Mould in renovated homes: why it happens and what ventilation costs

The mould pattern we find most often in Portugal isn’t in old, neglected houses. It’s in renovated ones. New windows, sometimes insulation, sealed gaps, and no ventilation put in to replace the draughts the old house relied on. By the second winter there’s black mould in the bathroom ceiling, behind the wardrobes and around the window reveals.

Why it happens

Showers, cooking, drying clothes and breathing put litres of water into the air every day. An old house with leaky windows lost that moisture through the gaps, along with a lot of heat. Seal it up without giving the moisture a way out and it condenses on the coldest surfaces: corners, window reveals, ceilings under an uninsulated roof, the backs of wardrobes on outside walls.

Opening the windows helps, but nobody does it properly in January. A split air-conditioning unit doesn’t solve it either. It recirculates the air already in the room.

What the fixes cost

• Extract fan in a bathroom, ducted properly to outside, ideally humidity-controlled with a run-on: €350–600 per bathroom.
• Humidity-controlled extract throughout a house (bathrooms and kitchen): €2,500–4,500.
• Decentralised heat-recovery units cored through the outside wall, one per room: about €1,800–3,600 for a small apartment and €7,000–13,000 for a house. No ductwork, so they suit stone and lime-finished walls.
• Continuous extract system for a whole house: €2,800–7,500 in the cases we priced.
• Full MVHR (VMC double-flux) with heat recovery: €3,500–11,000 for an apartment and €7,000–12,000 for a house, depending on duct routes. False ceilings make it far easier.

In the right order

Ventilation first. Let the house dry for a season. Then treat the mould with a fungicidal wash and repaint in a breathable paint. Repaint before fixing the cause and it comes back through the new paint within a winter, which we see all the time. If there’s also a leak or rising damp, that has to be dealt with before any of it.

What to look for when buying

• An extract fan in every bathroom and the kitchen, and check it runs.
• Fresh paint on ceilings or in corners.
• A musty smell in wardrobes on outside walls.
• New windows with no trickle vents in a flat with no other ventilation.

Every pre-purchase inspection includes moisture readings and thermal imaging, and costs the ventilation if it’s missing. More on where damp comes from: damp in Porto, explained and why Portuguese houses are cold and damp.

Figures are from our own inspection reports in 2025 and 2026, ex IVA, with names and addresses left out. Every house is different, so treat them as a guide to scale, not a quote.

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