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GLASS & SEALED UNITS

Condensation Between Window Panes

Condensation trapped between double-glazed panes usually means the sealed unit has failed and moisture has entered the cavity.

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Treat cracked or loose glass as hazardous

Do not press, lever or dismantle glazing unless the pane is safely supported. Large, high-level and safety-critical glass needs professional handling.

MOST LIKELY CAUSES

Start with the glass, seals and frame support

Failed perimeter seal The edge seal no longer keeps moisture out.
Age-related breakdown Sealed units gradually lose performance.
Frame drainage problems Standing water accelerates seal failure.
Poor installation or movement Stress can damage the unit edge seal.
◫ Symptom variations Match the misting, cracking, rattling or water pattern. ⌄
  • Mist remains after wiping both sides
  • Moisture appears only inside the glass cavity
  • Clouding worsens in cold weather
  • Water droplets form between panes
  • One window is affected while others are clear
✓ Quick checks Safe checks before disturbing the glazing. ⌄
  1. Clean both accessible glass surfaces first.
  2. Check whether the mist is truly inside the unit.
  3. Inspect the frame for standing water.
  4. Look for cracked beads or damaged seals.
  5. Check whether the unit rattles or moves.
Important: Avoid pressure near cracked glass and never block window drainage slots.
1 Step 1 — Confirm sealed-unit failure Make the smallest safe correction and retest. ⌄

Rule out normal room-side condensation.

2 Step 2 — Check frame condition Make the smallest safe correction and retest. ⌄

Make sure drainage and glazing support are sound.

3 Step 3 — Replace the sealed unit Make the smallest safe correction and retest. ⌄

The glass unit can often be replaced without changing the whole frame.

⌁ Why the problem happens How sealed units, glazing support and drainage affect performance. ⌄

Double glazing relies on a sealed dry cavity. Once the edge seal fails, moisture enters and condenses between the panes.

⌘ Advanced checks Further diagnosis for persistent glazing faults. ⌄
  1. Measure visible glass and overall unit thickness.
  2. Identify toughened, laminated or patterned glass requirements.
  3. Check spacer bar type.
  4. Inspect glazing packers.
  5. Check frame drainage slots.
◇ Tools and information needed Useful items for accurate checks. ⌄
  • Tape measure
  • Torch
  • Plastic trim tool
  • Glazing information
  • Safety gloves
▣ Parts that may need replacing Common glazing components linked to this fault. ⌄
  • Replacement sealed unit
  • Glazing packers
  • Beads
  • Gaskets
  • Frame drainage components
! When to call a professional Conditions that should end basic DIY diagnosis. ⌄
  • The glass is large or above ground level.
  • Safety glass may be required.
  • The frame is distorted or rotten.
  • The unit is cracked as well as misted.
  • Glazing beads are external or security-sensitive.
? Frequently asked questions Common glass and sealed-unit questions. ⌄

Can the moisture be dried out permanently?

Temporary drilling or venting rarely restores full performance. Replacement is the proper fix.

Do I need a whole new window?

Usually not. The sealed glass unit can often be replaced in the existing frame.

Does misted glass reduce insulation?

Yes, because the sealed cavity is no longer performing as designed.