You clean the window and it still looks dirty. A milky haze sits between the two panes, or little droplets show up on cool mornings and dry into white streaks. That's a failed seal. Dual-pane windows are two sheets of glass with a sealed gap between them, and once that seal lets go, damp air gets inside and the insulating gas leaks out. You can't wipe it off because it's on the inside. We see this constantly in homes around Sebastopol, where the damp west county air works on seals, and in the subdivisions of Rohnert Park where the first wave of dual-pane windows went in decades ago. The good news is the frame is often fine. Only the glass needs replacing.
Below we cover how to tell a failed seal from other problems, how a sealed glass unit is built, how we swap the glass and when a full replacement makes more sense. If several windows have already fogged and the frames are aluminum, it's worth looking at vinyl window replacement too, since the cost gap narrows when it's half the house.
Signs the Seal Has Failed
The clearest sign is haze or condensation you can't reach, between the panes. It often starts faint, as a light fog that comes and goes with the weather, and gets worse until the glass looks permanently cloudy or stained. Rainbow-colored streaks and white mineral spots between the panes are later stages. Some people notice the window feels colder than its neighbors on a winter night, since the insulating gas is gone. Condensation on the inside face of the glass, the side you can touch, is a different issue: that's indoor humidity, and new glass won't fix it. A quick test is to wipe the inside surface. If the fog stays, the seal has failed.
What's Inside a Dual-Pane Glass Unit
The part we replace is called an insulated glass unit, or IGU. It's two panes of glass held apart by a spacer bar that runs around the edge. The spacer is filled with a drying agent that soaks up stray moisture, and the whole edge is sealed with layers of butyl and silicone. The gap usually holds argon gas, which insulates better than air, and one pane often carries a Low-E coating. Over years of heating and cooling, the seal flexes. Eventually it lets moisture in faster than the drying agent can handle, and fog appears. "Defogging" services that drill holes and dry the gap can clear the view for a while, but they don't restore the gas or the seal. A new IGU does.
How We Replace Foggy Glass
First we confirm the frame is worth keeping: no rot, no cracks, sashes that open and lock. Then we measure the glass precisely, note the thickness and any coatings or grids, and order a new unit built to match. Most take a week or two to arrive. On install day, we remove the stops or glazing beads that hold the old glass, cut the old unit free and lift it out. The opening gets cleaned, new glazing tape or setting blocks go in, and the new unit is set and secured. If your home is in a fire hazard zone, we can order the new unit with a tempered pane. Each window goes quickly, and nothing changes about the outside of your house except the glass is clear again.
Glass Only or a New Window: Which Applies
Glass-only replacement makes sense when the frame is in good shape and just one or a few windows have fogged. It costs a fraction of a full window, and you keep your existing frames and trim. It's the wrong choice when the frame is rotting or cracked, when the windows are single-pane aluminum with no room for a thicker dual-pane unit, or when most of the house is fogging at once, since that usually means the whole batch is reaching the end of its life. Check whether your windows are still under warranty too. Many manufacturers cover seal failure for years, and a free replacement unit beats paying for one.
If the frames turn out to be too far gone, our window replacement page explains the next step, from retrofit installs to full-frame jobs.
Common Questions
The swap itself is quick, often less than an hour for each window. The wait is for the glass: each sealed unit is built to your measurements, so plan on a week or two between the measuring visit and install day. We can do several windows in one visit, and nothing about your frames, trim or paint changes.
Often it is. Many window makers warranty the glass seal for a long stretch, sometimes for as long as you own the home. Look for a brand name or code etched in a corner of the glass or printed on the frame, and keep any paperwork from the original install. We can help identify the brand and check before you pay for new glass.
Yes, though less dramatically than a broken one. Once the seal fails, the argon gas that insulated the gap leaks out and is replaced by ordinary damp air, so the window lets more heat through in summer and loses more in winter. One fogged window won't change your bill much. A whole wall of them will.