Condensation or Fog Between Panes: Sealed Unit Failure vs. Dirty Glass Explanation Tree

Why this matters

A customer who sees fog on a window assumes the fix is cleaning. If a technician cleans the glass and charges for it without explaining that the fog is trapped inside the unit and will be back in an hour, the customer will call to dispute the invoice and lose trust in the company. The correct call at the door is to confirm where the condensation is, explain the cause in plain language, and document what was and was not in scope. This tree gives the technician a repeatable method to make that call in under two minutes on-site.

Step 1 - confirm where the moisture is

Before any other diagnosis, confirm the fog is between the panes and not on a surface that can be cleaned.

Test procedure:

• Press a dry microfiber cloth firmly against the interior face of the glass for 10 seconds. If the cloth picks up moisture or the fog pattern shifts and clears on the interior surface, the moisture is on the interior face. Clean as normal.
• Wipe the interior face dry and look again. If the fog pattern is unchanged after wiping the interior surface clean, the moisture is inside the cavity and the unit has failed.
• Repeat on the exterior face if you are uncertain whether the source is exterior condensation (dew, overnight humidity) versus interior seal failure.

What between-pane fog looks like:

• A hazy, milky, or oily film that does not move when you wipe the interior or exterior surface.
• Moisture beads or droplets that are visible in the cavity but cannot be felt on either face.
• A fog pattern that is worse in the morning and slightly better in the afternoon when the sun warms the unit and partially re-evaporates the trapped moisture - this thermal cycling is a classic sign of a failed IGU.

Step 2 - confirm the cause to the customer

A double-pane window is an insulating glass unit (IGU). The two panes are bonded at the perimeter with a spacer bar filled with desiccant, then sealed with a primary seal (polyisobutylene, PIB) and a secondary structural seal (silicone or polysulfide). The desiccant absorbs any moisture trapped inside the cavity during manufacturing. Over time, the perimeter seal can fail from UV exposure, thermal cycling, or frame movement. Once the seal fails, warm moist air from outside the unit infiltrates the cavity, the desiccant becomes saturated, and moisture condenses on the inner face of whichever pane is cooler.

Explain to the customer in plain terms:

• The fog is inside the unit, not on the glass surface.
• No cleaning tool can reach it. The two panes are bonded; there is no way to clean between them without destroying the unit.
• The fix is replacement: either the IGU itself (if the frame is in good condition and the IGU is separately replaceable) or the full sash or window unit if the IGU is integral to the sash.
• Cleaning the exterior face of that pane will make the glass around the fog look cleaner, which may make the fog patch more visible by contrast, not less. Be clear with the customer about this before cleaning so they are not surprised.

Step 3 - scope the cleaning visit

Even when an IGU has failed, the rest of the windows on the job are still in scope. Do not abandon the visit because one unit is uncleanable between panes.

For the failed unit:

• Clean the interior and exterior faces as normal. The surfaces you can reach are still dirty and the customer paid to have them cleaned.
• Write a note on the invoice that the fog between the panes was found on [location/description of window] and is outside cleaning scope. Recommend the customer contact a window company for IGU or sash replacement.
• Do not charge a separate diagnostic fee unless your pricing structure includes one. The identification takes two minutes and is part of professional service.

For the rest of the visit: proceed normally.

Why customers confuse this with dirty glass

The confusion is understandable. Fog on the outside of a window in the morning looks exactly like condensation that will wipe off, and it usually will. Customers extend that mental model to fog that is persistent and does not respond to weather changes. The other driver is that window cleaning is often triggered by visible haze on windows - and a failed IGU produces exactly that haze. The technician is often the first person to look at the window carefully enough to confirm where the haze sits.

A brief, non-technical explanation at the door - "the fog is sealed inside the glass, no cleaner can reach it, you need a glazier to replace the pane" - resolves the customer's question and prevents a dispute at invoice time.

References

• ASTM E2188 (Standard Test Method for Insulating Glass Unit Performance) - covers IGU seal performance requirements.
• IGMA (Insulating Glass Manufacturers Alliance) TB-3001, Guide to Understanding Sealed Insulating Glass Unit Failures.
• NFRC 100 (Procedure for Determining Fenestration Product U-factors) - background on IGU thermal performance and why seal failure degrades it.
• ANSI/AAMA 101 (Voluntary Specifications, Performance Requirements and Test Procedures for Windows, Doors and Unit Skylights).