A chill hits the glass, and the glass hits back. Wet sills, musty corners, and the heating never quite feels right.
Across the country, people wake to fogged panes and damp frames. The culprit sits in plain sight. Warm, wet indoor air meets a cold surface, drops its load as droplets, and sets off a chain of mould, smells and peeling paint. A tiny adjustment to airflow can shift that balance, keep the glass clear, and make rooms feel healthier.
Why your panes drip when the heating goes on
Warm air carries more water vapour than cold air. When that air touches a chilled pane, it releases water on the glass. That’s condensation. The temperature where this change starts is the dew point. It moves with humidity. At 20 °C and 60% relative humidity, the dew point sits near 12 °C. At 70%, it climbs to about 14 °C. A pane that falls below those numbers will mist and bead.
At 20 °C, 60% RH gives a dew point near 12 °C; at 70% RH it nears 14 °C. Keep glass above that, or lower the humidity.
Homes generate a lot of moisture. A family of four releases around 10–12 litres a day from breathing, cooking, showers and drying laundry. Modern windows seal well, so the moisture can build up fast. Heating alone does not remove water vapour. It only holds it in warmer air, which then dumps it on the next cold surface.
The simple handle position that dries the glass
Many modern PVC or aluminium frames include a micro‑vent position. You set the handle around 45 degrees. The sash shifts by a few millimetres, roughly 2–3 mm. That small, steady gap refreshes the air without a draught. Leave it engaged while the heating runs. The moisture level drops into a safer band. The panes often stay clear, even before sunrise.
Set the handle to micro‑vent and keep it there during heating hours. A 2–3 mm gap can stop the morning tears.
How to set it
- Turn the window handle about 45° until the sash sits very slightly off the frame.
- Check the seal: you should feel a faint movement, not a visible opening.
- If your hardware lacks this feature, fit a 2–3 mm restrictor or use trickle vents where available.
- Keep curtains 3–5 cm off the glass so air can wash the pane.
- Do not cover radiators under windows. Heat must reach the glass to lift its surface temperature.
What it changes
The tiny gap raises the air change a little, which whisks away vapour as you produce it. The drier indoor air holds less moisture, so the dew point falls. The glass surface also warms better when the radiator can breathe under it. Together, these two shifts keep the pane above the dew point for more hours of the day.
Check it with a £10 hygrometer
Place a small digital hygrometer near a window. Aim for 45–55% relative humidity in living spaces. That range helps prevent mould growth, slows dust mites, and still feels comfortable.
Target 45–55% RH at 18–21 °C. That band protects health, paintwork and window frames.
Aim for healthy humidity, not desert air
Use humidity readings to steer your venting. Small, steady actions beat big, daily purges that waste heat.
- Below 45% RH: reduce the micro‑vent gap a touch to avoid overly dry air.
- Between 45% and 55% RH: hold your settings steady.
- Above 55% RH: run a 10‑minute cross‑vent with two windows wide open, then increase the micro‑vent gap slightly.
Cut moisture at the source as well:
- Fit pan lids and run the cooker hood to outside while cooking.
- Shut the bathroom door, open the window or run the extractor during and after showers.
- Dry laundry outside or use a vented or condenser dryer; avoid racks in small rooms.
- Vacuum and dust extractor grilles; unblock trickle vents.
Dew point quick guide at 20 °C
| Relative humidity | Dew point | What your pane must beat |
|---|---|---|
| 50% | ≈ 9.3 °C | Keep glass above 9–10 °C |
| 60% | ≈ 12.0 °C | Keep glass above 12 °C |
| 70% | ≈ 14.0 °C | Keep glass above 14 °C |
| 80% | ≈ 16.4 °C | Keep glass above 16–17 °C |
When the fog persists, upgrade ventilation and glazing
Stubborn moisture points to a bigger mismatch between humidity and cold surfaces. You can fix both sides.
Ventilation that reacts to moisture
- Humidity‑sensing fans or dMEV/MEV (the UK equivalent of VMC) boost extraction as RH climbs in wet rooms.
- MVHR (heat‑recovery ventilation, similar to “double‑flux”) replaces stale air and transfers warmth to the incoming air.
- Service fans and ducts annually. Clean filters so the system keeps its airflow.
Warmer glass, fewer cold bridges
- Upgrade to double or triple glazing if panes feel icy to the touch in mild weather.
- Choose warm‑edge spacers to reduce edge‑of‑glass cold spots that start the beads.
- Replace tired gaskets and check hinges so the sash seals properly when closed.
Use a portable dehumidifier as a tactical tool in small, damp rooms. Set the target to about 50% RH and drain it often. Treat any black spots with a suitable cleaner and fix the moisture source to stop regrowth.
Musty odours, clouding between double‑glazed panes and flaking paint signal chronic moisture. Act early to protect lungs and finishes.
Energy and health: what this tweak costs and saves
Small, steady ventilation does carry a heat cost. For a typical 70 m² flat with 2.5 m ceilings (≈175 m³ air volume), a micro‑vent might lift infiltration by roughly 0.1 air changes per hour. With a 10 K temperature difference, that adds around 60 W of heat load, or about 1.4 kWh per day. Across a 180‑day heating season, the total lands near 250 kWh.
You trade that for fewer mould patches, less repainting, fewer damp smells, and clearer glass. People with asthma or allergies often breathe better when RH sits near 50%. Carpets and clothes dry faster. Rooms feel fresher in the morning, so you avoid long, cold bursts of purge ventilation that dump much more heat in one go.
Practical checks people often miss
- Security: some frames lock in micro‑vent mode, some do not. Test yours before leaving home.
- Noise and pollution: time your purge vents when traffic is low; keep micro‑vent and use a HEPA purifier on bad days.
- Radiator balance: bleed radiators and set flow so the one under the window runs warm. Cold rads leave glass vulnerable.
- Curtain fit: use shorter or lighter fabrics around bays so air can rise freely across the pane.
If you need a quick diagnostic at home
Stand by the window at dawn, when panes run coldest. Note the pattern. Beads at the top edge suggest high indoor RH. Beads at the lower corners often point to cold bridges or blocked convectors. Condensation between two panes means the unit has failed and needs replacement.
Run a two‑week test with a hygrometer. Log morning and evening RH. Try the micro‑vent setting, lids and extract fans. Aim for 45–55% RH on most days. If you cannot reach that band, plan a ventilation upgrade or call a pro for a survey.
Extra help for winter living
Set bedroom temperatures to 17–19 °C and keep doors cracked slightly to share dry air from living areas. In kitchens and bathrooms, fit auto‑run fans with overrun timers. In older homes, add trickle vents where frames allow, and seal obvious drafts at skirtings while keeping planned ventilation open.
For landlords and blocks, schedule seasonal checks: clear communal ducts, confirm fan flow rates, and educate residents on RH targets. A simple card by the window that says “45–55% RH, handle at 45° in winter” prevents complaints and protects the building fabric.
Just tried the 45° handle trick and my bedroom window stopped crying this morning. Who knew 2–3 mm could save paintwork and my sanity? The £10 hygrometer tip is gold—sitting nicely at 48% now. Also moved curtains 4 cm off the glass and unblocked the rad: night-and-day difference. My towels even dry faster 🙂
Interesting, but is the ~250 kWh seasonal loss assuming 0.1 ACH realistic for older leaky flats? Feels like we’d be venting much more already. Also, how do you balance security if micro‑vent doesn’t lock—any recommended restrictors that actually meet insurance standards?