Wind-Driven Rain at Stucco Window Corners
A 12-year-old two-story house has three-coat stucco over OSB sheathing on its east elevation. After a two-day storm with wind-driven rain from the east, the owner noticed water staining at the lower corners of two second-floor windows and bubbling paint on the interior drywall below. A pin-type moisture meter reads 26–32% at the drywall around those window openings and 10–12% at a reference point 4 feet into the room. The windows were replaced three years ago, and the owner says the installer 'sealed them with tape and caulk.' There is no visible weep screed at the base of the stucco, and no kick-out flashing at the roof-wall intersection above. The owner wants to know whether to call a roofer, a window contractor, or a mold remediator first.
1. Identify the likely water-entry paths and explain why mold growth may be present inside the wall even though the interior visible staining is limited.
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The pattern points to water entering at the window perimeters and at the roof-wall junction, not to a roof leak alone. Wind-driven rain from the east loads the east elevation; the window corners are the most vulnerable because the replacement windows appear to have been sealed without pan flashing, back-damming, or a drainage plane. Missing weep screed at the stucco base and missing kick-out flashing above allow water to run behind the stucco and down the OSB. The drywall readings of 26–32% are well above the 10–12% reference and indicate active wetting. Mold can grow on the paper facing on the back side of the drywall and on the OSB, where it is hidden from the room. Bubbling paint is a sign of moisture migration through the drywall, not just surface condensation. The interior visible staining understates the extent.
2. Sequence the diagnostic and repair work. What must happen before any mold remediation or clearance sampling?
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First, stop and control the water. Do not install new drywall, caulk, or mold-resistant paint until the envelope defects are corrected. Use infrared imaging and a pin or pinless meter to map the wet zone; remove baseboard and a section of drywall at the window corners to inspect the cavity and sheathing. A water test or spray rack on the exterior, if practical, can confirm the entry point. The repairs should include pan flashing and back dams at the windows, proper sealant joints, a drainage plane behind the stucco if absent, weep screed at the base, and kick-out flashing at the roof-wall intersection. Only after the envelope is watertight and the framing is dried to normal moisture levels should mold remediation be scoped. Clearance sampling before the source is fixed is meaningless.
3. The owner asks whether the windows can simply be caulked again and the drywall painted with mold-resistant paint. Respond.
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Caulking the window perimeter again and painting over the drywall would trap moisture and hide the problem. Mold-resistant paint does not stop mold growth on the back side of drywall or on the OSB sheathing, and it does not correct missing flashing or a missing drainage plane. The owner should hire a stucco or building-envelope contractor, not a mold remediator first. A roofer should evaluate the roof-wall kick-out detail, but the window corners are a separate defect. After repairs and drying, any mold-contaminated drywall and insulation should be removed, and the cavity cleaned and verified dry before closing.