Most Likely Causes (Ranked)
The bead of caulk between the window frame and the exterior siding is your primary weather barrier. It dries, cracks, and separates in 5-10 years from UV exposure and thermal cycling. Once it gaps, wind-driven rain enters the wall cavity. This is the simplest cause and the first thing to check — walk outside and inspect every window's caulk.
Metal or membrane flashing above the window (head flashing) deflects water running down the wall away from the window opening. If flashing was never installed (common in tract housing), installed incorrectly, or has corroded, water enters above the window and runs down inside the wall. This is the #1 hidden cause — you can't see it without removing siding.
Vinyl and aluminum windows have small weep holes at the bottom of the frame track that drain water that enters the track. When these clog with dirt, paint, or insect nests, water backs up inside the frame and overflows into the house. This is extremely common and takes 30 seconds to fix.
Double-pane windows have a seal between the glass panes. When this seal fails, the window fogs between panes. In some cases, the failed seal also allows water entry around the glass-to-frame junction, especially on older single-pane windows with dried putty glazing.
House settling can shift the window frame enough to create gaps between the frame and the rough opening. The gap may be invisible from inside because trim covers it, but wind-driven rain finds any opening. Settling also causes windows to bind or become difficult to operate.
How to Fix It
Recaulk the Exterior
$5-15 DIY- 1Remove all old caulk with a utility knife and caulk removal tool.
- 2Clean the surfaces with rubbing alcohol. Let dry completely.
- 3Apply a continuous bead of exterior-grade silicone or polyurethane caulk (OSI Quad or DAP Dynaflex Ultra) around the top and sides of the window.
- 4Do NOT caulk the bottom — water that enters must be able to drain out through the weep holes.
- 5Smooth with a wet finger and let cure 24 hours before rain exposure.
Clear Weep Holes
Free (2 minutes)- 1Locate the weep holes at the bottom exterior of the window frame (usually 2-4 small slots or holes).
- 2Clear with a toothpick, small wire, or compressed air.
- 3Ensure nothing blocks the holes from outside (mulch, soil, or paint).
- 4Check all windows in the house — if one is clogged, others probably are too.
Install or Replace Head Flashing
$50-200 DIY / $200-500 Pro- 1This requires removing the siding course above the window to access the flashing area.
- 2Install self-adhering membrane flashing (like Grace Vycor or ZIP System flashing tape) across the top of the window, extending 6+ inches up the wall.
- 3Layer the siding over the flashing so water sheds outward, not behind.
- 4This is an intermediate-to-advanced repair — incorrect flashing makes the problem worse.
When to Call a Professional
Never caulk the bottom of a window frame — this traps water inside and causes worse damage.
If the interior wall below a window is soft or discolored, open it up to check for mold and rot before just recaulking.
Wind-driven rain leaks only happen during specific conditions — test with a garden hose aimed at the window to confirm the repair worked.
Ice dams in winter can force water up under flashing, causing similar symptoms to a window leak.
How to Prevent It
Inspect and recaulk all exterior window caulk every 3-5 years.
Clear weep holes during spring and fall cleaning.
Ensure grading slopes away from the house — water pooling near the foundation splashes up to windows.
Trim bushes away from windows so air circulation helps caulk and frames dry.
Frequently Asked Questions
Wind direction matters — the window facing the prevailing storm direction gets hit hardest. Also, one window may have worse caulk, missing flashing, or clogged weep holes while others are fine.
Wind-driven rain damage is often excluded unless the wind first created an opening (like a missing shingle). Gradual deterioration of caulk and flashing is maintenance, not covered.
Usually not necessary. In 90% of cases, recaulking, clearing weep holes, and fixing flashing solves the problem for $5-50. Replace only if the frame itself is rotted or warped.
Have someone inside watching while you spray a garden hose at the window from outside. Start at the bottom and work up. This isolates the leak source and confirms the repair.