Just Installed and Still Drafty: Post-Service Decision Tree
Why this matters
A customer who calls within 30 days of a window replacement saying "it is still drafty" is the highest-stakes callback in this trade. The technician needs to decide whether the draft is real, where the air is entering, and whether the cause is the new unit, the install around it, or a pre-existing condition the customer is now noticing. Each cause has a different remedy and a different conversation about responsibility; the wrong answer at the first visit produces a second callback.
Step 1: Verify the draft is real
Customers describe drafts based on perceived cold, not actual airflow. A new high-performance window has a colder interior glass surface than the single-pane it replaced (more heat is kept in the room, less conducts through the glass), and the cold glass produces a downdraft along the interior surface that feels like a draft but is not air leakage. Confirm:
- Use a smoke pencil, incense stick, or thermal anemometer at the sash perimeter, jamb-to-frame joint, sill, and head. Look for actual smoke deflection, not just a sense of cool air.
- A thermal camera image with the heat on and the outside cold shows the temperature gradient. Real air leakage produces a sharp linear cold line tracking the leak path; a glass downdraft produces a uniform cold field across the glass surface that fades up the room.
- Measure the interior glass surface temperature with an infrared thermometer. A modern double-pane with low-e and argon should read within 10 to 15 degrees F of room temperature in cold weather; if the glass reads close to outdoor temperature, the unit may be defective.
A real draft will move the smoke; a cold-glass complaint will not.
Step 2: Identify the leak path if a real draft is present
A real draft enters through one of five paths. Test each in order:
- Sash-to-frame seal: weatherstrip at meeting rail, jambs, head, sill. Dollar-bill drag or smoke at the sash perimeter under a small pressure differential.
- Frame-to-rough-opening: perimeter where the new window meets framing. Pull interior trim and inspect foam, backer rod, and sealant in the install gap.
- Sill pan and flashing: water-side seal that also serves as air seal. Inspect from exterior for gaps in head, jamb, and sill flashing.
- Exterior siding penetrations near the window: vents, cables, hose bibs, old fastener holes all leak air; the customer attributes it to the nearest visible change.
- Wall-cavity bypass: air entering through outlets, switches, or floor band joists tracking up to be felt at the window perimeter.
A pressure-differential test (turn on a bathroom exhaust) makes detection easier because all leaks become active at once.
Step 3: Decide what to open
The repair scope depends on which path is leaking:
- Sash seal: replace weatherstrip, adjust sash compression, or warranty-return the unit if the seal is defective. Do not open the trim.
- Install-gap seal: pull interior trim, inspect foam or backer rod, re-foam with low-expansion window foam (AAMA recommends low-expansion to avoid bowing the jambs), re-tape interior side, re-install trim.
- Flashing or pan: open the exterior siding. This is the largest scope and should be presented to the customer as a corrective opening of the install, not as a touch-up.
- Existing wall condition: not the window. Document for the customer and either propose a separate scope to address the wall, or refer.
Step 4: Match findings to the correct action
| Finding | Action |
|---|---|
| Smoke shows no movement; glass surface cold | Not a draft. Document IR readings, explain glass-downdraft physics, suggest cellular shade or interior curtain to break the downdraft. |
| Sash weatherstrip compressed unevenly or missing at a corner | Replace weatherstrip per manufacturer; adjust sash for proper compression. Verify sash latches engage fully. |
| Air felt at trim, foam visible in install gap is over-expanded and bowing jamb | Pull trim, cut back over-expanded foam, re-foam with low-expansion product per AAMA 2502. Verify operating sash still slides freely. |
| Install gap shows foam absent or undersized backer rod | Re-foam the gap with appropriate low-expansion window foam, tape interior with vapor-aware tape, re-install trim. |
| Air felt at exterior wall above window | Likely head-flashing detail. Open exterior siding above window, inspect flashing, re-flash per ASTM E2112. |
| Air felt at sill from below | Likely sill-pan or floor-band-joist condition. Verify sill-pan presence and integrity; if pan was not installed, this is a corrective opening at the install team's cost. |
| Air felt at an outlet near the window, not at the window itself | Wall cavity bypass; separate from the window scope. Recommend gasketed outlet cover and air seal at the rough-opening of the receptacle box. |
Step 5: Distinguish callback work from warranty work from change-order work
Three buckets, each with a different invoicing conversation:
- Callback (install defect, within warranty): re-foam an under-foamed gap, re-set a racked sash, replace a missing weatherstrip. No charge.
- Warranty (unit defect): defective IGU, failed weatherstrip, warped sash. Manufacturer warranty processed.
- Change order (out-of-scope condition): pre-existing flashing, wall bypass, glass downdraft. New quote, customer signs.
Identifying the bucket before lifting a tool is the difference between a recovered relationship and a billing dispute.
Step 6: Document and re-test before leaving
After the corrective work, repeat the smoke or thermal anemometer test under the same pressure-differential condition. The customer should see the test result, not be told the work is done. Photograph the as-found and as-corrected condition for the file.
Do not seal a window perimeter air leak with caulk or sealant on the interior trim. Interior sealant masks the leak path and traps moisture against the rough opening, leading to hidden rot. The correct air seal is foam or compressible backer in the install gap, behind the trim. Surface caulk over a leaking install gap is a 6-month fix that becomes a hidden-rot callback the next season.
References
- AAMA 2502-19, Comparative Analysis of Residential Window Air Infiltration Performance
- ASTM E2112-19, Standard Practice for Installation of Exterior Windows, Doors, and Skylights
- ASTM E283/E283M-19, Standard Test Method for Determining Rate of Air Leakage Through Exterior Windows
- NFRC 400-2020, Procedure for Determining Fenestration Product Air Leakage
- IRC R609, Exterior Windows and Doors (installation requirements)
- IGMA TM-3000-90, North American Glazing Guidelines for Sealed Insulating Glass Units