We've covered the "golden rule" of winter ventilation: 5 minutes of cross-ventilation, 3-4 times a day. Perfect theory. But what happens in practice?
You leave at 8:00 for work and return at 19:00. The home stays hermetically sealed for 11 hours. Moisture gets trapped, the air goes stale and conditions for mould become ideal. The solution? Trickle vents.
A trickle vent is a very discreet, narrow slot (aluminium or plastic) at the top of the window frame. It allows a minimal, controlled amount of fresh air to pass even when the window is fully locked.
Most trickle vents need no electricity. They work passively, exploiting the pressure and temperature difference between indoors and out (stack effect). A small adjustable flap lets you open them fully, halfway, or close them in a storm. Most also include an internal insect mesh, so you needn't worry about mosquitoes or dust.
The evolution of basic trickle vents - they feature a humidity sensor (polyamide strip) that reacts automatically:
If you shower or cook and humidity climbs to 70%, the vent opens by itself fully to flush out vapour. The polyamide strip reacts to changes in relative humidity in under 10 minutes.
When the room dries (humidity below 45%), the vent closes almost entirely so you don't lose heat. The home literally breathes on its own, with zero intervention from you.
In the UK they are mandatory by law. In Greece, the KENAK doesn't yet require them for simple renovations. But are they worth it? Let's weigh up:
Mould prevention: 24/7 background ventilation. Security: ventilate without open windows - ideal for ground floors. Hygiene: expels CO2, odours and VOCs from furniture and cleaning products.
Slight noise reduction loss: any opening in the frame, however small, lets some noise through. On busy roads, choose acoustic trickle vents with internal absorbent material that reduces noise by 6-8 dB. Minor draught: on very cold days, sitting directly below may cause a slight cool draught.
The vent is usually fitted at the top of the frame or the sash. Some models are wall-mounted just above the window (wall slot vents) - ideal for renovations where you don't want to modify the aluminium.
Each vent is characterised by its airflow rate in m³/h. For a 15 m² bedroom you need around 8-12 m³/h. A 30 m² living room may require two vents on different windows.
If you're planning a deep energy retrofit (e.g. "Exoikonomo"), request trickle vents in your new windows from the start. Retrofitting them later costs 30-50% more due to modifications required. Choose hygro-adjustable vents (Aereco type or equivalent) for maximum energy savings.
If you're sealing your home with Class 4 airtight windows and external insulation, the answer is a clear yes. Without trickle vents or mechanical ventilation, mould is mathematically certain. Trickle vents are the cheapest insurance policy against moisture.
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