Why Is There Condensation Between Double-Glazed Panes?

First locate the moisture

Wipe the room-side and exterior glass surfaces. If the haze remains inaccessible, it is inside the sealed insulating glass unit. If it wipes from the interior surface, indoor humidity and low glass temperature are producing condensation. Exterior dew can occur on an efficient window when the outside pane radiates heat to a clear night sky; it is not necessarily a defect.

This distinction prevents unnecessary work. Ventilation and humidity control can address room-side condensation, while failed between-pane seals normally require replacement of the insulating glass unit.

How an insulating glass unit works

Two or more panes are separated by a spacer and sealed around their edges. The cavity contains dry air or a gas such as argon, while desiccant absorbs limited residual moisture. The edge seal must prevent moisture vapour entering and retain the intended cavity condition.

When the seal loses integrity, the desiccant eventually becomes saturated and temperature changes reveal fog, droplets or mineral deposits between panes. The glass may still be physically intact, but its original thermal and visual performance can no longer be assumed.

Why seals fail

Ageing is one cause, but edge conditions matter. Standing water in glazing pockets, blocked frame drainage, insufficient edge cover, incompatible sealants, incorrect setting blocks and excessive thermal or structural movement can shorten service life.

Strong sun and temperature cycling increase expansion and contraction. A distorted sash or poorly supported heavy unit can also stress the edge. When replacing glass, investigate these conditions so the new unit is not placed back into the same failure environment.

Why drilling is not restoration

Some services drill holes to vent or dry the cavity. This may reduce visible fog temporarily, but it does not recreate the original hermetic seal, restore gas concentration, renew saturated desiccant or verify coating condition. Treat it as a cosmetic intervention, not a return to the specified insulating-glass performance.

The normal durable remedy for a failed sealed unit is replacement with a matching or approved new IGU while retaining a sound frame and sash.

Specify the replacement correctly

Record overall thickness, pane thicknesses, cavity width, Low-E or solar-control coating, gas, spacer color, safety treatment and glass appearance. A visually similar clear unit may not match the thermal, solar or safety function of the original.

During re-glazing, use compatible setting blocks and gaskets, maintain edge clearances and keep drainage paths open. Confirm that glazing beads and the frame can accept the selected unit without edge pressure.

Prevent repeat failures

Clean tracks and weep openings, repair failed exterior perimeter joints and correct sash alignment. Do not seal over designed drainage holes. Manage indoor humidity and use exhaust ventilation in kitchens and bathrooms to reduce interior-surface condensation.

Keep photographs, the new glass specification and warranty record. If several units fail in the same elevation, look for a shared exposure, drainage, manufacturing or installation cause rather than treating every pane as an isolated event.

When an inspection is worthwhile

Arrange a closer inspection when staining reaches adjacent finishes, the sash no longer closes squarely, or several panes begin fogging within a short period. The reviewer should distinguish glass-edge failure from water entering around the outer frame. These conditions can look similar from indoors but require different repairs. A replacement IGU will not stop water that originates at an unsealed wall joint, and resealing the wall will not clear moisture already trapped inside a failed unit.