Which to Check First on Condensation: Glass vs Frame vs Install Sequencing Decision Tree

Why this matters

Condensation calls split three ways: a failed insulated glass unit (IGU) seal, a cold frame or interior-humidity problem at the frame, or an install/flashing defect letting water and cold air in. Each is a different fix and a different conversation about cost and warranty. The trap is checking in the wrong order: a tech who assumes "fogged window equals bad glass" replaces an IGU when the real issue was a humidifier running at 60 percent indoor RH in winter. Sequence the checks by what is cheapest to observe and most decisive, and you stop guessing.

The right first check is always where the moisture sits, because that single observable separates the three causes faster than any other test.

Symptom presentation

• Fog or moisture between the panes, cannot be wiped off either side: failed IGU seal. Decisive on its own.
• Water droplets on the interior surface of the glass or on the frame, wipeable: surface condensation from indoor humidity and a cold surface.
• Water at the sill, jamb line, or exterior face paired with staining or wind-rain timing: install/flashing intrusion masquerading as condensation.

Quick checks

Sequence them so the cheapest, most decisive observation comes first.

1. Locate the moisture (free, decisive). Wipe the interior glass. If it clears, the moisture is on the room side (frame/humidity branch). If it does not clear and the exterior is also dry, it is sealed between the panes (IGU branch). If the moisture is at the sill/jamb and tracks rain, it is the install branch.
2. Read indoor relative humidity (cheap). A hygrometer reading. High interior RH in cold weather makes surface condensation on any cold surface, regardless of window quality.
3. Check the cold-surface pattern (cheap). Surface condensation concentrates at the coldest points: bottom edge of glass, corners, and the frame, especially on metal or non-thermally-broken frames. A uniform film at the bottom edge in cold weather is classic surface condensation.
4. Inspect for water entry (moderate). Only if the sill/exterior read flagged it: pull the stool or inspect the exterior for flashing defects.

Isolation tree

1. Moisture is between the panes (will not wipe off).
   • Failed IGU seal. The desiccant in the spacer is saturated and the sealed gas/air space is now venting room or outdoor moisture. Confirm and move to glass remediation. No amount of humidity control fixes this; the unit is the defect.
2. Moisture wipes off the interior glass or sits on the frame.
   • Read indoor RH and surface pattern.
   • High RH (winter), condensation at glass edges/corners/frame: surface condensation. This is a building-science / humidity problem, often worsened by a cold or non-thermally-broken frame, not a defective window. Verify the NFRC U-factor and condensation-resistance expectations the customer was sold against.
   • Normal RH but persistent frame condensation localized to the frame: cold-frame issue, possibly a thermal bridge at the frame or a metal frame without a thermal break.
3. Moisture at the sill / jamb / exterior, tracks rain or wind.
   • This is not condensation; it is water intrusion. Move to install/flashing logic. Inspect the sill pan, end dams, head flashing laps, and WRB integration per ASTM E2112; reproduce with a directed water test per ASTM E1105 if needed.

Confirming diagnosis

• Glass (IGU) confirmed when fog persists between panes and cannot be wiped from either surface. Edge-of-glass haze, a visible film, or seasonal fogging that disappears in dry conditions all point to a breached seal per the IGU durability framework in ASTM E2190.
• Frame / humidity confirmed when the moisture wipes off, concentrates at the coldest surfaces, and correlates with measured high indoor RH or a cold/thermally-bridged frame. Lowering indoor RH or improving the surface temperature eliminates it.
• Install confirmed when the wetting is at the sill/jamb, tracks wind-driven rain, and a directed water test reproduces entry traceable to a flashing or WRB defect.

Remediation

• Glass: Replace the failed IGU (or sash, depending on construction). A sealed-unit failure is typically a warranty item under the glass/IGU coverage; verify against the manufacturer's IGU warranty terms before quoting.
• Frame / humidity: Address indoor RH (ventilation, source control, humidifier setpoint) and, where the frame is the cold surface, evaluate thermal-break upgrades or interior storm/insulating treatments. Set the customer's expectation against the unit's published condensation resistance and U-factor.
• Install: Correct flashing to ASTM E2112, restore the sill pan with end dams and back dam, fix WRB shingle-laps, and re-verify with a water test. This is not a glass problem and replacing glass will not stop it.

A note on sequencing in practice: when more than one branch looks plausible, resolve the cheapest and most decisive first. The wipe test costs nothing and eliminates the between-pane branch outright. The RH read costs a hygrometer and a minute and eliminates or confirms the humidity branch. Only the install branch requires invasive inspection or a water test, so it goes last in the sequence even though the customer often assumes it first. Working the order this way means you almost never disturb trim or run a water test on a window whose problem was a 60-percent indoor RH in January. Record the indoor RH and outdoor temperature at the visit; a single reading documents the dew-point math for the customer and for any warranty file, and turns a "your window is bad" argument into a measured fact.

References

• ASTM E2190, Standard Specification for Insulating Glass Unit Performance and Evaluation (IGU seal durability and failure criteria).
• ASTM E2112, Standard Practice for Installation of Exterior Windows, Doors, and Skylights.
• ASTM E1105, Standard Test Method for Field Determination of Water Penetration of Installed Exterior Windows.
• NFRC 500, Procedure for Determining Fenestration Product Condensation Resistance; NFRC 100 (U-factor).
• IGMA (Insulating Glass Manufacturers Alliance) technical bulletins on IGU performance and condensation.