Home Improvement

Condensation Between Double Pane Windows: Fix or Replace

Mist you cannot wipe off from either side is inside the sealed glass unit, and it means the seal has failed. The frame can usually stay. Only the glass unit needs replacing.

Condensation between double pane windows means the seal around the glass unit has failed. Damp air has got into the gap between the two panes, and nothing you wipe from either side will reach it. It is not dangerous, and the frame can almost always stay. What needs replacing is the sealed glass unit inside it, a job a glazier does in the existing frame. Before you call one, rule out the two kinds of condensation that are not a fault at all.

Key takeaways

Wipe the glass on the room side, then on the outside. Mist that survives both is inside the unit, and the seal has failed.
A failed seal is not a safety problem, but the window insulates less and the fogging usually gets worse.
The frame can usually stay. A new sealed glass unit fits into the existing frame.
Demisting services clear the view but do not repair the seal.
Check your installation paperwork first. A failed unit may still be under guarantee.

Inside, outside or between the panes

Three kinds of condensation look alike from across the room, and only one of them is a fault. A minute with a dry cloth tells them apart.

Where it is	How to tell	What it means	What to do
Room side of the glass	Wipes off on a cloth from inside	Damp indoor air meeting cold glass	Ventilate and lower indoor humidity
Outside of the glass	Wipes off from outside, clears as the day warms	The window is insulating well	Nothing
Between the panes	Survives wiping on both sides	The seal on the glass unit has failed	Replace the sealed unit

Anglian, which makes and fits windows, is plain about the third row. Condensation between the panes means the seal has a leak and the gas cavity is no longer sealed. The unit has to be replaced.

Look at the window at different times of day before you decide. A seal that has only just failed may fog only when sun warms the glass and stay clear the rest of the time.

Why the seal fails

A double pane unit is two sheets of glass held apart by a spacer bar around the edge. The gap is sealed and usually filled with argon or dry air. The spacer contains a desiccant, a drying agent that soaks up the small amount of moisture that gets in over the years.

InterNACHI's guidance for home inspectors puts most failures down to what it calls thermal pumping. Sun heats the gas between the panes during the day and it expands. At night it cools and contracts. That daily push and pull stresses the seal until small fractures form, and air starts moving in and out. Once the desiccant is saturated, the moisture in that air condenses on the inside of the glass.

Some windows fail sooner than others:

Windows that face the sun go through bigger temperature swings every day.
Vinyl frames expand and contract more than other materials, which InterNACHI links to more seal failures.
Units from a faulty production run can fail together, years ahead of the rest. Several windows fogging within the same season is a sign of this.

Does a foggy window matter?

Not urgently. The window still keeps out rain and wind, and it is no less secure. The problems build slowly.

It insulates less. The argon that made it more efficient leaks out as damp air leaks in. Even a demisted unit, InterNACHI notes, may not insulate any better than single glazing.
The view gets worse. Fogging that comes and goes at first usually becomes more frequent.
The haze can become permanent. Repeated condensing and drying can leave a cloudy film on the inner faces of the glass. Nobody can reach that to clean it, so it stays even if the window is later demisted.

So there is no need to rush. Equally, the fault will not clear on its own, and a unit that goes on fogging for years ends up cloudy for good.

Condensation between double pane windows: fix or replace

Option	What it involves	What it fixes	When it makes sense
Replace the sealed glass unit	A glazier removes the old unit and fits a new one in the same frame	The seal, the insulation and the view	Almost always: the frame is sound, only the glass has failed
Replace the sash or the whole window	A new opening section or a new frame	Everything	Only when the frame or sash is rotten, warped or broken
Demisting service	Holes drilled in the glass, cleaning fluid in, moisture out, a small vent fitted	The view, mostly	A unit you cannot easily replace, accepting that the seal stays broken
Live with it	Nothing	Nothing	A window you plan to replace soon anyway

Replacing the sealed unit is the usual answer. Industry figures reported by InterNACHI put a glass-only replacement at about three cases in four. Only a small share of failures need a whole new window. Replacing the unit gets back everything the window had, without touching the frame, the sill or the wall around it.

