New Window Fogs Only in Cold Weather: Seal vs Condensation Decision Tree

Why this matters

A newly installed window that fogs only in cold weather generates one of the most misdiagnosed callbacks in the trade, because two completely different problems look identical from across the room. Surface condensation on the inside of the glass is a room-humidity-and-temperature issue the window is correctly reporting, not a defect, and replacing the unit will not fix it. A failed insulated glass unit (IGU) seal, where moisture is trapped between the panes, is a warranty defect that requires a new sash or IGU. The cost and liability stakes are opposite: ordering a warranty replacement for surface condensation eats your margin and Andersen, Pella, or Marvin will reject the claim once they see the fog wipes off; while dismissing a real seal failure as "just humidity" leaves a defective unit in the wall and a customer who escalates. The diagnosis is fast and definitive once you know where the moisture sits and when it appears.

Symptom presentation

Fog or condensation forms on a cold day and clears when the weather warms or the room dries. The decisive observation is which surface holds the moisture. Condensation on the interior room-side glass that you can wipe with a finger is surface condensation, the warm humid indoor air meeting the cold inner pane and dropping below dew point. Moisture, droplets, or a permanent haze trapped between the two panes that you cannot reach or wipe is IGU seal failure. Note timing: surface condensation tracks indoor humidity and the coldest outdoor mornings, worst at the glass edges and bottom; seal-failure fog appears as the unit cycles cold and may show a faint film, mineral spotting, or visible droplets sealed inside. Note location: condensation concentrated low and at edges is a thermal and humidity pattern; fog uniformly trapped between panes regardless of which side you view is seal failure.

Quick checks

Wipe the inner glass. If the moisture comes off on a cloth, it is surface condensation, not a seal failure. View the fog from both the inside and outside; trapped IGU moisture is visible from both sides and cannot be wiped from either. Measure indoor relative humidity and compare to the outdoor temperature; high indoor RH on a cold day is the recipe for surface condensation, and a humidifier, new construction drying out, or a sealed-up tight house drives it. Check whether new heavy drapes, blinds, or a deep sill is blocking warm air from washing the glass, which locally chills the pane and triggers condensation. For suspected seal failure, look for a desiccant haze or spotting between panes and check the unit's date stamp against the IGU warranty.

Isolation tree

Branch on where the moisture sits. If it wipes off the interior pane, the branch is surface condensation, fix is reduce indoor humidity and improve air movement across the glass; this is not a window defect. If it is trapped between the panes and visible from both sides, the branch is IGU seal failure, fix is warranty replacement of the sash or glass unit. Within surface condensation, if RH is high from a humidifier or new-build drying, the branch is excess indoor moisture, fix is dehumidify and ventilate. If condensation is only behind blinds or a deep sill, the branch is blocked warm-air washing, fix is improve airflow and pull treatments off the glass. If it appears only at the very bottom edge, the branch is the colder edge-of-glass and frame area, normal on the coldest days at high RH. Within seal failure, if it is a recent install with trapped moisture, the branch is a defective or damaged IGU, fix is claim and replace.

Confirming diagnosis

Confirm surface condensation definitively with the wipe test: if a dry cloth clears the inner glass, the diagnosis is settled and no replacement is warranted. Corroborate by logging indoor RH and outdoor temperature; condensation that appears and clears with humidity and weather swings is the signature. Confirm IGU seal failure by viewing the trapped haze or droplets from both interior and exterior, the moisture is sealed inside and unreachable, and by noting it persists after the room is dried and warmed. A spacer-edge desiccant smear or a permanent internal film also confirms seal failure. For borderline cases, observe over a full cold-to-warm cycle: surface condensation comes and goes with conditions; trapped seal-failure moisture stays. Do not file a warranty claim until the wipe test and both-sides view rule out surface condensation, because the manufacturer will reverse the call.

Remediation

For surface condensation, the fix lives in the room, not the window. Lower indoor relative humidity (turn down or remove humidifiers, run bath and kitchen exhaust fans, ventilate, address new-construction moisture), improve warm-air movement across the glass, and keep heavy window treatments off the pane so room air washes it. Educate the customer that a tight, energy-efficient window reports indoor humidity by condensing first, this is physics, not a defect, and chronic high RH also risks mold and rot elsewhere. For confirmed IGU seal failure, document with photos from both sides and the unit date stamp, then file the manufacturer warranty claim for a replacement sash or IGU; most quality residential units carry a long or limited-lifetime glass warranty against seal failure. Do not attempt to drill, vent, or defog a sealed unit in place; replace the IGU. Verify the new unit holds clear through a cold cycle before closing the call.

References

• ASTM E2190 Standard Specification for Insulating Glass Unit Performance and Evaluation (IGU seal and dew-point performance)
• ASTM E546 Standard Test Method for Frost Point of Sealed Insulating Glass Units (detecting seal failure)
• AAMA / NFRC condensation resistance guidance (NFRC Condensation Resistance rating and surface condensation behavior)
• Andersen, Pella, and Marvin IGU limited warranty documents (seal-failure coverage terms by manufacturer)