Condensation Location Tells the Cause: Interior vs Between-Panes vs Exterior Single-Observable Decision Tree
Why this matters
Window condensation has exactly one fast, free diagnostic: which surface is wet. The pane has three surfaces a customer can fog up, interior, exterior, or the sealed cavity between panes, and each one points to a completely different cause and a completely different fix. A tech who learns to read the location in ten seconds never misdiagnoses a humidity problem as a glass defect or a normal exterior dew event as a warranty claim. This article is built around that single observable so the answer is decided before any tool comes out of the truck.
The financial stakes: between-pane fog is a glass replacement, interior fog is usually a humidity conversation, and exterior fog is frequently a sign the new high-performance glass is working as designed. Confusing them costs either a wrong part or a needless callback.
Symptom presentation
The customer reports a "foggy," "sweaty," "wet," or "cloudy" window. Your only job at intake and on arrival is to pin the location:
- Interior surface (room side): wipeable from inside, dries when you towel it.
- Between the panes (sealed cavity): cannot be wiped from either side; sits inside the glass sandwich, sometimes as a haze, a visible film, or droplets that come and go with temperature.
- Exterior surface (outdoor side): wipeable from outside, typically appears on cool clear mornings and burns off as the day warms.
Quick checks
- The wipe test (decisive). Wipe the interior. Clears? Interior surface condensation. Does not clear and the exterior is dry? Between the panes. Wet only on the outside and clears as the sun hits it? Exterior dew.
- Time-of-day and weather note. Exterior condensation appears overnight/early morning on clear, calm, humid nights and dissipates by mid-morning. Interior condensation appears in cold weather, worst at glass edges and corners. Between-pane fog tracks temperature swings and does not respond to wiping.
- Indoor RH read. A hygrometer; high winter RH drives interior condensation independent of window quality.
- Edge inspection. For suspected IGU failure, look for haze, mineral deposits, or a visible film at the spacer/edge of glass that does not clean off.
Isolation tree
- Between the panes (will not wipe off either side).
- Failed IGU seal. The hermetic seal has breached and the desiccant is spent. This is a glass-unit defect; humidity control and install corrections do nothing. Confirm and route to IGU replacement. Evaluate against the seal-durability criteria of ASTM E2190 and the unit's IGU warranty.
- Interior surface (wipes off inside).
- Surface condensation from indoor moisture meeting a cold surface. Read indoor RH. High RH in cold weather, condensation concentrated at glass edges, corners, and frame: room-humidity problem. The window is not defective; the dew point of the indoor air exceeds the glass surface temperature.
- If RH is moderate but the frame or glass surface runs unusually cold, evaluate the unit's U-factor and condensation resistance against what was specified.
- Exterior surface (wipes off outside, morning dew pattern).
- Exterior condensation on the outdoor pane is normal behavior for energy-efficient low-E IGUs: the outer glass stays cool because the coating keeps interior heat in, so outdoor humidity dews on it like grass on a clear night. This is evidence the glass is performing, not failing. Reassure and document; no repair.
Confirming diagnosis
- Between-panes confirmed when moisture is inside the sealed cavity and resists wiping from both sides, often with edge haze or a film. Per ASTM E2190, this is a documented seal failure mode and a warranty/replacement path.
- Interior confirmed when the moisture wipes off the room side, concentrates at the coldest points, and correlates with measured high indoor RH and cold weather. Reducing indoor RH or raising surface temperature eliminates it.
- Exterior confirmed when the moisture is on the outdoor face, appears on clear calm humid mornings, burns off with sun, and the customer has high-performance low-E glass. No defect.
Remediation
- Between panes: Replace the IGU or affected sash. Pursue under the glass/IGU warranty where the unit is within term; document the failure mode (between-pane moisture, non-wipeable).
- Interior: Manage indoor humidity (exhaust ventilation, source control, humidifier setpoint, air movement at the glass). Where the surface itself is too cold, evaluate a higher-performance glass package or interior storm. Set expectations against published U-factor and NFRC condensation resistance.
- Exterior: Educate the customer that exterior dew on low-E glass is normal and transient. It typically clears within hours of sunrise and warmer/breezier conditions. No corrective work.
A field shortcut that ends most of these calls in under a minute: stand at the window and ask the customer to wipe the inside while you watch. The three outcomes map one-to-one to the three causes, and the customer sees the diagnosis happen rather than taking your word for it. If the inside clears and the glass re-fogs while you stand there, that is a live demonstration of the humidity branch and a natural lead-in to the ventilation conversation. If it will not clear and you can show the haze sits inside the spacer, the customer accepts the IGU-replacement path without an argument. And if the wetness is only on the outside on a clear morning, walking them through why their efficient glass behaves like a car windshield turns a complaint into a selling point. The wipe test is the whole diagnosis; everything after it is matching the observed surface to the remedy.
References
- ASTM E2190, Standard Specification for Insulating Glass Unit Performance and Evaluation (sealed-unit failure criteria).
- NFRC 500, Procedure for Determining Fenestration Product Condensation Resistance; NFRC 100 (U-factor).
- IGMA (Insulating Glass Manufacturers Alliance) technical bulletins on interior, exterior, and between-pane condensation.
- AAMA / WDMA / CSA 101/I.S.2/A440 (NAFS) for fenestration performance context.