How do you tell rising damp from condensation?
Look at the height and the season. Rising damp climbs from below: a stain or tide line up to a metre or a metre and a half from the floor, with a white crust of salts, and it is present all year, summer included. Condensation shows on cold spots: corners, window reveals, behind a wardrobe on an outside wall. It arrives with cold weather and closed windows, and black mould grows on it.
To be sure you need data, not intuition: a moisture meter on the wall at several heights and a temperature and humidity logger in the room for at least two weeks. If relative humidity stays above 65 to 70 percent and the wall feels cold, it is condensation. If the air is normal but the wall is wet from the bottom, the water comes from the ground or from a leaking pipe.
In Barcelona a mix is common: an old building, a ground floor, very airtight windows after a refit and little ventilation. Then both causes are treated, otherwise one brings the other back.
Why does paint peel on a ground floor?
Moisture moves out of the wall and carries dissolved salts with it. At the surface the water evaporates and the salts crystallise under the paint, lifting it in blisters. An acrylic paint film does not let vapour pass, so pressure builds beneath it until it bursts.
Repainting that wall is pointless: next season the blisters return and the surface underneath is even more crumbly. First the loose layer and the affected plaster are removed, then the wall is allowed to dry, and only then is a breathable finish applied. Drying time depends on masonry thickness: a 40 to 50 cm wall takes months, and the renovation schedule has to include that margin.
What do cement render and plastic paint do to an old wall?
They lock the water in. Old masonry (brick in lime mortar, stone, rammed earth) breathes: moisture that enters the wall leaves through its surface. Cement render and plastic paint close that exit, so the water stays in the wall and rises higher, until it finds a weak point: the next room, a joint or the base of a partition.
The issue is stiffness and density. Cement is harder and less permeable than the historic mortar, and when the masonry moves it cracks, letting water in but not out. So the first step in restoring a ground-floor wall is usually the same: strip the cement to roughly 30 to 50 cm above the visible mark and let the wall breathe.
Lime paint and plaster: a fix or cosmetics?
Lime does not cure the cause; it lets the wall live with moisture. Lime plaster is vapour-permeable, absorbs and releases water, and salts settle in its pores instead of breaking the surface. For moderate damp it is a working, durable system; for heavy damp the source must be dealt with first.
The sequence that works: find the cause, take the water away (external drainage, pipe repair, rainwater downpipes), remove the cement, apply a lime-based renovating plaster, in two coats with drying time between if needed, and finish in lime or silicate paint. In a luxury interior the result is also beautiful: a soft, living surface that acrylic cannot give. But if the water source has not been found, lime cannot work miracles and the wall will have to be redone.
Do dehumidifiers and ventilation help?
They help against condensation, not against rising damp. A dehumidifier lowers air humidity and stops mould when the cause is excess vapour: showers, cooking, drying laundry, closed windows. But it will not stop water that climbs from the ground through the wall: you will empty litres from the tank and the wall will stay damp.
For a home that is not occupied all year, regular airing or mechanical ventilation with heat recovery that runs in your absence matters more. In a shut, empty flat humidity builds up, and mould arrives precisely during the months away. Controlled ventilation with a humidity sensor solves this more cheaply than fighting the consequences.
What not to cover with furniture and finishes
Do not push wardrobes or built-in joinery tight against exterior or ground-floor walls without a 3 to 5 cm air gap and vent openings, because a cold pocket forms behind them and mould follows. Do not glue vinyl wallcovering, panels or tiles onto a damp wall, and do not close the base of the wall with a plastic skirting that cannot breathe.
For luxury work there are handsome solutions: panels and slatted timber cladding are fixed on a frame with an air gap and access slots, stone and marble go on a verified dry substrate, and built-in wardrobes stand off the outer wall with a perforated back. Keep the order in mind: dry wall first, finish second. The reverse ends in a second renovation at your expense.
Where to start and how BCN YARD can help
Begin with an inspection and measurements: moisture meter, logger, photos of the walls in raking light, a check of pipes and downpipes. It takes a few days and saves months of arguing about the cause. After diagnosis a plan is drawn up: what is fixed in the building, what inside the flat, with which materials and in what order.
BCN YARD runs these projects in Russian and Spanish: a visit and measurements, then a phased estimate with a separate line for wall drying. One person responsible for the site coordinates the plasterers, plumbers and the designer.
Frequently asked questions
Can mould just be painted over?
No. Paint hides the stain, but the spores and the cause remain, and the mould returns within months. You need to remove the moisture source, treat the surface and dry the wall. If the mould covers more than a hand and the problem is condensation, first correct ventilation and the insulation of the cold spot, or the patch comes back.
How long does a wall take to dry once the cause is fixed?
It depends on thickness and material: a 40 to 50 cm brick or stone wall takes months, a thin partition far less. It is measured with a moisture meter at several depths, not by eye. Plaster and paint go on when readings fall steadily, otherwise moisture is trapped under the finish and everything has to be redone.
Does every old building need lime finishes?
Not everywhere, but on brick or stone walls laid in lime mortar, especially on lower floors and exterior walls, it is the safest choice. Gypsum plaster and plasterboard inside are fine where the wall is dry and protected from damp. The decision depends on the diagnosis of each wall, not a rule for the whole building.