Condensation is the most common kind of damp in homes and the least understood. No water is getting in from anywhere: the house itself produces it, through the people living in it, and it settles on the coldest walls. That's why it appears in winter, concentrates in bedrooms and doesn't improve however much you change the paint unless you also deal with the air. This guide explains how it works, the usual causes and how to reduce it layer by layer.
How condensation forms
Warm air holds more water vapour than cold air. When the air in a room touches a surface colder than its dew point, the surplus vapour turns to liquid on that surface. An example: air at 20 °C and 70% relative humidity has a dew point of around 14 °C. Any part of the wall at 14 °C or lower gets wet. And a home has plenty of spots like that.
A family of four produces between 10 and 15 litres of vapour a day. Breathing and sweating at night account for half a litre per person; a shower, about a litre and a half; cooking, two or three litres; drying clothes indoors, another two or three. If that vapour doesn't get out, it stays in the air and finds the coldest wall.
The causes, from most to least common
- Lack of ventilation. Modern PVC or thermally broken aluminium windows seal very well. That's good for noise and energy, but it removes the air exchange that gaps used to provide. Many renovated homes start getting condensation right after the windows are replaced.
- Thermal bridges. Columns, slab edges, lintels, shutter boxes and corners are where insulation is interrupted or missing. They're several degrees colder than the rest of the wall. Mould traces those lines: a vertical band where there's a column, a horizontal strip where the wall meets the ceiling.
- Uninsulated walls. Concrete block or brick with a coat of render and little else. The whole north-facing wall is a thermal bridge.
- Furniture against outside walls. A wardrobe against a north wall stops air circulating and keeps the wall from getting any warmth from the room. Behind the wardrobe the wall is colder and damper.
- Vapour produced without extraction. Kitchens without an externally vented hood, bathrooms without an extractor, laundry hung in the hallway.
MATE5000 AntiHumedad
Breathable, anti-condensation interior paint with a deep matt finish.4 L and 15 L · 12 colours · 5–7 m²/L
Why it appears in winter, in the Canary Islands as well as the north
On the mainland, condensation comes with the heating: indoor air warms up and fills with vapour, windows are kept shut, and the temperature difference between that air and the outside wall shoots up. The colder outside and the more closed up inside, the more condensation.
In the Canary Islands the mechanism is different but the result is the same. Hardly anyone has heating, so indoors in winter it's 16–18 °C with 70–80% relative humidity, because the outside air is already loaded by the trade winds. Uninsulated block walls drop to 13–14 °C at night in the midlands and on north-facing slopes. With those numbers, the dew point is reached effortlessly. That's why in La Laguna, Tacoronte, La Orotava or upper Las Palmas, mould in bedrooms is almost routine between November and March. We go into detail in the guide to damp at home in the Canary Islands.
How to recognise it
| Sign | What it indicates |
|---|---|
| Drops on the windows in the morning | Too much vapour in the air, not enough ventilation |
| Mould in upper corners and on the ceiling | Thermal bridge where wall meets slab |
| Vertical band of mould | Uninsulated column |
| Mould behind wardrobes or headboards | Cold wall with no air circulation |
| Scattered black spots, no tide mark | Condensation, not water ingress |
| Improves in summer | Confirms a thermal origin |
If the stain has a clear edge, grows after rain or there's white powder low down on the wall, it isn't condensation: check water ingress and salt bloom, or use the guide how to tell what kind of damp you have in 5 minutes.
A layered solution
There's no single fix. What works is adding up several measures, from cheapest to most expensive, until the problem eases.
Layer 1: produce less vapour
- Put lids on pans and use a hood that vents outside.
- Shower with the door shut and the extractor on, or the window open, during and after.
- Don't dry clothes indoors; if there's no alternative, do it in a closed room with the window open or with a dehumidifier.
Layer 2: ventilate effectively
- Cross-ventilation for 5 to 10 minutes, twice a day, opening windows on opposite sides. It works better than leaving a window ajar all day, and cools the house much less.
- Air the bedroom as soon as you get up: that's when the air holds the most vapour.
- In very airtight homes, consider mechanical ventilation (humidity-controlled extractors or MVHR). It's the measure with the biggest impact in the medium term.
Layer 3: control the humidity in the air
A €10 hygrometer tells you where you stand. The aim is to stay between 40 and 60% relative humidity. If you can't get below 70% even with ventilation, a dehumidifier in the bedroom overnight makes a real difference, especially in the Canary Islands where heating isn't realistic.
Layer 4: the wall surface
This is where paint comes in. A plastic paint forms a closed film that water sits on; a paint with biocide only delays the fungus while the biocide lasts. MATE5000 is a breathable, anti-condensation interior paint with a deep matt finish: according to its technical data sheet, it helps reduce the surface conditions that encourage condensation. A 4 L tin covers around 10–14 m² in two coats (5–7 m²/L per coat) for €19.02. For a single room, Pared Seca in a 2 L DoyPack does the same job. They don't remove the vapour from the room: that's why they go together with layers 1 to 3, not instead of them. We explain it in anti-condensation paint: what it does and doesn't do.
Layer 5: building work on the thermal bridge
If one particular area condenses every night despite all of the above, the problem is thermal and the solution is insulation: insulation panels on the inside of that wall, or external wall insulation (ETICS) if the building allows it. That's building work and should be assessed by a professional. A breathable paint is still the right finish afterwards.
Recommended order
- Clean off the existing mould (the procedure is in how to remove mould from a wall).
- Start layers 1 and 2 today; they cost nothing.
- Paint with MATE5000 once the wall is dry. Keep furniture 5 cm away from outside walls.
- Measure humidity for a month. If it stays above 65%, add a dehumidifier or mechanical ventilation.
- If one area keeps condensing, ask a technician to assess the insulation of that wall.
You'll find the full treatment, with products and quantities, on the condensation solution page; if you want it all together, the Condensation Kit combines Sellador Total L44 750 ml and MATE5000 4 L for €26.69, 6% less than buying them separately. And if you'd rather we took a look before you buy, send photos of the area and the window: that usually makes it fairly clear whether it's condensation or something else.
Guía escrita y revisada por el Equipo Humix Paint con los datos de las fichas técnicas de cada producto. La pintura actúa en la superficie: si el agua sube del suelo o entra de fuera, primero se trata la causa.
