A window can appear completely closed and still let in cold air, street noise, dust, or rainwater.  The problem is not always visible. In many cases, the cause is hidden in the gasket, sash pressure, drainage system, or the installation joint between the frame and the wall.

Before blaming the entire window, it is worth understanding where the leak actually begins.

1. Weak Sash Clamping Pressure

When the window is closed, the sash should press evenly against the frame. If the pressure is too weak, small gaps remain and air can pass through.

This may happen because of misaligned locking points, worn hardware, or incorrect adjustment.

A simple paper test can help: close the window on a sheet of paper. If it slides out easily, the clamping pressure may be insufficient.

2. Worn or Damaged Seals

Rubber and foam gaskets form the airtight barrier between the sash and the frame. Over time, they can harden, shrink, crack, detach, or lose their shape.

Strong sunlight, temperature changes, dust, and aggressive cleaning products can accelerate wear.

If the seal is damaged, drafts may appear around one corner, one side, or the entire perimeter of the window.

3. Failed Glazing Seal

Double- and triple-glazed units rely on sealed edges to keep the space between the panes insulated.

If the glazing seal fails, you may notice:

• condensation between the panes;
• colder glass surfaces;
• reduced thermal insulation;
• weaker sound insulation.

This does not usually create a direct air leak, but it makes the window area feel colder and less comfortable.

4. Cracked or Dried Exterior Sealant

Exterior sealant protects the installation joint from wind and rain. Over time, sunlight, frost, heat, and building movement can cause it to crack or pull away from the frame.

Even a narrow gap can become a path for cold air or wind-driven rain.

Check the exterior perimeter for cracks, missing sections, brittle material, and areas where the sealant has lost adhesion.

5. Poorly Sealed Installation Joint

The gap between the window frame and the wall is one of the most common hidden sources of drafts.

Problems occur when installers:

• use too little foam;
• leave empty cavities;
• apply insulation unevenly;
• fail to protect the foam from moisture;
• rely only on decorative silicone.

In this case, the window itself may be functioning properly, while air enters through the wall joint and appears around the sill, reveals, or trim.

6. Missing or Faulty Flashing

Flashing directs rainwater away from the window opening. If it is missing, damaged, or incorrectly installed, water can enter behind the frame or facade.

Typical signs include:

• damp corners;
• stains around the window;
• peeling paint;
• leaks during heavy rain;
• mold around the reveals.

The visible moisture may appear far from the actual entry point because water can travel behind plaster, insulation, or cladding.

7. Blocked Drainage or Weep Holes

PVC windows often include small drainage openings at the bottom of the frame. These allow water inside the profile channels to escape outdoors.

Dust, insects, old sealant, paint, or construction debris can block them. Water then accumulates and may eventually leak indoors.

These openings should be cleaned carefully and must never be sealed with silicone.

8. Frame Distortion or Warping

If the frame becomes bowed, twisted, or warped, the sash can no longer press evenly against it.
Possible causes include:

• incorrect fastening;
• poor shimming;
• excessive tightening;
• temperature changes;
• structural pressure;
• aging or low-quality profiles.

Common symptoms are difficult locking, sash rubbing, uneven gaps, and stronger drafts on one side.

9. Building Settlement

cracks around the frame;
• damaged sealant joints;
• sash misalignment;
• uneven pressure;
• leaks that appear years after installation.

If several windows or nearby doors become difficult to close, the problem may be structural rather than limited to one window.

10. Hidden Damage in the Surrounding Wall

Sometimes the window is not the real source of the problem.

Hidden damage around the opening may include:

• cracked masonry;
• deteriorated plaster;
• wet insulation;
• previous water damage;
• unstable framing;
• rot in timber elements.

Warning signs include recurring stains, mold, crumbling material, musty odors, or leaks that return after resealing.

In such cases, simply adding more sealant will not solve the underlying issue.

Final Takeaway

A closed window is not always an airtight or watertight window. Drafts and leaks may come from worn seals, weak sash pressure, failed glazing, blocked drainage, damaged sealant, poor installation, or movement in the surrounding wall.

Start with the simplest causes first: check the gasket, clamping pressure, exterior sealant, and drainage openings. If the problem appears to come from the installation joint, flashing, frame distortion, or the wall itself, professional inspection is usually the safest solution.

Correct diagnosis often makes it possible to repair the problem without replacing the entire window.