Why Windows Fog Up Between the Panes, and Why Cleaning Never Works

You have cleaned it twice. You have cleaned it from the outside. The haze is still there, and in certain light it looks like a film that will not come off.

It is not dirt, and you are not going to reach it. Here is what is actually happening inside that window.

What a Modern Window Is Made Of

The glass in your window is not one sheet. It is a sealed unit: two panes of glass, sometimes three, held apart by a spacer around the edge, with the gap filled with gas and the whole perimeter sealed shut.

The glass is not what insulates. The trapped gas layer is. The glass is there to hold the gas still.

Along the edge of that unit sits a sealant, and inside the spacer sits a desiccant, a drying agent whose job is to absorb the small amount of moisture sealed in at the factory so the inside surfaces stay clear.

What Fails

Over years of expansion and contraction, every day, summer and winter, the edge seal eventually gives way. Once it does, two things happen at once.

The gas escapes, so the unit stops insulating the way it was built to. And outside air starts moving in and out, carrying moisture. The desiccant absorbs what it can until it is saturated, and after that the moisture has nowhere to go. It condenses on the inner surfaces of the glass, where nothing can reach it.

That is the haze. It is on the inside faces of two panes that are sealed together, which is why no amount of cleaning will touch it, and why it cannot be dried out.

What Makes It Happen Sooner

Direct sun. South and west facing windows cook all afternoon and fail earlier than north facing ones in the same house.

Big temperature swings. Ontario is hard on sealed units. A thirty degree summer day and a minus twenty winter night work the seal in both directions every year.

Age. Sealed units have a service life. Most windows in a GTA home were installed at the same time, which is why they tend to start fogging within a few years of each other.

Installation. A unit that was handled badly or installed under stress fails early.

Why It Costs You More Than Appearance

People tend to treat fogging as a cosmetic problem and live with it for years. It is not cosmetic.

A failed sealed unit has lost the gas layer that did the insulating. That window is now performing closer to a single sheet of glass than to the double pane you paid for. In January, that is heat leaving the house through the window all day and all night.

It is also the reason that room always feels colder than the rest of the house, and the reason there is sometimes condensation on the room side of that window in winter when no other window in the house has it.

What the Repair Actually Involves

This is where most people are relieved. The frame is not the problem. The sash is not the problem. The hardware is not the problem.

The sealed unit is removed from the existing frame and a new one goes in. Your window stays where it is. From inside the room, nothing changes except that you can see through it again.

The new unit has to be made to the exact dimensions of the old one, so there is an ordering period between the measurement and the install. That is normal and it applies to every company.

When Replacing the Glass Is Not the Answer

We would rather tell you this up front than on site.

Replacing just the glass is the wrong call if the frame is rotted, warped, or no longer square; if the window is a very old single pane with no sealed unit to replace; if you are already planning to renovate that wall; or if you want a different window altogether, a different size or a different opening style.

In those cases you are better off replacing the window, and glass alone would be money spent twice.

How to Check Your Own House This Week

Walk the house on a bright day and look at each window at an angle rather than straight on. Seal failure shows up far more clearly off-angle, and in its early stages it looks like a faint smear near one corner rather than full fog.

Finding it early does not change the repair. It does mean you stop paying to heat the outdoors sooner.

If you want to understand which glass spec to put back in, see our guide on double, triple, Low-E and argon. For the winter picture of this problem, see our guide on window condensation in winter.

Send us a photo on WhatsApp and we will tell you whether you are looking at a failed seal. Call 437-344-8490.