Short answerMould appears in the corner of the bedroom ceiling because it's the coldest spot in the room and, at night, the vapour you produce while sleeping condenses right there. It's condensation, not a leak. To help keep it from coming back: clean off the mould, let it dry, prepare the area with a sealer, paint with a breathable, anti-condensation paint and ventilate every morning.
If the stain has a clear tide mark or grows when it rains, it isn't condensation: the water getting in has to be stopped first.
It's the question we get most in winter: "I clean the corner and a few weeks later the black spots are back". It isn't bad luck or bad cleaning. It's physics. This guide explains why mould always picks that corner, how to confirm it's condensation and the order of work to help keep it from reappearing in the same place.
Why the corner of the ceiling
A high corner brings together three things that encourage condensation:
- It's the coldest area. Where an outside wall meets the ceiling there's usually a slab edge or a column. The wall loses more heat there and stays several degrees colder than the rest. That's what's called a thermal bridge.
- Little air reaches it. Air moves less in corners. If there's also a wardrobe or a curtain nearby, the area barely warms up or gets ventilated.
- It gets the night-time vapour. Two people sleeping with the door and window shut fill the bedroom air with moisture. That warm air rises, touches the cold corner and the vapour turns to water.
With air at 20 °C and 70% relative humidity, the dew point is around 14 °C. If the corner is at that temperature or below, it gets wet every night. Mould spores are in the air of every home; they just need a surface that stays damp for a few days to grow. That's why, if you clean but change nothing else, mould comes back to the same spot.
Check that it's condensation
Before treating anything, make sure the water is coming from the air and not from outside. It's condensation if:
- It's scattered black spots, with no clear edge.
- It appears in autumn and winter and improves in summer.
- There are drops on the windows in the morning.
- It also appears behind furniture against outside walls.
- It doesn't grow after rain.
If the stain has a yellowish or brown tide mark, grows after rain or you're on the top floor under a flat roof, suspect water ingress. In that case, start with the roof: we explain it in how to waterproof a flat roof and in water ingress. If in doubt, the guide how to tell what kind of damp you have will clear it up, or send us a photo and we'll tell you for free.
Pared Seca
Breathable, damp-resistant and anti-condensation. It lets water vapour out of the surface instead of trapping it.2 L DoyPack · 12 colours · 10–12 m²/L per coat
Step by step to help keep it from coming back
1. Clean off the mould thoroughly
Put on gloves and a mask and open the window. Use a product made for cleaning mould stains and follow the instructions on its label: contact time, dilution and rinsing. Scrub with a sponge, not dry with a brush, so you don't stir up spores. Never mix cleaning products. There's more detail in how to remove mould from a wall.
2. Let the wall dry
It's the step most people skip. Ventilate the room and let the area dry completely before applying anything. The technical data sheets for our products call for a dry, sound and clean surface. In winter and with high humidity, expect it to take longer.
3. Prepare with Sellador Total L44
When a wall has already had fungi, it's worth preparing it. Sellador Total L44 is a water-based sealing primer that, according to its technical data sheet, regulates how much the surface absorbs and improves adhesion of the finish, without affecting breathability.
- As a primer, dilute 1:1 with water; undiluted it works as a sealer.
- Apply by brush, roller or spray, between +5 °C and +40 °C.
- Second coat or finish after 24 h.
- Coverage: 9–10 m²/L per coat. For the affected corner and a margin around it, the 750 ml tin (€9.57) is enough; for a whole wall and the ceiling, the 4 L tin (€46.17).
4. Paint with a breathable, anti-condensation paint
Paint the whole wall, corner to corner, not just the stain: otherwise the touch-up will show. Pared Seca is designed for walls and ceilings with condensation in bedrooms, bathrooms and kitchens. It lets water vapour escape from the wall instead of trapping it and, according to its data sheet, helps reduce the surface conditions that encourage condensation.
- Ready to use. 2 or 3 even coats, without overloading the roller.
- Second coat after 2–4 h (at 23 °C and 50% humidity). In colder or more humid conditions, wait longer.
- Coverage: 10–12 m²/L per coat, around 6 m²/L in two coats. The 2 L DoyPack (€23.90) covers around 12 m² in two coats.
If you're painting the whole bedroom, MATE5000 AntiHumedad in a 4 L or 15 L tin is the interior alternative, with a deep matt finish. The differences are in anti-condensation paint: what it does and doesn't do. With MATE5000, the Condensation Kit combines Sellador Total L44 750 ml and MATE5000 4 L for €26.69, 6% less than buying them separately.
5. Change what caused the mould
Paint works at the surface. You control the vapour in the room:
- Ventilate when you get up. Five or ten minutes of cross-ventilation (windows on opposite sides) replaces the stale night air without cooling the house.
- Move the furniture out. Leave about 5 cm between the headboard or wardrobe and outside walls.
- Measure. A hygrometer tells you the relative humidity. Somewhere between 40 and 60% is reasonable.
- If it won't come down, dehumidify. A dehumidifier in the bedroom overnight helps a lot, especially in places without heating like much of the Canary Islands.
- Don't dry clothes indoors and leave the door open during the day so air can circulate.
What it costs to treat a corner
Example: the outside wall of a bedroom, 3.5 m wide by 2.5 m high (8.75 m²), with mould in the corner of the ceiling.
| Product | Quantity | Price |
|---|---|---|
| Sellador Total L44 | 750 ml for the affected area and around it | €9.57 |
| Pared Seca | 1 × 2 L DoyPack, two coats on the whole wall | €23.90 |
| Total | €33.47 |
Prices include VAT. If you're also treating the ceiling or more walls, measure the total area and calculate with the guide how much paint do I need.
When this isn't enough
If, after cleaning, preparing, painting and ventilating daily, the same corner still gets wet every night, the thermal bridge is too strong to solve from the surface. That needs building work (insulating that wall or that junction with the slab) and should be assessed by a technician. A breathable paint is still a good finish afterwards. It isn't enough either if there's actually water ingress: the water has to be stopped first.
If you want to understand the whole problem, with every layer of the solution, read condensation at home or visit the condensation solution page.
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.
