Leaky Window? Find the Cause and Fix It

A window that lets in air or water is more than an inconvenience — it’s a slow drain on your energy bill and potentially a source of moisture damage to your walls, sill, and surrounding structure. The good news is that most leaky windows have a straightforward fix once you identify the source.

Is the Leak Air or Water?

These are different problems with different causes.

Air leaks (drafts) — You feel air movement around a closed window. Energy is escaping, and outdoor air is getting in. This raises your heating and cooling costs and can make rooms uncomfortable near the windows.

Water leaks — You see water on the sill, stains below the window, or pooling inside during rain. This can lead to wood rot, mold, and drywall damage if not addressed.

A window can have one or both problems.

Common Causes of Leaky Windows

Failed or Missing Weatherstripping

Weatherstripping is the seal between the moving sash (the part that opens) and the stationary frame. Over time it compresses, tears, or falls off. When this happens, air flows freely around the sash even when the window appears closed.

Fix: Replace the weatherstripping. This is typically a DIY-friendly repair. Measure the old weatherstripping, buy a matching replacement type (foam, v-strip, felt, or rubber), remove the old material, clean the channel, and press or nail the new strip in place.

Cracked or Deteriorated Glazing Compound

On older wood windows, the glass is held in the frame by putty-like glazing compound. When this compound dries, cracks, and falls away, gaps appear between the glass and the frame — letting in both air and water.

Fix: Remove the old crumbling compound, clean the channel, apply new glazing compound, let it cure, and paint over it. This is a manageable DIY project but requires patience and attention to detail.

Failed Window Seal (Double-Pane)

Double-pane windows have a factory-sealed perimeter seal. When this seal fails, moisture enters between the panes (you’ll see fogging or condensation between the glass layers). Air infiltration often follows.

Fix: The insulated glass unit (IGU) must be replaced. There is no reliable way to re-seal a failed double-pane unit from the outside — the unit needs to be replaced in full.

Gaps in Exterior Caulking

The caulk around the outside of the window frame (between the frame and the exterior wall) can crack and pull away over time. This is a common entry point for both air and water.

Fix: Remove the old caulk with a utility knife, clean the joint, and apply fresh exterior-grade silicone or polyurethane caulk. A straightforward DIY repair that makes a significant difference.

Damaged Frame

If the wood frame is rotted or warped, it will no longer hold a proper seal against the sash or the wall. Damaged frames often require partial repair or full window replacement.

Fix: Assess the extent of rot. Small areas of rot can sometimes be filled with epoxy wood filler. Significant structural rot requires replacing the window or the frame section.

Cracked Glass

A crack in the glass itself provides a direct path for air. Even a hairline crack will allow noticeable air movement, and cracks allow water infiltration during rain.

Fix: Glass replacement by a glazing professional.

Diagnosing Your Leaky Window

To find the source of a draft: on a windy day, hold your hand around all four sides of the window — sash joints, frame perimeter, and glass edges. You can also use a candle or incense stick; the smoke or flame will react to air movement.

For water leaks: check during heavy rain. Note exactly where water appears first. Is it running down from the top? Coming up from the sill? Appearing on the frame? The location tells you where to look outside.

When to Call a Professional

Call a glazier when:

• The glass itself is cracked or broken
• You have a failed double-pane unit (fogging between panes)
• The frame shows significant rot or structural damage
• Caulking and weatherstripping fixes haven’t resolved the problem

Allnite Glass serves Clarksville, Nashville, and Hopkinsville for window glass repair and replacement. We’ll help you identify whether the problem is the glass, the seal, or something else — and give you an honest recommendation on the fix.

Call (931) 645-2464 or contact us online for a free assessment.