Why Do My Windows Fog Up or Collect Condensation
About half an inch of dry air separates the two panes in a typical insulated glass unit, and a failed edge seal fills that gap with room air. Every fogged unit pulled from a house in Carlsbad, CA and logged by window installers carlsbad fish tommy shows the same thing, a broken bond at the spacer and a mineral film on the inner faces of the glass. Sorting that from ordinary condensation decides whether a window needs a replacement window or only a fan and a wiped sill.
Fog between the panes
Moisture trapped between the panes sits where no cloth reaches. Haze or mineral spotting that stays put after both sides are wiped means the seal around the spacer has failed and moist air has entered the double pane unit. Cleaning does nothing for it, because the film is on the inside faces of the glass. The fault is in the sealant, not the surface, and the haze comes back after every wipe.
Water on the room side
Droplets on the interior face form when the glass surface is colder than the dew point of the indoor air. Humid air off Agua Hedionda Lagoon and Batiquitos Lagoon, along with the marine layer, raises the chance of this on cool mornings. Water that wipes away and does not return by midday points to the conditions in the room, not to a defect in the glass.
Moisture made indoors
Cooking, showers, and laundry drying indoors all put water into the air, and that water lands on the coldest surface in the room. Exhaust fans that vent outside, run during the activity and for a while after, cut the load noticeably. Frames matter too. An aluminum window conducts cold through the frame and tends to sweat at the edges first, while a vinyl window usually stays warmer along the frame, though the glass itself can still fog on the surface.
Water on the outside
Condensation on the exterior face happens when the outer pane cools below the outdoor dew point, usually on a clear, still night near the coast. It burns off as the sun reaches the glass. This does not mean the window is defective, and no repair applies to it.
Sills, drywall, and repair
Water that collects on a sill again and again damages the paint first, then the wood, then the drywall below the window. Check the corners of the sill, the track of a slider, and the lower rail of a casement for soft spots or lifting paint. A failed sealed glass unit can sometimes be replaced on its own, leaving the frame in place, provided the frame is square and the wood or vinyl is sound. A new unit will not remove every trace of condensation on a cold, damp morning in Carlsbad, CA, since surface moisture depends on indoor humidity as much as on the glass.