Fixed the Draft Now Condensation Appears: Second Fault Masking Decision Tree

Why this matters

A draft repair that resolves the customer's original complaint and then produces a condensation complaint two weeks later is the textbook second-fault-masking pattern. The original draft was venting humid interior air to the exterior, which kept the interior glass and frame surfaces dry by keeping the local humidity low. Once the leak was sealed, the same interior humidity that was always there now has nowhere to go, and it condenses on the coldest surfaces in the room: the new glass, the frame edges, and any cold corner of the wall. The customer reasonably believes the new condensation is a new defect, but the underlying condition (interior humidity above the dew point of the cold surfaces) was present all along and was simply being masked by air leakage. The diagnostic is to confirm this pattern before warranty-returning a unit that is performing as designed.

Step 1: Verify the timeline before measuring anything

The pattern only fits if the condensation post-dates the draft repair. Ask:

• Date the original draft repair was performed.
• Date the customer first noticed condensation.
• Whether the customer ran a humidifier, did indoor renovation work involving wet materials, or had a household change (new occupants, more cooking, more showers) between the two dates.
• Whether the condensation appears at the same window that was draft-repaired, at adjacent windows, or throughout the home.

If the condensation pre-dates the repair, this is not second-fault masking; it is an independent condensation complaint and should be diagnosed on its own. If the condensation appeared after the repair, the masking hypothesis is the working theory.

Step 2: Measure indoor humidity and surface temperatures

Carry a digital thermo-hygrometer and an infrared thermometer. Record:

• Indoor temperature and relative humidity in the affected room at the time of inspection.
• Outdoor temperature.
• Interior glass surface temperature at the center, edge, and corner of the affected windows.
• Interior frame surface temperature at the meeting rail and at the sill.
• Wall surface temperature at the cold corner nearest the window.

Calculate the dew point from temperature and humidity using a psychrometric chart or a smartphone calculator. Any surface colder than the dew point will collect moisture. A room at 70 F and 50 percent RH has a dew point of about 50 F; any surface colder than 50 F will sweat. A modern double-pane low-e window in a 25 F outdoor environment commonly has a center-of-glass temperature around 58 F and an edge temperature around 50 F. The edge is the critical zone.

Step 3: Identify whether condensation pattern matches the masked-leak theory

The second-fault-masking pattern produces a specific signature:

• Condensation forms on the window that was draft-repaired, often heaviest on the repaired side.
• Adjacent unrepaired windows may also show condensation but less.
• Condensation appears first on the edge of the glass and the lower frame, where the interior surface is coldest.
• Condensation pattern follows the cold weather, fading on warmer days and returning on colder ones.
• Indoor RH reads above 45 percent in winter conditions.

Patterns that do NOT match second-fault masking:

• Condensation between the panes of glass (IGU seal failure, warranty issue).
• Condensation only on the exterior surface (normal cold-night radiative cooling, not a defect).
• Condensation forming hours after the room is heated, with no humidity source identified (likely vapor migration from a wall cavity).

Step 4: Identify the actual humidity source

The masking theory only works if there is interior humidity to mask. Find the source:

• Bathroom exhaust fans not running during or after showers, or vented into the attic instead of outside.
• Range hood not vented or vented through a charcoal recirculating filter.
• Whole-house humidifier set above 35 percent in winter operation.
• Indoor plants in volume.
• Unvented combustion appliances (gas log fireplaces without dedicated venting, kerosene heaters).
• Wet basements or crawl spaces with no vapor barrier.
• Recent indoor renovation: drywall, paint, mortar, or wet wood curing.
• More occupants than the home was designed for, or new aquariums, hot tubs, or indoor pools.

Each of these produces a corrective recommendation. Without addressing the humidity source, no window repair will resolve the condensation.

Step 5: Match findings to the right action

Step 6: Explain the masking to the customer

This conversation is hard because the customer hears "the draft repair caused the condensation." It did not; the repair revealed the condensation by removing the air-leak vent path. The right framing is:

• The original draft was leaking conditioned air to the exterior and bringing the local humidity down to outdoor levels.
• Sealing the leak restored the wall to its design performance and brought the local humidity back up to the rest-of-room reading.
• The condensation has always been present at the dew point on cold surfaces; it just was not landing on this window before the seal was tight.
• The actual fix is reducing indoor RH or improving the cold-surface temperature with an interior treatment, not undoing the draft repair.

Document the indoor RH reading on the invoice and recommend a target winter RH of 30 to 40 percent for cold-climate homes. The customer needs the number on paper.

Step 7: When to warranty-test the unit anyway

References

• ASHRAE Standard 55-2020, Thermal Environmental Conditions for Human Occupancy (indoor humidity targets)
• ASTM E2128-12, Standard Guide for Evaluating Water Leakage of Building Walls (humidity diagnosis context)
• AAMA 1503-09, Voluntary Test Method for Thermal Transmittance and Condensation Resistance of Windows, Doors, and Glazed Wall Sections
• NFRC 500-2017, Procedure for Determining Fenestration Product Condensation Resistance
• IRC R303.4, Mechanical Ventilation (whole-house ventilation requirements)
• IGMA TM-3000-90, North American Glazing Guidelines for Sealed Insulating Glass Units