Condensation and mould at home: why they appear and how to get rid of them

Black patches in bedroom corners, grey spots on the ceiling near the window, clothes in the wardrobe smelling musty. In Portugal, condensation mould is probably the most common defect in homes — and it often gets worse after the windows are replaced. The good news: once you understand the physics behind it, it can be solved.

The physics in two lines

Air contains water vapour. The warmer the air, the more vapour it can hold. When humid air touches a cold surface it cools down; if it drops below the dew point, the vapour turns into liquid water — just like a glass of cold water in summer or window panes in the morning.

In a home, vapour comes from people (breathing), showers, cooking and drying clothes indoors. A family produces several litres of water as vapour every day. If that vapour cannot escape, it seeks out the coldest surfaces to condense on.

Mould doesn’t need running water to grow: it is enough for the relative humidity next to the surface to stay high for long periods (an international reference criterion points to average values from around 80% at the surface). Dust, paint and wallpaper provide the food.

Why certain areas get mould

• Thermal bridges: concrete columns, beams, slab edges and roller-shutter boxes conduct heat better than brick walls; they are colder and are the first to show stains, often in the exact shape of the concrete element.
• Corners: more external surface losing heat and less air circulation.
• Behind furniture and inside wardrobes placed against exterior walls: air doesn’t circulate and the wall stays colder.
• Rooms that are barely heated (cold bedrooms next to a warm living room): vapour migrates there.

Why it got worse after replacing the windows

Old window frames let air in through every gap: uncomfortable, but they ventilated. New, airtight windows cut off those air inlets. Unless controlled air inlets (trickle vents in the frames or walls) and proper extraction in kitchens and bathrooms are provided, moisture gets trapped inside the home.

Diagnosis: is it really condensation?

• It gets worse in winter and eases in summer.
• Black or greenish mould, without white salts.
• It appears on any floor, not just the ground floor.
• Wet window panes in the morning.
• An (inexpensive) thermo-hygrometer often shows indoor relative humidity above 60–65% in winter; the comfortable and healthy range is around 40–60%.
• A thermal imaging camera reveals the thermal bridges and cold spots.

If the stain is at the base of ground-floor walls, with salts, read our guide to rising damp; if it appears after rain, suspect a leak.

Solutions, from the simplest to the most complete

1. Produce less vapour (and remove it at source)

• A kitchen extractor hood vented outside, running while cooking (and for a few minutes after); lids on pans.
• Bathroom extraction during and after showering; door closed.
• Avoid drying clothes indoors; if you must, do it in a closed, ventilated room.

2. Ventilate continuously and in a controlled way

• Open windows on opposite sides for a few minutes a day, even in winter — short, intense ventilation loses little heat.
• Install self-regulating trickle vents in bedroom and living-room window frames.
• Continuous low-rate extraction in bathrooms and the kitchen.
• In major renovations, consider MVHR — mechanical ventilation with heat recovery: it renews the air constantly and recovers much of the energy from the extracted air.

Portugal’s residential energy performance regulation (REH, part of the national Energy Certification System) requires a minimum air change rate precisely for reasons of air quality and condensation control.

3. Warm up the cold surfaces

• External wall insulation (ETICS, known locally as “capoto”): the most effective solution, because it wraps columns, beams and slab edges and eliminates thermal bridges.
• Internal insulation: possible when the façade can’t be touched, but it requires care with continuity and vapour control (a vapour check or barrier), so as not to move condensation inside the wall.
• Insulated roller-shutter boxes and thermally broken frames with double glazing — always combined with ventilation.
• Keep heating more steady rather than in short bursts.

How to clean mould (and when it isn’t enough)

1. Protect yourself: gloves, mask and an open window.
2. Apply a proper fungicidal product or a solution of diluted bleach; leave it to act and wipe clean. Never mix bleach with ammonia or acids.
3. Let it dry completely.
4. Paint with a fungicidal paint, preferably vapour-permeable.

This treats the symptom. Without correcting ventilation or thermal bridges, the mould will be back next winter.