Why Are My Windows Foggy Between the Panes?

It means the seal has failed

Modern double- and triple-pane windows have a gas-filled space between panes of glass, sealed at the edges and containing a desiccant to keep the space dry. When that seal fails, outside air and moisture get in, and you see condensation or a permanent haze trapped between the panes — not on the inside or outside surface you can wipe.

How to confirm that's what you're seeing

Try wiping both the interior and exterior surfaces of the glass. If the haze or moisture doesn't clear because it's actually between the layers, the seal has failed.

Unfortunately, this isn't repairable

Once the seal fails, there's no way to re-seal or dry out the space between the panes. The only real fix is replacing the glass unit (the sash or the insulated glass unit itself), not necessarily the entire window frame, depending on the window type.

Does it need to happen right away?

Cosmetically, no — a foggy window still functions as a window. Functionally, a failed seal means you've lost the insulating gas fill between the panes, so that window is now less energy-efficient than it was. Whether to prioritize replacement depends on how many windows are affected and your energy costs.

Preventing it in newer windows

Seal failure is mostly a function of age and manufacturing quality rather than something homeowners cause, but extreme temperature swings and physical stress on the frame (settling, impacts) can accelerate it.