Condensation Between Panes Only on North Windows After Install: Decision Tree

Why this matters

Condensation or fog trapped between the two panes of an insulating glass unit (IGU) is, by definition, a seal failure: the only moisture that can sit inside a sealed cavity got there because the perimeter seal lost its airtight integrity and the desiccant in the spacer saturated. When this appears only on north-facing windows shortly after an install, the orientation is a clue, not a coincidence. North glass runs the coldest because it gets no direct solar gain, so its interior glass surface drops below dew point first and a marginal or damaged IGU fogs there before the sun-warmed elevations ever do. The job is to confirm it is true interior-cavity fog (a seal/IGU problem) versus surface condensation (a humidity/indoor-air problem), because the fix and the warranty owner are completely different.

Symptom presentation

The homeowner reports cloudy, hazy, or foggy north windows that will not wipe clean from either the inside or the outside, often with a faint ring or streaking visible in raking light. It shows up on cold mornings, worse on the north and shaded elevations, and the south/west units stay clear. Because it followed a recent install, the homeowner suspects defective glass; sometimes they are right (shipping/handling damage to the IGU edge seal) and sometimes the fog is on a surface they have not wiped.

Quick checks

Settle the surface-versus-cavity question first, on site, in two minutes.

Wipe both accessible surfaces. Clean the interior face (surface 1) and the exterior face (surface 4) of the suspect pane. If the haze wipes away from either surface, it is surface condensation or a film, not a seal failure. Surface-1 fog on a cold north window is an indoor-humidity problem; surface-4 (exterior) dew on a high-performance low-E unit on a clear cold morning is actually normal and a sign the glass is insulating well.

If it will not wipe from either side, it is between the panes. That is a cavity-side seal failure by definition; nothing you can clean reaches surface 2 or 3.

Read the indoor humidity. A hygrometer reading well above roughly 50 to 60 percent RH in heating season, combined with surface-1 fog that wipes clean, points to the home, not the glass.

Isolation tree

Branch on where the moisture lives.

Wipes clean, surface 1 (interior). Indoor humidity is condensing on the coldest glass. North/shaded units fog first because they are coldest. This is not an IGU defect. Cause is high indoor RH (new construction drying out, humidifier, unvented combustion, cooking/bathing without exhaust) meeting a cold surface. The replacement window did not break; it is doing its job and revealing a moisture load.

Wipes clean, surface 4 (exterior). Transient exterior dew on an efficient low-E IGU on a radiant-cooling clear night. Self-resolves as the sun comes up. Normal, document and educate.

Does not wipe, between panes. Seal failure of that IGU. Now sub-branch on whether the failure is install-related or unit-related. Inspect the IGU edge for handling damage: a chipped corner, a cracked edge seal, or a glazing bead that pinched the unit. Check the setting blocks and that the sash drainage/weeps are open; a unit sitting in standing water in the glazing pocket fails its edge seal early. Pattern across the house matters: a single fogged unit suggests a damaged or marginal individual IGU; multiple north units fogging together early can indicate a shipping/handling batch issue or a glazing-pocket-drainage problem the install introduced.

Confirming diagnosis

Confirm cavity fog and assign the cause.

Cavity fog confirmation: the haze persists after cleaning both exposed surfaces, and it changes with temperature (heavier when the glass is coldest, lightening as the glass warms). A persistent ring of mineral residue at the glass edge after repeated thaw cycles is a classic late-stage seal-failure marker.

Install-induced vs unit defect: open the stops/glazing bead on the suspect sash and inspect the glazing pocket. Standing water, blocked weeps, missing or wrong setting blocks, or visible edge damage point to handling/installation. Clean, drained, undamaged pockets with a fogged unit point to a defective IGU.

Indoor-humidity confirmation (for the wipes-clean branch): track interior RH over a day. If lowering humidity (running bath/kitchen exhaust, reducing humidifier output) clears the surface-1 fog, the home is the cause.

Remediation

Between-panes seal failure: the IGU is not field-repairable. The fix is replacement of the sealed glass unit (or the sash, depending on the product line) under the glass/seal warranty. Most manufacturers warrant IGU seal failure for a defined term (commonly 10 to 20 years on residential lines); document the install date, capture photos of the fog and any edge damage, and file the warranty claim. If the failure traces to a glazing-pocket drainage problem the install introduced, correct the weeps and setting blocks when reglazing so the replacement does not fail the same way.

Surface-1 (interior) condensation: this is a humidity remediation, not a glass replacement. Recommend running kitchen and bath exhaust fans, reducing or eliminating humidifier output, addressing unvented moisture sources, and improving air circulation across the cold glass. Set expectations that north and shaded glass will always be the first to show condensation if indoor RH is high.

Surface-4 (exterior) dew: educate only; it indicates a well-insulating unit and clears on its own.

References

• ASTM E2190, Standard Specification for Insulating Glass Unit Performance and Evaluation. Defines IGU durability and the seal-performance basis behind seal-failure warranties.
• IGMA (Insulating Glass Manufacturers Alliance) technical guidance on IGU seal failure, desiccant saturation, and condensation diagnosis.
• NFRC ratings (U-factor, SHGC) for understanding why north/shaded glass runs coldest and condenses first.
• AAMA/WDMA/CSA 101/I.S.2/A440 (NAFS), North American Fenestration Standard, condensation-resistance context.
• Manufacturer glass/seal warranty terms and reglazing instructions (Andersen, Marvin, Pella, Milgard, Simonton): IGU replacement procedure and warranty filing.