How to Stop Condensation Forming Between Double Glazing Panes

Condensation trapped between the panes of a double glazed unit is different from the morning mist you can wipe off the room side. Internal misting means the hermetic seal has failed — and once that happens, no amount of ventilation or heating will clear it permanently.

What the seal actually does

A sealed unit is built as a closed system: two panes, a spacer bar, a primary seal of polyisobutylene and a secondary structural seal. The spacer bar is filled with desiccant beads that mop up the small amount of moisture left inside during manufacture. As long as the seals hold, the cavity stays dry for decades.

Why units mist up

Daily temperature swings make the panes flex — glaziers call it thermal pumping. Over years, this works at the seals. When a seal opens, each pumping cycle draws in humid air. The desiccant absorbs it at first, which is why early seal failure often goes unnoticed. Once the desiccant is saturated, moisture condenses on the coldest glass surface inside the cavity.

Can it be repaired?

Drill-and-dry services vent the cavity and can clear visible mist for a while. But the argon fill is long gone, the desiccant is spent, and the seal is still broken — thermal performance does not come back. For the cost, a replacement unit into your existing frames is nearly always the better answer.

Ordering the right replacement

• Measure tight to the aperture, then follow the measuring guide for the correct deductions
• Match the spacer width printed inside the cavity
• Upgrade while you are at it — a low-E, argon-filled unit outperforms most units fitted ten years ago; compare options with the U-value calculator

Sized correctly, the swap takes minutes per window and restores both clarity and insulation in one visit.