Window Condensation - Source vs Symptom Decision Tree

Why this matters

Window condensation is the symptom every homeowner can see, which is why it generates window-replacement calls when the real problem is humidity load. Replacing the window without solving moisture lowers the dew point of the new glass slightly and the condensation returns next winter on a new $40,000 install. Conversely, dismissing fogged or sweating windows as "just humidity" misses failed insulating glass units (IGU seal failure) that genuinely need replacement. The four buckets - excess indoor humidity, IGU seal failure, exterior surface condensation, and a building envelope air leak hitting cold glass - each have distinct field signatures and different correct trades.

Symptom presentation

Five intake questions: condensation on the inside surface of glass, between the panes, or on the outside, time of year (winter only, summer only, both), is it every window or specific ones, has it always been this way or did it start recently, and what is the indoor RH reading. Winter inside-surface condensation on a few windows is normal at low outdoor temperatures and high indoor RH; condensation on every window in winter is a humidity problem; condensation between the panes any time is a seal failure.

Cross-trade quick checks

• Condensation between the panes, never dries: IGU SEAL FAILURE. Window trade. Not a humidity problem.
• Inside-surface condensation in winter, only on coldest nights, only at corners: NORMAL or minor humidity tweak. No trade needed if RH under 50%.
• Inside-surface condensation every winter, all windows, RH over 50%: INDOOR HUMIDITY. Source diagnosis - plumbing, ventilation, combustion.
• Inside-surface condensation only on north-side or single-room windows: ROOM-LEVEL air leak / cold pocket. Envelope diagnosis.
• Outside-surface condensation in summer mornings: NORMAL. New high-performance low-E glass with cold interior, warm humid morning outside. Customer education, no trade.
• Frost on inside glass below 32F outside: severe humidity issue, urgent diagnosis.
• Condensation around frames, dripping onto sills: high-RH AND envelope air leak. Mixed trade.

Isolation tree by source

Indoor humidity path. Confirm indoor RH with hygrometer at multiple rooms. Target 30-40% in winter, 40-50% in summer per ASHRAE 62.2 guidance. Sources of excess winter humidity: unvented combustion (gas log set, unvented heater), oversized humidifier on HVAC, hidden plumbing leaks, basement / crawlspace moisture migrating up, occupants who shower without bath fan use. Diagnose by elimination - shut humidifier, log RH for 48 hours, then add sources back one at a time. Bath / kitchen / dryer exhaust must terminate outdoors per IRC M1502, M1505 - venting a dryer into the garage or attic is a primary winter humidity source.

IGU seal failure path. Condensation between the panes of a sealed insulating glass unit means the perimeter seal lost integrity. Argon gas escaped, ambient air entered, water vapor follows, and the unit fogs. Cleaning will not fix it; only IGU replacement (some manufacturers warranty 20 years, check). The window frame and sash stay; only the sealed glass insert is swapped. Diagnostic: confirm the moisture is genuinely between the panes by wiping interior and exterior - if both wipe dry and the fog remains, it is IGU. The whole-window-replacement upsell on an IGU-only failure is a callback risk and reputation killer; do the IGU swap.

Exterior condensation path. Modern low-E glass on a cool morning after a warm humid night reads cold on its outside surface; ambient humidity condenses there until the sun warms the glass. This is a sign the window is performing well (interior surface stays warm). Customer education, no work scope. Often appears in spring and fall in humid climates.

Envelope air leak path. Condensation only at specific windows often points to a building-side defect rather than the window itself. Diagnostic: feel for drafts at the frame perimeter and sash; thermal-image (if available) to find cold spots fed by air leakage from the rim joist, the wall cavity, or an attic chase. Recessed lights above are a common path - warm humid indoor air rises through them into a cold attic and the wall cavity gets colder than the window. Air-sealing the cavity above (top plate gaps, can lights) and at the rim joist below cuts the cold pocket; sometimes adding cavity insulation. Blower-door testing on the room before / after quantifies the change.

Confirming which trade on site

The first tech confirms in 20 minutes with three tools: a hygrometer, an IR thermometer, and the ability to disassemble a window stop / sash for fog confirmation. Step one: hygrometer at three locations. Step two: IR-thermometer the glass center, glass edge, and frame; surface temperatures over 5F below dew point will condense. Step three: wipe the inside and outside of the suspect window with a clean cloth - if condensation returns and is locked between panes, IGU. Step four: if humidity-driven, walk the moisture sources (humidifier setting, bath fan use, dryer vent termination, plumbing pan check, basement / crawlspace humidity).

Calculation gut check: with indoor air at 70F and 40% RH, the dew point is approximately 45F - so any surface colder than 45F will condense. A single-pane window or a metal frame in cold weather sits well below that. A modern low-E IGU sits within 5-10F of indoor temperature and will not condense at 40% RH. If condensation happens at 30% RH on a modern window, an air leak is the answer, not humidity.

Hand-off protocol when the trade you sent is wrong

Window contractor arrives, finds IGU is sealed (no inter-pane fog), indoor RH is 60%: document RH and IR-temperature readings, decline the window-replacement scope, refer to HVAC / building-science / plumbing for humidity diagnosis. HVAC tech arrives, finds the customer also has 3 IGUs with seal failure: complete humidity work and refer to window trade for IGU swap. Either trade walks into a recessed-light + cold-pocket scenario: refer to insulation contractor for air-seal and cavity work. Document the actual surface temperatures so the next visit has data. The most common wrong answer is "replace your windows" - sell only what the diagnostic supports.

References

• ASHRAE 62.2: residential ventilation and IAQ; informs target indoor RH and ventilation requirements.
• IRC M1502: dryer exhaust must terminate outside the building, no screens, specific length limits.
• IRC M1505: kitchen and bath exhaust termination requirements.
• AAMA / WDMA / CSA 101 / I.S.2 / A440 (NAFS): North American Fenestration Standard - basis for IGU and unit performance ratings.
• NFRC 100/200: window U-factor and SHGC certification - relevant for diagnosing center-of-glass surface temperature.
• IICRC S500: moisture meter readings and action thresholds in materials.