New Window Fogs Between Panes Within Three Months: Defective Seal vs. Install Damage vs. Condensation Decision Tree
Why this matters
A fogged window within the first few months of installation erodes customer confidence fast, and the wrong diagnosis can void a manufacturer warranty or leave a real installation defect unrepaired. There are three fundamentally different causes - factory seal failure, installer-induced seal failure, and ordinary exterior condensation misread by the customer - and each requires a different response. Getting this right before filing a warranty claim protects your relationship with the supplier and keeps callbacks from becoming chargebacks.
Step 1: Confirm the fog is actually between the panes
Before anything else, touch the interior glass surface in the fogged area.
- If the glass surface feels wet or you can wipe moisture off with a cloth, the condensation is on the interior glass face. This is normal in cold weather when indoor humidity is high. The IG unit is not failing. Skip to the Exterior vs. Interior Condensation section below.
- If the glass surface is dry and the fog cannot be wiped away, the moisture is trapped between the two panes. The IG unit seal has failed. Continue to Step 2.
A flashlight held at a low angle to the glass can help confirm: true inter-pane fog will look hazy or milky in a way that does not shift when you change your viewing angle, while surface condensation will show small droplets.
Step 2: Inspect the sash for twist or distortion
Install-induced seal failure is caused by stress on the sash. When a sash is shimmed unevenly or screws are over-driven into the frame, the sash can rack or twist. A twisted sash applies constant tension to the perimeter spacer, and the seal cracks progressively over weeks rather than all at once.
Check for distortion:
- Open the sash (for casement or awning units) and look at the sash corners under direct light. Racking shows as a visible gap or non-square corner at the sash frame joint.
- Measure the sash diagonals with a tape measure. More than 1/8 inch difference indicates distortion.
- Inspect the installation screws along the frame. Dimpled or countersunk frame material around any screw indicates over-driving.
- Look at the rough opening shimming from inside. Shims should contact the frame uniformly; a shim that drives hard at one corner and is absent at the opposite corner will produce the torque that breaks a seal.
If distortion is present, the cause is installation error, not a defective unit. Document with photos before any repair attempt.
Step 3: If no distortion is found, assess for factory defect
A seal that fails with no installer-induced stress and no handling damage is a manufacturing defect. Before concluding this, also check for:
- Shipping or handling damage to the spacer edge (dents, kinks, or deformed corners visible when you look at the glass edge).
- Any caulk or glazing compound applied directly onto the spacer during installation, which can chemically attack the seal.
- Chips or cracks at the glass corners, which propagate from the edge and compromise the seal bond.
If the spacer edge is clean, no distortion is present, and the fog appeared with no thermal stress event (hail, impact), treat this as a factory defect and proceed with a warranty claim.
Exterior vs. Interior Condensation (no defect)
When the customer has identified condensation on the outer glass surface as fog between the panes, a brief explanation resolves the concern without a service call:
- Exterior condensation on the outer pane happens in spring and fall when the outdoor air is more humid than the cold glass surface. It is a sign the window is insulating well - the outer pane is staying cold because little heat is passing through from the inside.
- Interior condensation on the inner pane means indoor humidity is too high relative to the glass surface temperature. The window is not defective; a dehumidifier or improved ventilation resolves it.
Neither condition requires a warranty action. If the customer cannot be certain which surface is fogged, ask them to run a fingernail across the fogged area and report whether they leave a track in the moisture.
Repair and warranty path summary
| Finding | Root cause | Action |
|---|---|---|
| Fog is between panes, sash is square, no handling damage | Factory seal defect | File warranty claim with manufacturer; replace IG unit under warranty |
| Fog is between panes, sash is distorted, shims or screw pattern is uneven | Installer-induced seal damage | Replace IG unit at installer cost; correct frame installation before installing replacement |
| Fog is between panes, handling damage visible at spacer edge | Shipping or site handling damage | File freight or handling claim if applicable; replace IG unit |
| Moisture wipes off interior glass surface | Interior condensation, high indoor humidity | Customer education; no window defect |
| Moisture on exterior glass surface only | Exterior condensation, window is well-insulated | Customer education; no window defect |
Do not file a manufacturer warranty claim when distortion is present. Most warranties explicitly exclude installer-caused seal failure, and submitting a distorted sash for warranty replacement can result in the supplier denying all future claims from your account or requiring an inspection fee. Document the distortion first and confirm your repair path internally before any supplier contact.
References
- AAMA 2101 (standard for voluntary specification for insulating glass units in windows)
- National Glass Association installer training guidelines for IG unit installation clearances
- Standard industry guidance on IG unit seal failure classification (factory vs. field)