Foggy windows between the panes: why it happens and why cleaning won't fix it

What's happening inside the glass

A double-pane window is two sheets of glass held apart by a metal or foam spacer and sealed around the edges. The gap is meant to be dry, and often holds a gas that slows heat travelling through. That seal is the whole point of the window.

Over the years, it gets worked. The glass expands and contracts with every sunny day and cold night, rain gets driven against the edge, and on the coast, salt air adds to the wear. Eventually the seal opens, damp air gets in and the dry gap isn't dry any more. When the air cools, the moisture condenses on the inner faces of the glass. You'll see haze, beads of water or a milky film, and you can't wipe it away because it isn't on any surface you can reach.

If the water is on the room side instead, that's a different problem. See our guide to condensation on windows to tell which you have.

Why defogging services are temporary

You'll find companies that offer to defog sealed units for less than a replacement. The usual method is to drill a small hole, dry out the gap and fit a vent or a plug. It can make the view clear again, and for a window you're selling, or a low-priority room, that might be reasonable.

It doesn't restore the seal. A unit that can breathe is a unit that can fog again, usually within a couple of seasons. The sealed gas that gave the window its insulating value is gone, so the window is no warmer than before you paid. And the repair doesn't touch the frame, the weatherstripping or the gap around the window. If your sills are cold and your frames tired, you've fixed the least of it.

The other cheaper option is a new sealed unit in the old frame. That can be good value if the frame is sound and the window is younger. On the 1970s to 90s aluminum and early vinyl windows we see across Vancouver Island, the frame has usually aged as much as the glass.

How to tell the age from the spacer stamp

Most sealed units are stamped with a manufacturer code and a date. It's on the metal or foam spacer, the thin strip between the two sheets of glass, along one edge. You'll need good light and sometimes a flashlight held at an angle, and it's easiest to see from the room side with the sash open or the blind raised.

• Look for a short line of letters and digits, sometimes a month and year, sometimes a code
• Photograph it with your phone, zoomed in, so you can read it later
• Send the photo when you book, or have it ready when we measure. We'll read it with you

This matters for two reasons. It tells you whether the unit might still be covered by the maker's warranty, and it tells you whether the rest of the window is the same age and about ready for the same fate.

What replacement involves

Here is how it goes with us, from start to finish.

• Measure: a free 45-minute visit. We check every window, not just the foggy one, and look at the frame, the sill and the trim for damage
• Quote: a written price, line by line, good for 30 days.
• Build: your windows are made to order by Thermoproof in Cobble Hill, the Island's longest-running locally owned window maker, since 1977
• Install: our crew removes the old unit, checks and insulates the opening, fits the new window, finishes the trim and hauls the old one away
• After: a 5-year installation warranty from us. We also confirm which products met the rebate requirements and give you the invoice details the claim needs../retrofit-vs-full-frame-window-replacement/">retrofit insert or a full-frame replacement. Many homeowners start with the foggy and drafty windows (see drafty windows) and do the rest in a later year. For a ballpark, use the online estimate. In Nanaimo and nearby, see the Nanaimo page.