Draft Returns Only in Wind After Foam and Flash: Decision Tree

Why this matters

A draft that only shows up when the wind blows, after the crew already foamed the rough opening and flashed the unit, is a wind-pressure-driven air-leakage path the static install did not close. Still-air days mask it because there is no pressure difference to drive flow; a 15 to 25 mph gust loads the windward wall and finds the one gap that the low-expansion foam, the sash weatherstrip, or the flashing lap did not seal. Chasing this with more caulk on the visible interior trim almost never works, because the leak is usually behind the trim or inside the sash perimeter. This tree isolates whether the path is around the unit (rough-opening/flashing) or through the unit (sash/weatherstrip/weep), so the second visit fixes it instead of guessing.

Symptom presentation

The homeowner feels cold air at the window only on windy days, often described as coming from a specific corner or the meeting rail. There is no draft on calm days, and a smoke pencil held at the interior on a calm day shows nothing. The complaint frequently follows a recent install where the crew did foam the gap and tape the flange, which is exactly why it is confusing: the obvious sealing steps were done. The distinguishing fact is wind dependence and, often, direction dependence (worse when wind hits that wall).

Quick checks

Reproduce the pressure before you diagnose. You cannot find a wind-driven leak in still air.

Confirm the lock is fully engaged. A double-hung or casement that is unlocked or only half-locked does not pull the sash tight to the weatherstrip; the homeowner using the window and leaving it unlatched is the simplest cause and the first thing to rule out.

Reproduce pressure with a blower door or, lacking one, a box fan in an adjacent door depressurizing the room. With the room under negative pressure, run a smoke pencil or your bare hand slowly around the entire perimeter: interior trim reveal, sash meeting rail, sash sides, sill, and the head. Mark every location that moves smoke. This converts a wind-only intermittent into a repeatable test.

Check the weep system on the exterior. Blocked or painted-over weep slots on a vinyl or clad unit can pressurize the sill chamber and push air inward under wind load. Confirm weeps are open.

Isolation tree

Use the smoke-pencil map to branch.

Smoke enters at the interior trim reveal (between drywall/casing and the window frame), not at the sash. The leak is around the unit, in the rough-opening seal or flashing, and the foam did not fully fill or the flashing lap is reversed/unsealed at a corner. Pull a section of interior casing on the leaking side and inspect the foam: look for voids, a skinned-over bridge with a hollow behind it, or foam that pulled away from the framing as it cured. Wind-driven water staining in that cavity confirms an exterior flashing path as well.

Smoke enters at the sash perimeter (between the operating sash and the frame). The leak is through the unit. On a double-hung, check the meeting-rail weatherstrip and the sash-to-jamb fin/pile; a compressed, missing, or mis-seated pile lets air past under pressure. On a casement, check the bulb gasket compression around the full perimeter and the lock-roller engagement. A casement that is not pulling tight at the top or hinge corner leaks there first under wind.

Smoke enters at the sill or weep area. Suspect the weep/drainage chamber: blocked weeps cause pressure buildup that vents inward, or the sill flashing pan is not continuous and wind drives air (and water) up under the unit.

Smoke enters at the head only. Suspect missing head flashing or a head shim gap the foam skinned over; the head is the hardest spot for foam to fill against gravity and the most common void location.

Confirming diagnosis

Prove the branch before you open the wall further.

Around-the-unit (flashing/foam): with the casing off, you should see the void or the unsealed lap directly, and the depressurized smoke test should now show air entering at that void, not at the sash. A targeted exterior water test (low-pressure hose at the head and sides, never high-pressure into the assembly) that produces interior moisture confirms a flashing path co-located with the air path.

Through-the-unit (sash/weatherstrip): close and lock the sash, then run the smoke test again at the sash perimeter only. If air still enters at the sash with the unit locked and the rough opening confirmed sealed, the weatherstrip or gasket is the path. Look for a gap you can see daylight through at a corner, a pile that does not contact, or a gasket that has taken a compression set.

Weep/drainage: temporarily clear the weeps and re-test; if the sill draft drops, the weeps were the cause.

Remediation

Around-the-unit: remove the bridged/voided low-expansion foam, re-fill in lifts with a window-and-door-rated low-expansion foam so the gap is fully filled without bowing the frame, and correct the flashing. Re-lap the flashing shingle-style (sill pan first, then jambs over the pan, then head over the jambs) so it sheds water and blocks air, and seal the corners per ASTM E2112 sequencing. Reinstall casing with a continuous interior air seal (sealant or backer rod plus sealant) at the frame-to-rough-opening joint.

Through-the-unit: replace the failed weatherstrip, pile, or bulb gasket with the correct OEM profile, and adjust sash hardware so the sash pulls tight to the weatherstrip (casement keeper/roller adjustment, double-hung balance and lock alignment). Verify even gasket compression around the full perimeter.

Weep/drainage: clear all weep slots, remove paint or debris, and confirm the drainage path daylights to the exterior.

Re-verify under negative pressure: the perimeter should be quiet with the sash locked and the casing resealed.

References

• ASTM E2112, Standard Practice for Installation of Exterior Windows, Doors and Skylights. Flashing sequence, sill-pan, and air-sealing requirements for the rough opening.
• ASTM E1105, Field Determination of Water Penetration of Installed Windows by Uniform or Cyclic Static Air Pressure Difference (companion field-pressure water test method).
• ASTM E783, Field Measurement of Air Leakage Through Installed Exterior Windows and Doors.
• AAMA/WDMA/CSA 101/I.S.2/A440 (NAFS), North American Fenestration Standard, for unit air-infiltration ratings and weatherstrip performance.
• Manufacturer installation instructions (Andersen, Marvin, Pella, Milgard, Simonton): low-expansion-foam and flashing-flange sealing requirements specific to the unit and warranty.