What’s Causing This?

Failed sealed unit (most common). Every double-glazed pane is a factory-sealed unit containing dry air or inert gas between two panes. When the perimeter seal deteriorates — through age, UV exposure, or poor original installation — moist air infiltrates the cavity and condenses on the inner glass surfaces. This is by far the most frequent cause and means the unit itself needs replacing. See our guide on fixing condensation in double glazing for full repair steps.

Exhausted desiccant strip. Inside the spacer bar around the perimeter of the unit sits a desiccant material that absorbs any residual moisture. Once it becomes saturated — usually after many years — it can no longer prevent misting. This happens earlier in units that had a minor seal breach, allowing a slow ingress of damp air over time.

Physical damage to the frame or glazing bead. A cracked frame, damaged glazing bead, or perished edge seal around the glass can allow water to track into the rebate and then into the unit cavity. Check the outer perimeter of the frame carefully. Replacing the window seal may resolve early-stage ingress before the unit itself fails.

Thermal stress cracking. Micro-cracks in the glass or edge seal can develop when a unit is subjected to repeated temperature swings — more common in south- or west-facing windows. These cracks are often invisible to the naked eye but allow slow moisture ingress.

Poor original installation. If the window was not correctly seated, if the wrong type of glazing tape was used, or if the unit was installed without adequate expansion tolerance, premature seal failure is likely. This is particularly relevant in newer properties where condensation appears within a few years of installation.

How to Diagnose the Exact Cause

1. Confirm it is between the panes. Touch the inner surface of the glass on both sides. If the misting disappears or moves when you wipe it, condensation is on the room-side surface only — caused by high indoor humidity, not a seal failure. Condensation that cannot be wiped from either side is trapped inside the unit.
2. Check the frame and glazing bead. Inspect the perimeter of the glass on both the interior and exterior. Look for cracked or missing glazing bead, perished rubber gaskets, or gaps where sealant has pulled away from the frame. Run a finger along the silicone line where the frame meets the wall — any softness or gaps can allow water to track in.
3. Look at the spacer bar. On the edge of the unit (viewed from inside the room at an angle) you should see the metal or warm-edge spacer bar. White powdery deposits or visible moisture around this bar indicate the desiccant is exhausted and the seal has broken.
4. Check the age of the unit. Sealed units typically last 15–25 years before seal failure becomes likely. If your windows are within this age range and showing misting, unit replacement is almost certainly the correct course. If they are newer, suspect installation fault or physical damage.
5. Inspect for frame damage. On timber frames, look for paint failure, soft wood, or rot around the rebate — particularly at the bottom corners where water pools. On uPVC frames, check for cracks at the corners and ensure drainage slots at the bottom of the frame are clear and unblocked.
6. Note the location of misting. Condensation that fills the whole pane suggests total seal failure. Misting only in corners can indicate a localised seal breach, often caused by frame movement or impact damage at that point.

How to Fix It

Failed sealed unit: The glass unit must be replaced. The frame itself is usually reusable. You need to measure the existing unit accurately — follow our guide on how to measure for replacement windows to get the dimensions right before ordering. A replacement unit is then fitted into the existing frame using new glazing tape and bead. Full steps are in our fix condensation in double glazing guide.

Perished or damaged window seal: If caught early — before moisture has entered the unit cavity — resealing the outer perimeter can stop further ingress. Remove the old silicone, clean back to bare material, and apply a fresh bead of frame sealant. See our guide on replacing a window seal for step-by-step instructions. Note: this will not clear existing condensation already trapped inside the unit.

Frame damage on timber windows: Repair or replace any rotten or damaged frame sections before fitting a new unit. A new sealed unit installed into a compromised frame will fail prematurely. If the frame damage is extensive, consult our repair a wooden window frame guide first.

Blocked drainage slots (uPVC frames): Clear any debris from the drainage slots at the base of the frame using a thin tool or airline. This prevents water pooling against the sealed unit edge — a common cause of premature seal failure in uPVC windows.

When to Call a Tradesman

Replacing a sealed glass unit in an accessible ground-floor window is a manageable DIY task for a competent homeowner. However, call a registered glazier or FENSA-registered window installer when: the window is above ground floor and cannot be safely reached from inside; the frame itself is damaged or rotten to the point where it needs replacement rather than repair; the unit is unusually large, heavy, or awkwardly shaped (bay windows, roof lights, or arched frames); or if the window forms part of a fire escape route, where Building Regulations govern what glazing specification must be used. Any replacement that changes the window’s energy performance rating in a way that affects the building’s compliance may require a FENSA or CERTASS certificate — a registered installer will handle this automatically. Check the GOV.UK Building Regulations guidance if you are unsure whether your replacement requires notification.

How to Prevent It Happening Again

• Control indoor humidity. High indoor moisture accelerates seal degradation from the inside out. Use extractor fans in kitchens and bathrooms, avoid drying clothes indoors without ventilation, and consider a whole-house mechanical ventilation system if condensation is a persistent problem across multiple windows.
• Inspect frame seals annually. Each autumn, check the silicone or gasket seal around every glazed unit on both the interior and exterior. Re-seal any gaps or cracks immediately — a small tube of frame sealant used early costs far less than a new sealed unit.
• Keep frame drainage clear. On uPVC and aluminium frames, clear drainage slots at the base of the frame at least once a year. Blocked slots cause water to pool against the unit edge, degrading the seal from the outside.
• Specify quality units when replacing. When ordering replacement units, choose those with warm-edge spacer bars rather than traditional aluminium spacers. Warm-edge technology reduces the temperature differential at the edge of the glass, which is where seal failure most often begins.