Demisting is a cosmetic fix. The service drills a small hole, usually from outside, and sprays in a cleaning solution. It draws the moisture out, then fits a vent so damp can escape as the unit breathes. InterNACHI notes that removing the water does not repair the seal itself. The gas is not replaced, so the insulation does not come back. It can make a fogged window look clear, and it may cut the risk of damp reaching the frame. Whether that is worth the price depends on how expensive the unit would be to replace.

Before you call a glazier

Check every window, not just one. Wipe each one on both sides. A glazier visits once whether they replace one unit or four, so finding them all now saves a second call-out. It is a ten-minute job to add to the fall home maintenance checklist.
Find the paperwork. Double glazing usually comes with a guarantee. Anglian's advice is to contact the company that installed the windows if they are still covered. The guarantee may cover the glass unit but not the labour to fit it, so read what it says.
Look at the spacer bar. The strip between the panes around the edge often carries the unit maker's name or a code. A photograph of it helps an installer or a glazier trace the unit.
Note any etched mark in a corner of the glass. It usually means safety glass, such as toughened or laminated. A replacement in a door or near the floor has to be safety glass too.
Check the frame while you are there. Press any wood around the glass for soft spots. Look for water collecting at the bottom of the frame. Anglian lists blocked drainage channels and failed sealant around the frame as separate causes of leaks. Either one changes the job.
Get the glazier to measure. They measure the unit's height, width and overall thickness before ordering. A new unit is made to size and has to fit the same beads. Your own measurements are only useful for comparing quotes.

Mistakes that cost money

Replacing the whole window for a fogged unit. It costs more and fixes nothing a new unit would not, unless the frame itself is failing.
Drilling the glass yourself. A hole in a sealed unit with no vent fitted lets more damp in. Drilling toughened glass shatters it.
Treating room-side condensation as a failed seal. If it wipes off from inside, the window is fine. The cause is damp indoor air, and a new unit will mist up just the same.
Waiting years. The fogging rarely stays where it is, and the cloudy film it leaves on the inner faces cannot be cleaned off later.
Ignoring the guarantee date. Many failures happen within the guarantee period, and the cover runs out whether you claim or not.

If the condensation is on the room side

Mist that wipes off from inside is not a window fault. It is moisture from cooking, showers, drying clothes and breathing landing on the coldest surface in the room. Anglian's advice is to improve ventilation and consider a dehumidifier, and stopping condensation on windows every morning gives the humidity to aim for and a two-minute routine. Left alone, the damp settles into the corners of the frame and the wall below. That is often where a musty smell in the house starts.

A window that feels cold around its edges as well as misting up is usually letting air through too. Sealing a drafty window deals with the frame and the sash. Stopping drafts is also one of the cheapest ways to lower the heating bill without turning down the thermostat.

Frequently asked questions

Can you fix condensation between double pane windows yourself?

Not in a way that lasts. The moisture is inside a sealed unit, and the seal is what has failed. Demisting services drill the glass, dry it and fit a vent, which clears the view but does not restore the seal. The lasting fix is a new sealed glass unit fitted into the existing frame by a glazier.

Is a foggy double glazed window a problem?

It is not dangerous, and the window still keeps out rain and wind. It does insulate less, because the gas between the panes has leaked out and damp air has leaked in. The fogging usually gets worse over time and can leave a cloudy film on the inner faces of the glass that nobody can reach to clean.

Do I need a new window if the glass is foggy?

Usually not. Only the sealed glass unit has failed, and a glazier can replace it in the existing frame. InterNACHI, the home inspectors' association, reports industry figures putting a glass-only replacement at about three cases in four. A whole window is only needed when the frame or sash is damaged too.

Why is there condensation on the outside of my windows?

Because the glass is insulating well. On a cold, clear morning the outer pane stays cold, since little heat from the room reaches it, and moisture in the outdoor air settles on it. Anglian, which makes windows, says it is not a fault and usually clears as the day warms up.