Thousands face damp season after season. Relief starts with habit.
When warm, moist indoor air meets a chilled pane, water appears as if from nowhere. It is physics, not a fault in the glass. The change that stops those morning tears is small and cheap. It also makes rooms healthier to breathe in and cuts the risk of mould taking hold behind furniture and curtains.
Why winter makes your windows cry
Indoor air stores moisture from cooking, showers, drying laundry and simple breathing. The cooler the surface, the easier that water drops out. At 20 °C with 60% relative humidity, the dew point sits near 12 °C. Nudge humidity to 70% and the dew point rises towards 14 °C. If your window surface falls below those temperatures overnight, droplets form.
A modern, airtight frame keeps heat in, but it also traps vapour. Heating does not remove moisture by itself. Without a steady escape route, humidity climbs, and the glass becomes the first cold surface to show it.
Keep living spaces near 45–55% relative humidity. Below this, skin dries. Above this, mould and mites thrive.
How much moisture do people add?
- Four people breathing overnight: 2–3 litres of water.
- Showers and baths: 1–2 litres per person.
- Cooking without lids: 1–3 litres per meal.
- Drying clothes indoors: 2–4 litres per load.
The small handle move that changes your mornings
Most recent PVC or aluminium windows include a micro‑vent or trickle setting. Turn the handle to a mid position (often around 45°) so the sash opens just a few millimetres. That slim gap renews air without a gale. If your frame lacks this feature, a 2–3 mm latch or ventilator strip achieves the same effect. Leave it running while the heating is on.
This gentle bleed carries out a little moisture all the time, so panes stay above the dew point more often. It also reduces the need for energy‑wasting purge openings later.
A near‑invisible 2–3 mm opening can end daily window mopping and cut the odds of mould by keeping RH stable.
Set and check, don’t guess
Put a small hygrometer by a window and watch the numbers settle through the day. Aim for 45–55% in living areas. Keep curtains 3–5 cm off the glass to allow air to wash the pane. Do not drape laundry over radiators. Clear trickle vents and dust the bathroom extract grille. Use pan lids and switch on the cooker hood. These minor steps add up.
When the fog stays: escalate wisely
Respond to what your hygrometer shows. Use this simple playbook:
- Below 45% RH: close the micro‑vent slightly to avoid over‑dry air.
- 45–55% RH: keep the current setting. You are in the comfort band.
- Above 55% RH: open opposite windows for a 10‑minute cross‑flow, then increase the micro‑vent a touch.
Hardware upgrades that calm condensation
If high humidity keeps returning, look at the building, not just habits. Target ventilation first, then cold surfaces.
| Measure | What it does | Effect on condensation | Energy note |
|---|---|---|---|
| Trickle vents / micro‑vent | Continuous low background air change | Prevents daily fogging if sized well | Small heat loss, steady comfort |
| Humidity‑sensing extract (VMC hygro) | Boosts automatically when rooms get damp | Targets bathrooms and kitchen peaks | Efficient, needs filter checks |
| Heat‑recovery ventilation (VMC double flux) | Exchanges stale air while warming incoming air | Strong control in tight homes | Recovers heat, higher upfront cost |
| Double or triple glazing | Raises inner pane temperature | Cuts surface cooling, less dew | Improves comfort near windows |
| Warm‑edge spacers | Reduces thermal bridge at glass perimeter | Less misting at edges | Best when replacing units |
Do a quick health check of your windows. If you see fog between the panes, the sealed unit has failed and traps moisture; replace it. Perished gaskets and cracked sealant allow cold air to leak across the frame and cool the glass. A plug‑in dehumidifier can help during a cold snap, but it treats symptoms, not causes. Treat any musty odour early; spores settle on dust behind wardrobes and under sills fast when RH sits above 60%.
Dew point governs the show: at 20 °C and 70% RH, any surface near 14 °C will mist. Warm the surface or lower RH.
Does a tiny gap waste heat?
Background airing does carry heat out, but the trade‑off often favours comfort and fabric. A small, steady flow limits extreme spikes of humidity after showers and cooking, so you avoid repeated wide‑open blasts that dump far more heat in a short time. In practice, households that keep RH stable report fewer fogged panes, shorter bathroom fan run‑times, and less need for dehumidifiers.
For context, consider a 70 m² flat with a volume near 175 m³. A very small micro‑vent might add roughly 15–25 m³ of fresh air per hour. With a 12–15 °C temperature difference, that equates to a modest, predictable heat load. A ten‑minute cross‑vent with two windows can shift several times that volume at once. The gentle option keeps temperatures even and moisture under control.
Practical winter checklist for clear panes and clean air
- Use micro‑ventilation continuously during the heating season; adjust by what your hygrometer shows.
- Shower with the door closed; keep the extract running for 15 minutes after you finish.
- Cook with lids; start the hood early and let it run on after heat is off.
- Dry clothing in a single room with the window slightly open, or use a vented space.
- Leave a small gap behind large furniture on external walls to avoid cold spots.
- Vacuum trickle vents and extract grilles each month to maintain airflow.
- Do not cover radiators; let warm air rise across the glass.
Health angles many people miss
Moist homes feel colder even when the thermostat reads the same. Damp air slows sweat evaporation on skin and encourages dust mites. Those mites flare allergies when RH sits above 60%. Keeping to 45–55% helps asthma sufferers, protects timber frames from swelling, and reduces black spotting in silicone joints.
Renters and shared homes
If you rent, you may not control the hardware, but you can still manage habits and measurements. Ask for working trickle vents, a serviced extract fan, and a simple hygrometer. Share a rota for lids, fans and drying to prevent one person’s laundry from raising humidity across the entire flat.
Spot and fix cold bridges
Edges and corners cool first. Look for grey shading or flaking paint near reveals. Consider thin insulating lining on cold external walls behind heavy furniture. When replacing a window, request warm‑edge spacers to keep the perimeter from chilling.
A final note on numbers: set your own baseline for a week. Log RH morning and evening in the living room and main bedroom. Note events that push the readings up. Then run a 2–3 mm micro‑vent for the next week, keep curtains off the glass, and repeat the log. Most homes see stable readings inside the 45–55% band and far fewer wet mornings. That stability protects furnishings, paint, and lungs, one quiet millimetre at a time.
