Why does fog between my double-pane window come and go?

What intermittent fog between the panes actually means

That haze living inside your glass, where you can't wipe it off from either side, is a failed insulated glass unit. The two panes are bonded around the edge into one sealed sandwich, and that seal has broken.

Humid air is now getting into the gap between the panes. When the glass gets cold, that moisture condenses into visible fog. When the glass warms up, it evaporates and the fog clears.

That's why it looks intermittent. Nothing is healing. The moisture is just cycling between liquid and vapor with the temperature.

Here's the part people miss: seeing the fog even once means the seal is permanently compromised. There's no product or trick that reseals a factory IGU in place.

Why the seal failed in the first place

Almost every case comes down to a seal that reached the end of its life. Factory IGU seals are good for roughly 15 to 25 years, and heat and UV shorten that.

South- and west-facing windows take the worst beating. The glass expands and contracts every day, and the sealant flexes with it thousands of times a year until it cracks.

There's a desiccant strip built into the metal spacer bar between the panes. Its whole job is to soak up stray moisture so the unit stays clear.

Early on, that desiccant hides the problem. It absorbs the first moisture that sneaks in, so the fog seems to come and go, then vanish for a while. That's the desiccant filling up, not a repair.

Once the desiccant saturates, the fog shows up regularly and stays longer each cold cycle. That's your cue the unit is fully done.

Water sitting at the sash edge speeds all of this up. Clogged weep holes or a bad glazing pocket keep the seal wet, and wet seals fail years early.

What to do right now, in order

First, confirm you're actually looking at a sealed-unit failure and not just dirty or humid glass. The wipe-both-sides test settles it in ten seconds.

Next, take a photo on a cold morning when the fog is heaviest. You'll want that for a warranty claim or to show a glazier what you're dealing with.

Then hunt down the window's age and maker. Look for a sticker on the frame edge, a stamp on the spacer bar, or the specs in your home purchase paperwork.

Check for a warranty before you do anything else. A lot of mid- and upper-tier windows carry 10 to 20 year glass warranties, and drilling or defogging first will void it.

If you're going to replace the glass, measure carefully: the visible glass opening and the full sash dimensions. A glazier needs both.

While you're there, clear the exterior weep holes at the bottom of the frame so water drains out. This won't fix the fog, but it protects whatever you install next.

• Wipe both glass surfaces to confirm the fog is internal
• Photograph it cold, when it's most visible
• Find the age and manufacturer
• Check warranty status before touching the unit
• Measure glass opening and full sash
• Clear the weep holes

What this costs in 2026

That's the sealed glass sandwich, ordered to your measurements, plus glazing supplies.

Hiring a glazier to supply and install a replacement IGU typically runs $200–$900 for a common window. Bigger units, tempered glass, and hard-to-reach openings push that higher.

Full window replacement (new frame and all) is a different tier entirely, usually $500 to $1,200-plus per window installed depending on material and size. You'd only go there if the frame is shot.

Argon-filled Low-E replacement units cost a bit more than plain double-pane glass, but the efficiency gain is worth it for a window you'll keep for another 20 years.

Defogging services often quote $100 to $250 per window. It's cheaper up front, but you're left with no gas fill and a drilled seal, so the value's questionable for most people.

DIY or call a glazier

This is genuinely DIY-friendly for the right window. A ground-floor, standard-size sash with removable glazing beads is very doable in an afternoon.

The job is: pop the beads, pull the old unit, set the new IGU on fresh setting blocks, and re-bed and re-bead it. The new unit gets ordered to your exact measurements from a local glass shop.

Where DIY stops making sense: large picture windows, anything on a second floor, and glass that has to be tempered. Those are heavy, awkward, and unforgiving if you drop them.

Vinyl and aluminum frames sometimes need the sash removed and taken to the shop, which changes the difficulty. Wood frames with putty glazing are more fussy and lean toward pro work.

If the window's under warranty, skip DIY entirely. Let the manufacturer or their authorized installer handle it so you don't blow the coverage.

Can you wait, or does this need fixing now?

You can wait a season without any real risk. Foggy glass isn't a safety hazard on its own, and the window still keeps rain and wind out.

But this symptom is rated high for a reason. Moisture is actively working inside the unit, and the mineral haze it leaves on the glass etches in permanently over months.

Wait a couple of winters and you'll find the inner glass surfaces have a cloudy film that stays even after you replace the seal. At that point you're definitely buying new glass, not just weighing it.

There's also the frame to think about. If water is pooling at the sash edge, wood rots and composite swells while you put it off.

The honest answer: if it's a minor back-bedroom window, low priority. If it's your main living-room view or the frame is getting wet, handle it this season.

How we diagnose fog between the panes

We start by ruling out the simple stuff: surface condensation from indoor humidity, which wipes away, versus internal fog, which doesn't.

Then we look at pattern and timing. Fog that appears on cold mornings and clears by afternoon is the classic signature of a breached seal cycling moisture.

Age and orientation tell us a lot. A 20-year-old west-facing unit fogging is textbook end-of-life; a 4-year-old unit fogging points to a defect or installation stress worth a warranty claim.

We check the sash and weep holes for standing water, because a wet glazing pocket both causes and accelerates seal failure.

From there it's a straightforward call between glass-unit-only replacement (frame is fine) and full window replacement (frame is compromised). Most cases land on glass-only.

The bottom line

Intermittent fog is a failed seal, full stop. It won't heal, and the desiccant that makes it look like it's clearing up is just running out of capacity.

The good news is the fix is usually the glass unit alone, not the whole window, and it's often DIY-able for a manageable-size opening.

Check your warranty first. Measure twice. And clear those weep holes so your next unit lasts its full 20 years.