A damp patch at a viewing arrives with an explanation attached. The seller says it is only condensation, or that a leak was fixed last year. The agent says every older flat in Portugal has some. Each claim may be true, and each one moves money: condensation stays with the flat, a leak has a source and often a second party, and a repair that failed returns after you own it. Since Decreto-Lei n.º 10/2024 nobody checks the building at the deed, and a defect found after completion must be proved by the buyer (Código Civil, Art. 342.º n.º 1). Our guide to mold and damp inspection and buyer rights covers the law. This guide tests the four claims against what the wall shows. An InspectOS Moisture Inspection settles what the wall cannot, before the CPCV.
Table of Contents
- Is "it's only condensation" a reason to relax?
- If the leak was fixed, why is the stain still there?
- Does every older Portuguese flat have some damp?
- Does airing the flat prove it is condensation?
- What settles the question before the CPCV?
- Frequently Asked Questions
Is "it's only condensation" a reason to relax?
Condensation damp comes from the building as well as its occupants: walls built before Portugal's first thermal rules in 1991 condense every winter.
The word "only" carries the claim. It suggests a habit of the last owners that leaves with them. Sometimes that holds: a bathroom without extraction, laundry dried indoors, one bedroom kept shut all winter. The cause can also be the wall itself. Portugal had no thermal regulation for buildings until Decreto-Lei n.º 40/90, de 6 de fevereiro, applied from 1 January 1991, and walls built before then were insulated only when a designer chose to. Pillars, beams, slab edges and roller-shutter boxes run colder still.
Condensation leaves a signature. It sits on the coldest surfaces, external corners, the strip below the ceiling on a façade wall, the border of a window, and appears as diffuse black spotting with no hard edge. The paint and plaster underneath tend to stay sound. It comes with cold nights and an occupied flat, and it does not wait for rain.
So test the claim by position. Mold that follows the frame of the building will be there for the next owner. Our guide to mold in Portuguese homes every winter sets out how to tell a building cause from an occupancy one.
If the leak was fixed, why is the stain still there?
A repaired leak leaves a dry stain with tide marks; an active one keeps a damp edge after rain, so dryness is the test.
Water that entered and dried leaves rings, the tide marks of each wetting. A stain can survive a good repair for years, which is why sellers repaint. The question is whether the patch is still taking on water.
Three kinds of evidence answer it. The first is the patch itself after a dry spell: a dead stain stays the same, an active one shows a darker, damp margin after the next heavy rain. The second is paper: an invoice for the repair, with a date and a description of what was done, a roof membrane, a terrace, a window seal, a pipe. The third is the building's own record. Repairs to a shared roof or façade are decided by the condominium, and the minutes of its meetings (atas) record the problem, the vote and whether the work went ahead.
Ask where the water came from. A leak from a common roof or façade involves the condominium, and a leak from the flat above involves its owner. A repair on either side that you cannot see documented remains a claim.
Does every older Portuguese flat have some damp?
No: in 2025, 32.1% of Portugal's population lived in dwellings with damp or infiltrations, so about two in three people did not (INE).
The line is offered as reassurance, and the national figure cuts the other way. INE's 2025 survey of income and living conditions put the share of the population living in dwellings with damp or infiltrations at 32.1%, and at 41.8% among people at risk of poverty (INE, Rendimento e Condições de Vida 2025, 11 December 2025). The survey counts people, not homes, but it shows a damp home is common and far from universal.
Age raises the odds without deciding the case. A 1960s flat with new windows and insulation added in a renovation can be dry, and a 2000s flat with an unsealed terrace above it can leak. Judge the flat in front of you.
Does airing the flat prove it is condensation?
Airing and heating reduce condensation within days but leave an infiltration or rising damp unchanged, so the response to airing is evidence short of proof.
A seller who advises opening the windows is making a claim about the cause. If the pattern fades once the flat is aired and heated, condensation is the likely source. If it does not, water is arriving from outside or from below.
The test has limits. A wall wetted by a leak or by capillarity insulates worse, runs colder and condenses on its surface as well. One patch can then carry two causes: mold on top from condensation, salt deposits or blistered paint underneath from water in the wall. Rising damp, a band at the base of ground-floor walls, is the third mechanism, and our damp survey cost guide sets all three side by side. After a storm, infiltration can appear anywhere on an exposed façade or top floor, which the storm and water damage inspection guide covers.
What settles the question before the CPCV?
An InspectOS Moisture Inspection measures moisture at the surface and within the wall, dated and room by room, which separates condensation from a leak.
A buyer can narrow the cause with position, timing, a dry-week comparison and paper. The mixed cases, and water behind a finish that looks dry, need readings.
In the 23 moisture inspections we carried out between April and September 2026, condensation was never the only cause: in the 10 where it was recorded, another source of water was always present as well. In 4 of those 23, there was active damp at spots that had already been repaired or repainted. Clients book a moisture inspection when they already suspect damp or a leak, so these figures describe the properties we were called to, not a national rate.
In an InspectOS Moisture Inspection the engineer uses thermal imaging to locate temperature anomalies on walls and ceilings, and a hygrometer then confirms whether those anomalies hold moisture. Readings at depth show whether water is inside the wall or forming on its surface, which is the line between a leak and condensation. The report records the readings, dated photographs, the cause identified and the extent of the damage. The engineer is registered with the Ordem dos Engenheiros, and the report arrives within 48 hours of the site visit.
One document will not help. The energy certificate that Decreto-Lei n.º 101-D/2020 requires before a sale (fines of €250 to €3,740 for individuals, Art. 35.º n.º 1) describes walls and windows and records nothing about leaks.
Frequently Asked Questions
The seller says a leak was fixed. What should I ask for?
The invoice, with its date and a description of the work, and the source of the water: roof, terrace, façade, window or a neighbour's pipe. If a common part was involved, ask for the condominium minutes that approved the repair. Then check the patch after heavy rain.
Is condensation damp a hidden defect I can claim for after buying?
It can be, when it comes from a feature of the building, such as uninsulated walls or thermal bridges, that a buyer could not detect at a viewing. The buyer must prove it (Código Civil, Art. 342.º n.º 1) and report it within one year of discovering it (Art. 916.º). Each case turns on technical evidence.
Can a dehumidifier running during the viewing hide a leak?
It can lower the moisture in the air and slow condensation, but a wall taking on water from outside stays wet. A dehumidifier in the flat is a sign the owners lived with damp; ask which room it served and since when.
Should a repaired stain lower my offer?
A documented repair with a dry stain after rain is a cosmetic matter. An undocumented repair, or a stain with a damp margin, is an open question about cost and about who pays, and that is worth settling before the CPCV.
Conclusion
Every explanation offered for a damp patch at a viewing can be tested: by where the patch sits, by what it does after rain and after a dry week, and by the paper behind any repair. Test them before the CPCV, while the answer can still change the offer. An InspectOS Moisture Inspection costs a fraction of the sinal you commit at signing.
→ Book a Moisture Inspection · get a quote at inspectos.pt/en/home-inspections
Updated October 2026 | InspectOS Portugal · Legal references last reviewed 27 September 2026
Sources: INE, Rendimento e Condições de Vida 2025 (11 December 2025) · Decreto-Lei n.º 40/90, de 6 de fevereiro (RCCTE, in force 1 January 1991) · Decreto-Lei n.º 10/2024, de 8 de janeiro · Decreto-Lei n.º 101-D/2020, de 7 de dezembro, Article 35.º n.º 1 · Código Civil, Articles 342.º and 916.º · InspectOS moisture inspection records, April to September 2026 (n = 23)