Why is condensation between my double-pane window getting worse?

What worsening condensation on a double-pane window actually means

Your window has two panes of glass with a sealed air or gas gap between them. That gap is kept bone-dry by a desiccant hidden in the metal spacer around the edge.

When the edge seal fails, humid outside air leaks into that gap. The desiccant soaks it up for a while, then gives up. After that, you get fog, haze, and sometimes water droplets or mineral streaks between the panes.

The reason it keeps getting worse is simple: once the seal is broken, moisture keeps coming in and has nowhere to go. There's no fixing that from the outside.

Here's the first thing to nail down. If you can wipe the moisture off with a rag, it's on the room-side surface, and that's a humidity issue, not a dead window. If your rag does nothing and the fog stays put, it's between the panes and the unit has failed.

That single check decides whether you're looking at an $80 dehumidifier fix or a $300-plus glass replacement.

Why your double-pane window is fogging

The overwhelming cause of fog you can't wipe is a failed IGU seal. Everything else is either a variation on that or a case of mistaken identity (surface humidity pretending to be seal failure).

Seals fail for a few reasons. Age is the big one. A 20-year-old window has been through thousands of hot-cold cycles, and the seal material eventually cracks.

Heat is the accelerant. Windows on the west and south sides bake every afternoon, so they usually fog years before the shaded ones on the north side.

Then there's early failure. Some units die in under 10 years from a weak factory seal or bad spacer bonding. If yours is young and fogging, check the warranty before you pay for anything.

If the fog is actually on the inside surface, you're dealing with indoor humidity. Tight modern homes trap moisture from showers, cooking, and even breathing. Cold glass just gives that moisture a place to land.

What to do right now

Start with the wipe test so you know which problem you actually have. Don't skip this. It's the difference between a cheap fix and a real repair.

If it's surface humidity, run your bath and kitchen exhaust fans longer, aim for indoor humidity between 30 and 40 percent, and move moisture sources (plants, drying laundry) away from the glass.

If it's a failed seal, find the manufacturer stamp. It's often etched in a bottom corner of the glass or printed on the metal spacer bar. That plus your purchase date tells you if a warranty applies.

Measure the sash opening and note the overall glass thickness. Most residential units run 1/2 to 1 inch thick total. A glass shop needs those numbers to quote an IGU.

Get two quotes: one for replacing just the sealed glass unit, one for a full window replacement. The gap between them usually decides which way you go.

While you're at it, check the sill and the wall directly below the window for soft spots, dark stains, or peeling paint. A long-failed seal can leak water into the frame.

• Wipe test to confirm interior fog vs surface humidity
• Locate manufacturer stamp and purchase date
• Measure sash opening and glass thickness
• Quote IGU-only replacement
• Quote full window replacement
• Inspect sill and wall below for water damage

2026 cost breakdown

That's the fix when the fog is on the surface, not in the glass.

Actual seal failure is a pro job. Replacing just the insulated glass unit typically runs $200–$900 per standard-size window installed, which lines up with the pro range for this repair. Bigger, tempered, or gas-filled units cost more.

Full window replacement is a different tier. A standard vinyl double-pane, installed, usually lands somewhere around $500 to $1,000 per window in 2026, and specialty or large units climb from there.

IGU-only replacement makes sense when the frame and sash are solid. You keep your existing window and just swap the glass, which is why it's the cheaper route.

Full replacement wins when the frame's rotting, the window's past 20 years, or you're redoing a whole wall and want matching new units with better efficiency.

DIY vs calling a pro

The humidity side is fully DIY. Ventilation, a dehumidifier, and better exhaust fan habits will clear surface condensation without anyone touching the glass.

The seal-failure side is not a DIY glass job. Cutting a sealed unit to size, matching thickness, and glazing it back in without breaking it or wrecking the frame takes the right tools and experience.

You can absolutely DIY the diagnosis and prep, though. Confirming the fog is between the panes, finding the warranty stamp, and measuring the opening saves you a service call and speeds up the quote.

Skip the DIY defogging kits. They drill the glass, dry it out, and add a vent hole, which clears the fog but kills the sealed gas fill for good. You trade a foggy insulated window for a clear uninsulated one.

Can this wait?

A fogged pane isn't going to hurt anyone tonight, so it's not a drop-everything emergency in the safety sense. But it won't heal, and it's costing you.

Every day the seal stays broken, you're losing insulation and letting moisture sit against the frame. In older wood windows, that trapped water leads to rot you'll pay far more to fix than the glass itself.

The realistic timeline is to fix it within the season. If the sill below is already soft or stained, move faster, because water is already getting where it shouldn't.

If the glass is also cracked (not just fogged), treat that as urgent, especially on upper floors where a failing pane is a real injury risk.

How your home's age changes the picture

Homes built or re-windowed in the early 2000s are hitting the tail end of their original seal life right about now. If yours is from that era, one fogged window is often the first of several.

Very old homes with original single-pane glass don't get inter-pane fog at all; if you're seeing it, someone installed newer double-panes at some point. Track down that install date for the warranty.

Newer tight-built homes (post-2015) can show heavy surface condensation even with perfectly good seals, because they hold indoor humidity so well. That's a ventilation fix, not a glass fix.

If you're in a house where windows were replaced piecemeal over the years, expect them to fail on different timelines. Budget for them the same way, one wall at a time.

Why it looks worse at certain times of year

Cold weather makes both types of condensation more obvious. The outer pane gets cold, and any moisture (trapped or surface) shows up as heavier fog.

Late summer is hard on the seals themselves. Peak afternoon sun on west-facing glass drives the temperature swings that crack edge seals over time.

Spring and fall aren't off the hook either. The big daily temperature swings in shoulder seasons flex the glass and spacer, and that repeated flexing is what eventually fatigues the seal.

If your fog appears and disappears with the weather, lean toward surface humidity. If it's there rain or shine, day and night, the seal is gone.

How we diagnose this

We start every fogged-window call the same way: figure out which surface the moisture is on. Between the panes means seal failure; a touchable surface means humidity.

Then we look at pattern and history. Which windows fogged first, their exposure, the home's age, and whether a warranty is in play all shape the fix.

We weigh IGU-only replacement against full window replacement based on frame condition and cost. A solid frame almost always favors glass-only.

Our cost ranges reflect real 2026 US pricing for standard residential units, installed. Oversized, tempered, and gas-filled glass runs higher, and warranty coverage can cut your bill to labor only.

The honest bottom line: fog between the panes can't be cleaned, and it only gets worse. The question isn't whether to fix it, just glass-only or whole window.