You’ve probably noticed it while cleaning your windows: a tiny hole at the bottom of the frame, sometimes two or three. Many people mistake it for a manufacturing defect or a crack in the PVC. The temptation to seal it with silicone to avoid drafts is strong. Big mistake: that hole is there to protect you, and blocking it can literally rot your home from the inside.
These openings are called drainage holes, sometimes also air inlets. They’re present on virtually all modern PVC windows and glass doors, deliberately installed by the manufacturer. Their purpose: to let water that naturally infiltrates the frame escape, and to allow minimal air circulation to prevent humidity from stagnating. Blocking them traps water inside the frame and deprives the room of essential natural ventilation.
What are these small holes at the bottom of your windows really for?
Drainage of rainwater and condensation
Even with properly installed double glazing, water always accumulates slightly at the bottom of the window frame. Heavy rain, water streaming down the pane, condensation forming at night: this water needs to drain out. The drainage openings are precisely drilled at the lowest point of the frame to let water flow outward instead of stagnating inside the profile.
Humidity regulation and natural ventilation
Beyond drainage, these air inlets contribute to discreet natural ventilation. They allow a small amount of outside air to circulate, which limits fogging on the glass and helps regulate indoor humidity levels. This isn’t a substitute for mechanical ventilation, but a complement that prevents completely confined air pockets near windows.
Why you should never block these openings
This answers the question everyone asks: no, you should never seal these holes, even with caulk, tape, or expanding foam—a mistake that ranks among the 5 errors that let moisture into your home when the first rains return. A window designed to drain water and suddenly unable to becomes an airtight trap, with consequences that appear within weeks.
Risks of humidity, condensation, and mold
Without drainage, water and vapor remain trapped in the frame. Humidity rises, condensation intensifies on the glass and adjacent walls, and mold often appears within months, particularly in corners and along joints. These fungi aren’t just unsightly: they degrade indoor air quality and can trigger respiratory issues in vulnerable people.
Water infiltration and costly damage
When water has no outlet, it eventually seeps back and infiltrates the home at the junction between the window frame and wall. Result: peeling paint, crumbling plaster, swollen wood on wooden windowsills. The expensive repairs that follow (mold treatment, resealing, sometimes complete frame replacement) far exceed any heating savings imagined by plugging the hole.
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The mistake that costs big money without you realizing it
A blocked hole today causes no visible damage. That’s precisely the problem: humidity works silently for months before mold spots or swollen wood appear. By the time it’s visible, the damage is already embedded in the wall.
Why so many people block them (and how to avoid this mistake)
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Scan Your Plant — Download Free AppThe reason is almost always the same: a sensation of cold air around the feet or legs in winter, combined with the idea that this hole causes heat loss (a simple paper test to detect which windows let in cold air actually lets you verify the real airtightness of your frames). In reality, these openings are calibrated to allow minimal airflow, far from representing significant heat loss on modern well-insulated frames. The real draft usually comes from worn seals or poor weatherproofing elsewhere on the frame, not from this little hole.
Before reaching for the silicone, it’s better to check the window seals, the condition of weatherstripping around the frame, and whether the handles close properly. In most cases, these elements explain the sensation of cold, not the drainage holes themselves.
How to maintain these holes without blocking them
These openings require minimal but regular maintenance. When cleaning your windows, a cotton swab or a vacuum with a fine attachment is enough to remove dust, pollen, and small debris that accumulate over time. Checking twice a year, in spring and fall, prevents them from becoming naturally clogged, which would produce exactly the same effects as intentional sealing.
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| Situation | Right approach |
|---|---|
| Hole visibly clogged | Clean with a cotton swab or fine brush |
| Sensation of air drafts | Check seals and handles, never block the hole |
| Strong wind and heavy rain | Switch air inlets to winter mode if your model allows it |
Alternatives to blocking: reduce drafts without risk
Some air inlets located at the top of the window have a winter mode, an adjustable position that reduces airflow without eliminating it entirely. This is a genuine alternative to limit discomfort during cold periods, unlike the drainage openings at the bottom which must remain free year-round. For the rest, thermal curtains, draft stoppers, or a general check of your frame’s overall airtightness address the cold problem without compromising water drainage.
In an energy renovation context, replacing old windows with modern double glazing remains the most effective solution for thermal comfort, far more so than sealing holes designed to protect the building’s structure against moisture, just as closing shutters at the right time to retain heat in your home can complement these insulation efforts.
