Condensation On Frame Versus Glass Decision Tree

Why this matters

Condensation on a window is the symptom most likely to drive a warranty call, yet the cause splits sharply between a defective unit and the surrounding building. Glass-surface condensation on the interior face of the indoor lite means the room dew point exceeds the inner glass temperature, which is almost always a humidity or air-leak problem and not a window defect. Condensation trapped between panes means the insulating glass seal has failed and the unit needs reglaze or sash replacement. Condensation on the frame surface only is the trickiest call because it can be either thermal bridging through the spacer or frame material, or a localized indoor humidity source, and the field test is different. Getting this branch right on the first visit prevents two-week warranty cycles where the manufacturer correctly refuses to replace IG units that are dry.

Symptom presentation

Start by isolating where the water actually sits. Pull a dry microfiber across the indoor face of the glass first; if it picks up moisture, glass-surface condensation is in play. Then look at the perimeter seal bead at the glass-to-frame interface. If condensation is only at the frame, check whether it follows the spacer line (about 0.5 inch in from the daylight edge) or sits on the painted or vinyl frame face. Finally, look between the panes by tilting your head and changing the light angle, water droplets or a fog that does not wipe away from the outside indicates seal failure inside the IG cavity.

The temperature pattern matters as much as the location. Glass-only condensation that appears on the coldest mornings and clears by mid-morning is usually room humidity overcoming a normal NFRC U-factor. Frame-only sweating that appears even on mild nights points to a thermal bridge or local air leak. Mixed presentations with both glass and frame wet, plus the indoor RH above 50 percent measured at the sill, almost always trace to a whole-house humidity load.

Quick checks at the unit

Walk the unit before testing. Run a hygrometer at the sill within 6 inches of the glass and read indoor RH and temperature. Read the outdoor temperature at the same height. Calculate the indoor dew point: at 70 F and 50 percent RH the dew point is about 50 F; at 70 F and 35 percent RH it drops to roughly 41 F. Then measure the inner-glass surface temperature with an IR thermometer aimed at the center of the daylight opening, holding the gun within 2 inches. If the glass surface is below the calculated dew point, the room atmosphere is over-humid for that glass package and the homeowner will see condensation on any window with that U-factor.

Check the weatherstrip continuity around the operable sash. A broken corner kerf or a compressed pile leaves a localized cold spot that pulls dew point onto the adjacent frame. Confirm the storm window, if present, is fully closed; a cracked storm bypass dumps cold air on the prime sash and condenses moisture along the lower rail. Confirm the room is not being pressurized by a bath fan or kitchen exhaust pushing humid air into the cavity around the rough opening.

Isolation tree

Branch 1 - condensation only between panes. The IG seal has failed. There is no field repair that restores the cavity gas fill or the original argon performance. Confirm the failure is past warranty terms by reading the data label on the spacer (most NAFS-compliant units carry a date stamp). If the unit is under warranty, document with a photo through a polarizing filter (cuts surface glare) and submit for reglaze or full sash replacement per the manufacturer process. Reference AAMA/WDMA/CSA 101/I.S.2/A440 for spacer performance expectations.

Branch 2 - condensation on the interior glass face only, clears by mid-morning, frame stays dry. Measure indoor RH and inner-glass temperature as described above. If the glass surface is below the indoor dew point and the unit has the labeled NFRC U-factor for that climate zone (per NFRC 100), this is a humidity problem, not a window problem. Walk the house: confirm exhaust fans vent outdoors (not into the attic), bathroom doors close and fans run during showers, the dryer vents outside through a tight termination, and there is no humidifier set above 35 percent in winter.

Branch 3 - condensation on the frame only, glass dry. Run the IR gun across the frame surface. A localized cold band along the meeting rail or sill points to weatherstrip bypass or a missing thermal break. A uniformly cold frame across the whole perimeter points to a thermal bridge through the spacer or to an aluminum frame without thermal break, which is original-spec performance and not a defect on legacy products.

Branch 4 - mixed condensation, glass and frame both wet, multiple windows affected. Whole-house humidity load. The window is the coldest surface and is condensing first. Solving requires source control (exhaust ventilation), an HRV or ERV in tight envelopes, and verifying the home does not have a hidden moisture source like an unvented crawl space or a plumbing leak.

Confirming diagnosis

Pull the IR temperature gun trace across the suspect window during the coldest part of the morning. Map the surface temperatures along a vertical centerline of the glass and a horizontal centerline of the frame. The glass center-of-glass temperature should sit within roughly 5 F of the value predicted by the labeled U-factor, indoor RH, and outdoor temperature using the NFRC 100 calculation. A center-of-glass reading that is more than 8 F colder than predicted indicates the unit has lost its low-emissivity coating performance or its gas fill, which is consistent with seal failure even before the cavity fogs visibly.

For frame-only complaints, run the same trace along the frame perimeter and look for a band 10 F or more below the rest of the frame. That band is the leak path. Smoke-pencil the gasket line on a windy day to confirm air bypass.

Remediation

References

• AAMA/WDMA/CSA 101/I.S.2/A440 North American Fenestration Standard, latest edition - performance class and rating requirements including spacer and seal durability.
• NFRC 100 Procedure for Determining Fenestration Product U-factors - the basis for the labeled U-factor on every certified window.
• NFRC 200 Procedure for Determining Fenestration Product Solar Heat Gain Coefficient - relevant when high SHGC contributes to summer condensation patterns.
• IRC R609 Exterior Windows and Glass Doors - residential code reference for installation and performance.
• ASHRAE 62.2 Ventilation and Acceptable Indoor Air Quality in Residential Buildings - the standard for whole-house and local exhaust ventilation that controls indoor humidity load.