than 20 years old, replacing only the glass may be money spent on a frame that will soon be the weak point. In that case, replacing the sash (on compatible systems) or the whole window makes more sense, and it is also your chance to upgrade to low-E glass, better spacers, or triple glazing.

Which one should you choose?

  • Frame is in good shape, fog is the only issue: replace the IGU (option 3).
  • Budget is tight and you just want a clear view: defogging (option 2).
  • Multiple windows are fogging at once and the frames are old: price a full replacement and compare it against several IGU swaps.
  • Windows are still under warranty: check the paperwork first. Many IGUs carry 10- to 20-year seal warranties, and the replacement may be free.

How to make the next set of windows last longer

  • Keep the frame drainage slots clear so water never sits against the glass edge.
  • Wash frames and glass with mild soap, not harsh solvents that attack sealant.
  • Manage indoor humidity (exhaust fans, dehumidifier) so the seal is not fighting constant moisture pressure from inside.
  • When buying replacements, ask about the IGU warranty and whether the spacer is a modern "warm edge" type, which runs less thermal stress on the seal.

Fog between panes is annoying, but it is one of the more predictable window problems: once the seal has failed, the fix is a straight choice between clearing the view, swapping the glass, or upgrading the whole unit. Get two or three local quotes for an IGU replacement before assuming you need new windows, and check your warranty paperwork first. You may be pleasantly surprised.

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