The Stain That Came Back Below the East Window

Entry point: the exterior sealant joint at the window flange, not the interior trim. The moisture gradient supports it - 22% at the drywall near the frame tapering to 11% one stud bay away - and the blistering on the exterior skin below the same window says the wall cavity has been wet long enough to push paint off from behind.

Why the interior caulk failed:

• Interior caulk is a decorative air seal, not a water barrier. It cannot stop water that is already inside the wall.
• Worse, it traps the water. Sealing the inside face keeps the cavity from drying inward, so the stain returns faster and the sheathing stays wet longer.
• Water entering at the flange runs down the housewrap, hits the window rough sill or a fastener, and tracks along the top plate to the ceiling-wall corner - which is why the visible damage is offset from the actual defect.

Diagnostic step before repairing: probe the exterior joint with a putty knife to see whether the sealant has lost adhesion or the joint was never given a backer rod, and check the siding above the window for a gap or a reversed lap that is funneling water to the same corner.

Sequence:

1. Today (before the rain): temporary control only. Tape poly over the exterior joint and lap it under the siding above so water sheds past the window, and set an air mover plus dehumidifier inside on the wet drywall. Do not smear the 40°F-rated sealant at 38°F overnight - it will skin over without adhesion and fail at the first storm.
2. When conditions are in range (dry, above 40°F and rising): remove the old sealant, install closed-cell backer rod to the correct depth so the joint is a proper hourglass shape, and apply the sealant to the specified thickness. Do not bridge a wide joint with a thin bead.
3. Address the siding: correct any reversed lap or open gap above the window, and verify the flange has a proper flashing detail and a drainage path out.
4. Verify. Hose-test the window from outside, low pressure, working upward, with a helper inside watching the meter at the stain. Re-meter after 24 hours. A leak test that only wets the glass proves nothing - you must wet the joint.
5. Only then repair the interior: cut out and replace the damaged drywall rather than spackling over it, treat any surface mold on the framing per your remediation protocol, seal the framing with an encapsulant, stain-block and prime, then paint to the break.

Why the order matters: painting or patching first hides the evidence and guarantees a callback after the next storm.

On timing: explain that painting today would trap moisture and fail within weeks, and give a real date tied to the weather window and the leak test. Offer the air mover and dehumidifier now so she sees progress.

On mold: describe what you actually observed rather than reassuring her blindly. Typical guidance: a small area of surface growth on drywall (roughly under 10 square feet) is normally handled in-house with PPE, containment, and an appropriate cleaner; anything larger, or growth on structural framing or inside the HVAC, goes to a remediation contractor. Say plainly that you will document it with photos and meter readings and that she can request the documentation.

On who pays: this is exterior envelope failure on the building, not resident negligence, so the repair is the owner's cost. Say that in writing, in the work order, and note the cause. If any fixture or personal property was damaged, direct her to the property manager and her renter's insurance rather than promising compensation you cannot authorize.

Documentation to leave behind: moisture readings before and after, photos of the exterior joint, the resident's report of the first occurrence, and the remedy applied. That file is what protects the property if the issue recurs or becomes a habitability claim.