Fogged windows: what a failed seal means and what it costs

A double pane window is two sheets of glass with a sealed air or argon gap between them, held apart by a spacer with desiccant in it. When the seal at the edge fails, humid air gets into the gap, the desiccant saturates, and moisture condenses on the inside surfaces where nobody can wipe it off.

How it shows up

Early on it is a faint haze that appears on cool mornings and clears by noon, so it is easy to miss. Later you get permanent cloudiness and a mineral film etched into the glass, sometimes with visible droplets running down the inside.

The giveaway is that cleaning both faces changes nothing. If you can wipe it away, it is surface condensation and that is a ventilation question, not a glass one.

Why it happens faster on some windows

Heat cycling kills seals. South and west facing glass takes the most sun, expands the most, and fails first. In a house where every window went in at the same time, the west elevation will be years ahead of the north.

Along the coast the seal failures come with salt deposition on the frames and, on aluminum frames, corrosion at the corners that opens the joint. Inland the driver is straight thermal load.

Units from the 1980s and early 90s used sealants that have mostly reached end of life. If a house has original glass from that era, we expect to find several.

What it costs to fix

You replace the insulated glass unit, not the whole window. A glazier measures the sash, orders the sealed unit, and swaps it, usually in the region of two to five hundred dollars per opening depending on size, coating and whether the sash has to come out. Tempered glass, required near doors, in bathrooms and low openings, costs more.

Whole window replacement only makes sense if the frame or the hardware is also finished, or if enough units have failed that the labor of doing them one at a time stops making sense.

On a purchase

Fogged units are a normal repair request and they are easy to negotiate because the cost is quantifiable. Ask for a glazier's quote rather than an allowance. We count and locate every failed unit in the report, and we note which openings need tempered glass, since that changes the price and some contractors quote it wrong.

One caution. On an overcast morning a lightly failed unit can look clear. We check window by window with a light held at an angle, but a seal that is marginal in December may be obvious in July.