Across the country, people are parking handles on the first notch of a tilt window to “let a little air in”. It feels sensible. It is not. That tiny opening creates a constant flow that bleeds warmth, churns damp, and makes your boiler and radiators work far harder than you think.
The tiny gap that drains your heat
The tilt position looks harmless. A 2 mm slit across the top edge seems like a neat compromise between stale air and comfort. In winter, that slit acts like a siphon. Warm air escapes high, cold air dives low, and the room never settles. You nudge the thermostat, pull on a jumper, and still feel a chill on the neck. The system fights a battle it cannot win because you have left a door open to the outside, even if it is only a finger’s breadth.
Meters move faster with a permanent gap than with a short, sharp blast of fresh air twice a day.
What a 2 mm gap really does
Physics sets the rules. The difference between indoor and outdoor temperature drives a continuous convective loop through that small opening. If the radiator sits under the window, it senses the cold stream and cycles rapidly. That local overheating near the valve fools the system, pushing it to fire more often while the rest of the room still feels cool. The result is higher consumption and a persistent draught that your body reads as discomfort.
Moisture complicates the picture. A thin trickle does not refresh the full room volume. Instead, it chills surfaces, which encourages condensation on the glass and colder walls. You see beads on the bottom edge of double glazing and a slight musty note by evening. The air feels both cooler and clammy because the envelope is cooling while the damp load remains.
The five-minute fix
There is a simple routine that works in homes, flats and student rooms alike. Open wide. Create a cross-breeze. Close firmly. Short, decisive ventilation removes humid air and stale odours without letting the structure cool down. Then the heating brings the space back to setpoint quickly, and it stays there.
- Open fully for 5–10 minutes, twice a day if possible, with doors ajar to create a through-current.
- Turn the thermostat or TRV down one notch while airing, then set it back once the window is closed.
- Close until the final click. Test with a sheet of paper; if it slides out easily, the window is not sealed.
- Keep curtains clear of radiators and sills; fabric over a heat source traps warmth behind glass.
- Leave trickle vents unblocked; they are designed for controlled background air, not a constant leak.
Better to shock the air for minutes than to leak heat for hours.
Money, moisture and a winter of small losses
How much can a small opening cost? Building physics offers a clear picture. In a typical 12 m² room with a 2.4 m ceiling, a 2 mm tilt gap along the head of a window can add roughly 0.5 air changes per hour in a cold spell. With a 15–20°C difference between inside and out, that equates to around 1.8–2.4 kWh of extra heat per day for that room alone. Multiply by a four‑month heating season and the sums start to sting.
| Window gap | Extra heat loss per day | Cost per day at £0.28/kWh | Approx. cost over 120 days |
|---|---|---|---|
| 2 mm (tilt first notch) | 1.8–2.4 kWh | £0.50–£0.67 | £60–£80 per room |
| 5 mm (generous tilt) | 3.8–4.5 kWh | £1.06–£1.26 | £127–£151 per room |
| Two rooms, mixed gaps | — | — | £180–£320 for the season |
These are scenario estimates. Wind exposure, window size, boiler type and actual indoor temperatures shift the final figure. Yet the direction is consistent: a small, permanent opening nudges costs up week after week while also inviting condensation and the mould risk that follows.
Why radiators work harder than they should
Thermostats, cycling and cold downdrafts
Most homes rely on thermostatic radiator valves (TRVs) near the window. Cold air falling from a tilt gap washes over the valve. The sensor thinks the room is cooler than it really is, so the radiator surges. Minutes later the valve throttles back as the area overheats, then repeats. That stop‑start pattern wastes gas or electricity and leaves you with alternating bursts of too warm and too chilly.
Boilers dislike this rhythm. Frequent short burns reduce efficiency and can lift maintenance issues over time. In flats with electric panel heaters, the effect is similar: rapid cycling with little gain in comfort. The quiet hiss you hear is money leaving the building.
Common signs you are leaking heat
- Condensation beads on double glazing by evening despite the heating running.
- A fine draught over the kitchen sink or down a hallway near the window line.
- The cat or dog avoids the sill because it feels cold even when the room reads 20°C.
- TRVs click and radiators pulse even at steady thermostat settings.
- You keep nudging the dial up through the day, then wake up to a stuffy bedroom.
Make the air work for you, not against you
Set a simple winter ritual. Two decisive airings beat a day‑long dribble. Aim for indoor humidity between 40% and 60%. A basic hygrometer costs a few pounds and tells you when to air. Dry laundry by a window only when you can open wide for ten minutes. Use lids on pans. Shut bathroom doors and use extract fans after showers. These small moves cut moisture, which means less condensation and less heat needed to feel the same comfort.
Check the hardware as well. Replace tired seals. Adjust hinges on a warped sash so the handle reaches its final detent. Lubricate the mechanism so you get that firm last click. If your window has trickle vents, keep them open; they provide a controlled, tiny supply without the chill of a permanent tilt. If you rent, report failed seals and loose handles; a 15‑minute fix can change your winter.
Numbers you can use in your own home
Want a quick test? Close the window fully and hold the back of your hand along the head of the frame on a windy day. If you feel a cold thread, your seal needs attention. Place a small desk fan by one open window and another window open across the room for five minutes; you will refresh the air faster and close sooner. Time how long your room returns to comfort after a full airing; note the thermostat setting needed. Repeat the same day with a micro‑opening left for an hour; the contrast in feel and in cycling noise will surprise you.
For a rough cost sense, pick a room and apply this rule of thumb. If a tilt gap adds around 2 kWh per day over 120 winter days, that is roughly £67 at a 28p unit rate. Two rooms doubles it. Add boiler inefficiencies from rapid cycling and you can reach £312 across a household that keeps several windows on the first notch. Close the gap and that money stays with you.
Open wide, cross‑ventilate, and close tight: short, crisp actions that raise comfort while lowering bills.
There is a bigger gain too. Air quality improves when you ventilate with intent. Carbon dioxide drops quickly during a full airing, and odours leave with it. Then the fabric warms evenly, the draughts stop, and the evening feels calmer. If you want to go further, set TRVs to 18–19°C in bedrooms and 19–20°C in living areas, and use door gaps to balance flow. The system settles, and so do you.
One last note for safety: never block fixed vents serving gas appliances, and always keep extractors working in kitchens and bathrooms. Controlled background air keeps combustion safe and reduces moisture loads. Combine that with short, decisive window openings and you will save energy without sacrificing healthy air.
