How do I know if a window is leaking through the wall?

The evidence gathers at the corners: stains and peeling at the head and lower corners inside, soft or stained trim and sheathing outside, and leaks that follow wind-driven rain from one direction. Water at the glass is a different, lesser problem than water at the wall.

What is a sill pan and why does it matter?

The waterproof tray formed at the bottom of the window opening before the unit goes in, sloped to drain outward. It is the assembly's last defense, catching whatever gets past the window, and its absence turns small leaks into rot.

Can a leaking window be fixed with caulk?

Rarely durably: flashing works by shingle-lapped layers, and caulk substituting for laps fails in seasons. Caulk belongs at specific joints; leaks at corners and heads usually mean the flashing sequence needs rebuilding.

Why does the wall leak but the window itself seems fine?

Because the window is often innocent: the leak rides the wall's drainage plane to the opening's flashing, which either sheds it past the unit or delivers it into the framing. Many window leaks are wall leaks that found the hole.

Window Flashing and Wall Leaks | Home Reference Guide

Page last reviewed: August 2026 · Reviewed by Harold R. Bennett, retired home inspector

Every window is a hole cut through the wall’s drainage plane, and window flashing leaks are what happens when the patch around that hole is wrong: water riding the housewrap behind the siding arrives at the opening and either sheds around it, the flashing’s job, or enters the framing at the corners, where the rot findings concentrate. This reference covers the shingle-lap sequence that works, the shortcut installs that fail, the evidence that convicts them, and the repair realities.

The principle: shingle laps around a hole

The wall’s water plan assumes water behind the cladding, so the opening’s flashing must integrate with the water-resistive barrier in shingle order, each layer lapping over the one below it so gravity always sheds outward: sill first, a pan flashing across the opening’s bottom, corners sealed and back edge dammed, sloped to drain out, the last-defense tray that catches whatever ever gets past the unit; jambs second, flashing up each side, lapping over the pan’s edges; the window in, on sealant at sides and head but with the sill’s flange left unsealed or gapped so the pan can drain (the counterintuitive detail that separates trained installs, since caulking the bottom flange seals the drain shut); head last, flashing over the top flange, and the barrier’s upper flap lapped over that, so wall water crosses the head on top of everything. The order is the entire technology, the flashing family’s laps-not-caulk law applied to a hole, and every failure below is a violation of it.

The installs that fail

The caulk-only install, flanges bedded and perimeter beaded with no flashing integration, is the volume failure: the sealant ages, the wall’s water finds the flange edges, and the corners drink, with the sealant-as-confession rule applying in full. Reverse laps, barrier tucked behind the head flashing or tape lapped uphill, route wall water inward by construction, invisible until the stains file. The missing sill pan converts every minor entry into framing contact, and its absence is the single most consequential omission, since heads shed by gravity while sills collect by it. Replacement-window wrinkles add their own: insert replacements inherit the original opening’s flashing, sound or not, and full-frame replacements re-open the sequence for better or worse, which is why a replacement quote that never mentions flashing or pans has answered the diagnostic question already. Around it all runs the cladding’s supporting cast, kick-outs, weep screeds, and drainage details by material, stucco and barrier-EIFS raising the stakes on every window they surround.

The evidence that convicts

Window-wall leaks testify at the corners. Inside: staining, bubbling paint, and soft drywall at the head corners (wall water crossing the top) and lower corners and stool (sill collection), swelling trim, and the wandering ceiling stain one floor down that started at a window above. Outside: stained or soft trim, peeling paint in the corner-and-sill geography, sealant joints split or improvised, and the probe test’s verdicts in sills and trim. Behavior corroborates: leaks correlated with wind-driven rain from one direction implicate the wall assembly, while all-weather dampness and between-pane fog file under the unit itself, and interior-humidity condensation, per the window-condensation reference, impersonates leaks every winter. The hose test settles disputes exactly as on roofs: low zones first, one zone at a time, a watcher inside, working up from sill to jambs to head to the wall above, the first arrival naming the layer.

Repair realities

Durable repair rebuilds the sequence, which means opening the wall around the window: cladding back, barrier laps corrected, pan and head flashing installed in order, and the practical mercy that most failures concentrate at one or two openings rather than the whole wall. Sealant maintenance holds its honest, small place, perimeter joints renewed at the caulk reference’s standard, and its dishonest large one refused: beads over failed laps buy seasons and hide the meter readings. Rot found at sills and framing repairs by the standard tiers while the wall is open, and the one-window lesson generalizes, since identical installs age identically, one convicted opening putting its siblings on the watch list.

The bottom line

A window is a hole in the wall’s raincoat, and the patch works only in shingle order: pan, jambs, unit, head, barrier over all, with the sill’s drain left open. Read corner stains as the flashing’s testimony, trust wind-direction correlation over the unit’s appearance, hose-test the disputes, and rebuild laps instead of buying caulk, because the water behind the siding was always coming, and the sequence is what decides where it goes.

Window Flashing and Wall Leaks