Rain on the glass is fine. Water on the sill is a problem. When wind drives storms against a wall, weak points in the frame, sealant, or wall cladding let water migrate inside and stain trim, bubble paint, and feed mold.

The fix starts with a calm check of where water enters, then a repair that matches the cause – no guesswork, no messy patches that fail at the next downpour. This guide explains how to tell a leak from indoor condensation, where water usually sneaks in, and which field tests prove the source.

The same steps help an apartment, a starter house, or an older place with wood windows. Keep the pace steady and the notes simple. A dry home needs three things: a sound path for water to drain, tight joints that move with the seasons, and glass that stays warmer than the wet air around it.

First question: is it a leak or inside moisture?

Look for water at edges, corners, and stool ends right after a storm. If wet spots appear only during rain, the issue matches the phrase window leaking when raining and points to a path from outdoors to indoors. Condensation behaves differently – it forms a uniform film on the coldest glass and shows up on calm, cold mornings even without rain.

A true leak leaves brown edges, soft spots in the stool, and streaks that trace down from the upper joints or from the head of the frame. Check the wall above the head for hairline cracks, failed caulk at siding joints, and clogged weep holes at the bottom of the frame.

If wind from one direction always triggers the problem, expect a flashing or siding issue near that face, not a glass fault. Good notes on timing, wind, and stain shape speed up the fix and prevent repeat damage.

Where water sneaks in during storms – and what that means

Most leaks come from a few repeat locations. Old or brittle sealant opens micro gaps at the frame–siding joint and pulls away from brick or stucco. Missing or misfit head flashing lets wind push water behind the top trim so it rides the sheathing and drops out at the interior stool.

Blocked weep holes trap water in the sill track until it rises and spills past the inner leg. Cracked glazing putty or failed IGU edge seals let water sit where it cools the frame and wicks inward along wood fibers.

On vinyl frames, a bowed or loose jamb liner breaks the air seal and channels water onto the stool. Poor slope on exterior sills holds puddles instead of shedding them. Each cause has a clear tell: stains above point to flashing; stains below and at the sides point to sealant or weeps; stains that mirror the sash rails hint at glazing issues.

A simple field checklist that proves the source

Before calling a crew, a short, careful check narrows the fix and saves time and cost. Work from dry to wet, top to bottom, and record every finding.

• Tape paper towels under the head, at both jambs, and on the stool. Mark the time as rain starts to track where wet shows first.
• Probe exterior sealant with a plastic pick; if it crumbles or pulls, plan to remove and reseal with a high-quality, paintable exterior sealant.
• Clear weep holes with nylon trimmer line; pour a small cup of water in the sill track to confirm a fast drain to the exterior.
• Inspect head flashing for a visible drip edge and a ¼–½ inch gap from siding; water should exit cleanly rather than track behind trim.
• Check interior humidity with a small hygrometer; if it sits above 40–45% during cold weather, address moisture sources so fog does not mask true leaks.
• Use a bright light and mirror to look up behind the head trim; water lines or dark streaks above the frame point to flashing or cladding issues.

Repairs that last through storm season

Match the fix to the cause and let each layer do its job. For tired sealant, remove the full bead, clean to bare, dry material, and rebuild the joint with backer rod where the gap exceeds ¼ inch. Tool a smooth, slightly concave bead so it stretches rather than tears as seasons change.

For clogged weeps, clean the track, verify slope toward the exterior, and keep bug screens free of lint that dams water. For missing or poor head flashing, add a proper metal flashing with a drip edge under the course above the window trim – it must kick water out and over the face, not hide behind caulk.

For failed glazing or IGU seals that fog and drip, replace the sash or unit rather than chase sealant around the glass. Where wood is soft, cut back to sound fibers, treat the area, and patch with a quality exterior epoxy before repainting. Each repair should end with a hose test: a low, steady flow on the area for a few minutes while an observer checks the paper towels inside. Stop, move, and repeat until every suspect path proves dry.

Dry sills, calm storms

Rain finds the weak link every time. A clear check, a focused repair, and a quick proof test put the problem to bed and protect the wall behind the trim. Keep the weeps open, watch the head flashing after the first big storm, and refresh exterior sealant when it starts to chalk or pull.

Windows that shed water, walls that breathe, and indoor air kept in a healthy range work together to stop stains before they start. With a steady routine, the next storm sounds like rain on glass – and leaves no trace on the sill.