Window Fogging Between Panes in Montgomery: Fix It?

Fog between your window panes means the seal on the insulated glass unit has failed. It cannot be repaired, but it usually does not require a new window either — replacing the glass unit inside the existing sash runs $150 to $400 when the frame is sound.

Before you do anything else, check the warranty. Insulated glass units frequently carry 10 to 20 year manufacturer coverage, and a surprising number of people pay for a replacement they were entitled to for free.

What actually failed

A double-pane insulated glass unit is two panes of glass separated by a spacer, sealed around the perimeter, with a desiccant packed inside the spacer to absorb any residual moisture. The cavity is usually filled with argon.

Every day that unit breathes. The sun heats it, the air inside expands and pushes on the seal; night comes, it contracts and pulls. Thousands of cycles a year. Eventually the sealant loses its grip somewhere along the perimeter.

Once that happens, humid outside air moves in and out with the cycle. The desiccant absorbs moisture until it is saturated, which is why a seal can fail eighteen months before you see anything. Then the water has nowhere to go, and it condenses on the inside faces of the glass. That is the haze, and the mineral deposits that etch into the glass over time are why an old failure often leaves a permanent ghost even after the water clears.

Why it happens sooner on some windows

Not random. The pattern is consistent.

• South and west exposures fail first. More heat cycling, more UV on the sealant.
• Bottom edges fail first, because that is where water sits in the frame channel if the weep holes are blocked.
• Large units fail earlier than small ones — more area, more pressure change per cycle.
• Blocked weep holes are a major cause. Those tiny slots at the bottom of the exterior frame drain water out. Painted over or packed with debris, the glass unit sits in standing water and the seal is attacked from below.
• Altitude changes on units shipped without capillary tubes.

That weep hole point is worth acting on. Take a look at the bottom exterior of your windows this weekend and clear them with a thin piece of wire. Five minutes, and it genuinely extends the life of the glass you have left.

Your four options, priced

Option	Cost	Honest verdict
Warranty claim	$0 to a service fee	Always check first
Glass-only (IGU) replacement	$150–$400 per unit	The right answer most of the time
Defogging service	$75–$200 per unit	Cosmetic only
Full window replacement	$400–$900 vinyl, more for other frames	Right when the frame is failing too

On defogging

The process: a small hole is drilled in the glass, the cavity is cleaned, sometimes a drying agent is applied, and a one-way vent is installed. The haze goes away and the view comes back.

What has not happened: the seal has not been restored, the argon is gone permanently, and the cavity is now deliberately vented to outside air. Thermal performance stays reduced. In humid weather the unit can fog again.

It is not a scam — it is a legitimate cosmetic service that costs less than glass replacement, and for a window you plan to replace in three years anyway it is a reasonable stopgap. Just buy it knowing what it is.

When to replace the whole window instead

Glass-only replacement is the default. Replace the entire unit when:

• The sash or frame has rot, or the sash no longer closes squarely
• The hardware is failing and parts are unavailable
• Four or more units in the house have fogged. Same age, same seal chemistry — you are on a conveyor belt, and paying $300 a pop as they fail one per year is worse than doing it properly
• They are single-glazed or old aluminum-framed and you were going to upgrade anyway
• The glass is obsolete enough that a replacement unit costs close to a new window

Finding the warranty

Worth twenty minutes before you spend anything.

1. Look for an etched code in a corner of the glass, or a small metal tag or sticker inside the spacer between the panes. Manufacturers stamp these with a code and often a date.
2. Check the frame channel. Open the sash and look at the top or side jamb for a label with a model and serial number.
3. Check your closing documents if the house is newer. Window warranties are frequently in the builder packet nobody has opened since move-in.
4. Call the manufacturer with the code. Some warranties are transferable to subsequent owners; some cover glass but not labor.

Even a warranty that covers the glass and not the labor turns a $350 job into a $120 job.

While you have someone out anyway

If you are paying for a glass replacement, this is the cheapest moment in the window's life to improve it. Going from clear glass to a proper low-E coating with argon costs modestly more on the unit and changes how the room feels — particularly on a west-facing window. Our energy-efficient windows guide explains which label numbers to specify.

You can also change the glass type at the same time: obscure glass for a bathroom, tempered where code requires it, laminated for a road-facing room where noise is the real complaint.

What you should not bother doing

Do not try to DIY a glass unit swap on a vinyl or clad sash. The glazing beads are unforgiving, the sealant matters, and a mis-set unit fails again in two years. The measurement alone has to be exact — glass shops build to your numbers and will not eat the error.

Do not replace all your windows because one fogged. One failure is one failure. Four is a pattern.

Do not ignore it indefinitely either. A vented, damp cavity degrades the low-E coating and etches the glass, so the longer you wait, the more likely a defog leaves you with a permanent haze.

Montgomery Window Replacement handles failed glass units and full replacements across Montgomery and the River Region — vinyl replacement windows, double-hung units, and energy-efficient upgrades — and we will tell you to call your manufacturer first if we think the warranty covers it. For the bigger decision, see our repair versus replace guide.

Get a free quote → · call (844) 511-2320