Condensation on windows is a common homeowner complaint, but where that moisture sits changes everything about what it means and how you must respond.

If water forms on the indoor surface, your room air is too humid. If water forms on the outdoor surface, your window insulation is working exceptionally well.

However, if moisture, fogging, or mineral stains appear between the two panes of glass, you are dealing with a permanent mechanical failure: a blown insulated glass unit (IGU) seal.

Anatomy of an Insulated Glass Unit (IGU)

A modern double-pane window is not just two pieces of glass dropped into a frame. It is a factory-manufactured hermetic unit consisting of five critical elements:

  1. Inner and Outer Glass Panes: Sized typically between 4 mm and 6 mm.
  2. Spacer Bar: A hollow aluminium, stainless steel, or warm-edge composite frame that separates the two panes.
  3. Desiccant Matrix: The hollow spacer bar is filled with thousands of tiny silica gel or molecular sieve beads. These beads absorb microscopic traces of moisture sealed in the unit during manufacturing.
  4. Primary Seal: A continuous bead of polyisobutylene (PIB) applied between the glass and the spacer, acting as the primary airtight and gas-tight barrier.
  5. Secondary Seal: A structural silicone or polysulfide sealant surrounding the outer perimeter, holding the entire assembly rigid.
  6. Gas Fill: Dry air or inert gas (such as argon or krypton) filling the sealed void to reduce conductive heat transfer.

The Mechanism of Seal Failure: Thermal Pumping

Double-glazed windows expand and contract every single day. When the morning sun strikes the window, the trapped gas heats up and expands, bowing the panes slightly outward. At night, the gas cools and contracts, creating negative pressure that pulls the panes inward.

This daily pressure cycle is known as thermal pumping. Over 10 to 20 years, thermal pumping places mechanical shear stress on the perimeter seals.

Seal failure accelerates when:

  • Drainage holes become blocked: Water pools in the bottom of the window sash. When the perimeter seal sits in stagnant water for months, the sealant breaks down hydrolytically.
  • UV degradation: Harsh solar radiation breaks down unshielded sealants.
  • Poor installation: If the glass unit is wedged tightly against frame screws without setting blocks, structural building movement cracks the seal.

What Happens When the Seal Breaches

The moment a microscopic fissure develops in the perimeter seal:

  1. Inert gas escapes: Argon or krypton gas leaks out, diminishing the window's thermal insulation (R-value).
  2. Moist air enters: During the cooling phase of thermal pumping, outdoor air is sucked through the fissure into the cavity.
  3. Desiccant exhaustion: The desiccant beads inside the spacer bar absorb the incoming moisture until their chemical saturation limit is reached.
  4. Permanent fogging: Once the desiccant cannot hold any more water, the relative humidity inside the cavity reaches 100%. As soon as the outer pane drops below the cavity's internal dew point, condensation forms on the glass.

Over time, this moisture reacts with the glass surface, depositing calcium and mineral salts that form permanent white streaks ("white haze" or glass etching) that cannot be cleaned even if the cavity is dried.

Comparison: The Three Locations of Window Condensation

Location Cause Does It Wipe Away? What It Signals Solution
Indoor Surface High room humidity & cold glass Yes (from inside) Indoor ventilation needed Run extractor fans, open trickle vents
Outdoor Surface Cold glass & high ambient dew point Yes (from outside) Excellent insulation (pane is cold) None needed; burns off with sunrise
Between Panes Blown IGU perimeter seal No (untouchable) Unit insulation failed Replace the insulated glass unit

Can You "De-Fog" or Repair a Blown Unit?

Some commercial services offer "window defogging"—drilling two tiny holes in the corner of the glass, injecting alcohol-based cleaning solutions, drying the air with a pump, and installing one-way breather valves.

While this may temporarily clear visible mist, it has serious drawbacks:

  • The original argon gas fill is permanently lost.
  • The desiccant inside the spacer remains saturated and inert.
  • If mineral salts have already etched the glass, the permanent cloudiness remains.

The industry-standard, cost-effective solution is glass-only replacement. A local glazier can measure the existing glass unit, order a replacement IGU with modern warm-edge spacers and low-E coatings, and swap the glass in less than 30 minutes without replacing your window frame.

For monitoring atmospheric dew points and condensation risks on building envelopes, learn more in our guides on What Is Dew Point? and Why Do Windows Fog Up?.