Why Is There Fog Between the Panes of My Window

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Fog between the panes means the edge seal has failed. On a window in Carlsbad, CA, the running record double pane windows carlsbad in spiraling holds shows the same pattern again and again: a unit clear in the morning, hazy by afternoon, and a homeowner convinced the glass just needs a better cleaning. Wiping the inside face does nothing, because the moisture sits in the sealed gap between the two lites of the window.

What the Seal Does

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An insulated glass unit is two sheets of glass held apart by a metal or foam spacer, sealed around the perimeter. The spacer is filled with desiccant, a drying material that absorbs any moisture trapped during assembly or that creeps in over the years. That desiccant has a limit. Once the seal breaks and humid air keeps entering, the desiccant saturates and has nowhere left to put the water.

How the Failure Shows Up

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Early on, condensation forms on the inner glass surfaces when temperatures change, then clears. Along the coast the marine layer makes this obvious, since a cold, damp morning gives way to warm sun by noon. The fog comes and goes through the day. Left long enough, repeated condensation deposits minerals on the glass. That leaves a permanent cloudy film that cannot be wiped off from either side.

Insulation Loss

A failed seal can also mean the gas fill, such as argon, has escaped and been replaced with ordinary air. The unit still keeps out wind and rain, so this is not a safety issue and does not demand a same-week replacement. It is a loss of performance and clarity. A drilled and vented unit may clear up, but it does not return to its original insulating value.

Why Seals Break

Heat cycling from direct sun is hard on the edge seal, especially on west-facing glass near Carlsbad State Beach. Frame movement and plain age add stress. Water makes it worse. Standing water on a sill, or weep holes blocked with paint, dust or debris, keeps the bottom edge of the glass unit wet. A wet edge breaks down faster, which is why the lower corners usually fog first on a slider or double hung.

Repair Options

The sealed unit can often be replaced on its own, keeping the existing frame and sash if they are in good condition. That works on a vinyl window with square, unwarped sashes, or a sound wood window. It makes less sense when the frame is soft, bowed or leaking at the corners. Check the sash first, clear the weep holes, and measure the unit thickness and overall size before ordering. A replacement unit made to those figures drops in without touching the trim.