August and early September are moving season — apartment-hunting time. At a summer viewing the windows are open, the walls are dry, and the first cold nights feel far away. Which makes this exactly the moment when it’s easiest to rent an apartment that turns damp and cold once winter comes.
Summer overheating and winter cold often come from the same causes: poor insulation, leaky windows, bad orientation, poor ventilation. According to INE data for 2025, 28.2% of people in Portugal lived in homes they couldn’t keep cool enough in summer; 32.1% — in housing with leaks, damp, or rotting windows and floors. Here’s what you can tell from a single viewing, how to read an energy certificate, and when repairs are the landlord’s responsibility to pay for.
Why one apartment is hot in summer and cold in winter
An apartment with poorly insulated walls and windows heats up fast in summer and loses heat just as fast in winter. In Lisbon, per IPMA’s 1991–2020 climate norms, there are on average 19.3 tropical nights a year — nights when the temperature never drops below 20°C. If the walls and floors have been soaking up sun all day, they keep releasing that heat indoors at night. The apartment doesn’t get a chance to cool down even once it’s cooler outside.
Winter works the other way. Lisbon’s average minimum January temperature is 8.6°C, but indoor comfort depends on how fast the home loses heat. Cold walls and windows lower surface temperatures; standing near them feels chilly even when the air temperature is normal. Condensation forms on those surfaces, and mould follows.
So even Portugal’s mild climate doesn’t by itself guarantee a comfortable home. What matters is the walls, the windows, the roof, the apartment’s orientation, sun protection, and whether you can ventilate it at the right times.
How Portugal’s housing stock is built
Most housing in Portugal was built before modern insulation requirements existed. About half the country’s buildings were put up before 1980, another 31.9% between 1981 and 2000, and only 17.9% in the 21st century.
But “built before 1980” lumps together very different houses. About a quarter of buildings date to before 1960, and roughly half of that older stock was built the traditional way — stone walls, timber floor structures. There was no modern insulation, so these homes stayed cold in winter. In summer, thick walls, small openings, external shutters, shading and night ventilation slowed the heat build-up and evened out temperature swings. Studies of houses in the Alentejo confirm how well these techniques worked.
In winter, people didn’t heat the whole house: the family gathered around the hearth or stove in the kitchen, and the rest of the rooms stayed cold (this is how, for instance, the traditional saloio house near Cascais was built). A steady twenty degrees in every room was never the standard these homes were designed around.
Then there’s the separate story of the 1960s–70s construction boom. Cities were growing, housing went up fast and at scale, and there were still no mandatory insulation requirements. Researchers class the apartment blocks of 1961–1980 in the typical low-energy-efficiency category: some traditional techniques had already disappeared, and modern solutions hadn’t arrived yet. Which is why a renovated 19th-century house can sometimes be more comfortable than a 1970s apartment. The build year is a clue, but what matters more is how the building is put together and how well it has been renovated.
Portugal only started regulating buildings’ thermal performance in 1990 (Decreto-Lei n.º 40/90). Mandatory energy certification arrived in 2006. But the new rules don’t automatically require existing buildings to be insulated. And there’s a separate problem with building condition: 35.8% need repairs, 4.6% of them major ones.
So choosing an apartment also means assessing the defects that have built up over the years.
What the state is doing about it
Rebuilding the old housing stock quickly isn’t possible. New buildings and major renovations are subject to energy-efficiency requirements, and housing being sold or rented has to come with an energy certificate. For a tenant, that’s the main standardised way to compare the thermal performance of different apartments. It doesn’t guarantee comfort.
A sale or rental listing has to state the property’s energy category, from A+ to F. Certificates for residential units are usually valid for ten years.
The category is calculated for a standard-use scenario and shows the building’s comparative energy demand for heating, cooling, ventilation and hot water — it says nothing about the actual temperature in a specific apartment during a hot or cold week. The result is also affected by the apartment’s position in the building, the condition of the windows, and whether the equipment works. A full certificate describes the walls, windows and systems, and lists possible improvements. Ask for the whole document and check its number, date and status in the open SCE/ADENE database. The landlord is required to provide the certificate before the lease is signed.
Poor insulation doesn’t just mean stuffy summers and a steep winter heating bill: moisture condenses on cold walls and windows, damaging finishes and belongings and creating good conditions for mould to grow. According to the World Health Organization, people living in damp or mouldy buildings can face up to a 75% higher risk of respiratory symptoms and asthma. Which is why signs of damp need checking separately.
Why damp and mould show up in an apartment
Damp and mould can have different causes — and need different fixes. If the ceiling or a section of wall darkens after rain, and the area around a window gets wet, you’re likely looking at a leak or damage to the façade, roof or window frame. If water collects on the glass in the mornings and black spots show up in corners and behind furniture, condensation is usually the cause. If mould keeps coming back in the bathroom or kitchen, especially in a windowless room, check the extractor fan and the airflow.
Ventilation lowers air humidity and helps with condensation. But it won’t fix a leaking roof, a damp façade, or a cold bridge — a section of the structure that cools down noticeably more than the rest. If you’re going to raise repairs with the landlord, note down where and when the mould appears: after rain, during cold spells, only behind furniture, or all the time.
The problem is widespread: in 2025, 32.1% of the population lived in homes with leaks, damp, or rotting frames and floors. Per Eurostat data for 2023, Portugal ranked second in the EU on this measure, after Cyprus — about three people in ten deal with it here, against one in seven EU-wide on average.
If you’re choosing an apartment
Before the viewing
Check the energy class in the listing and ask when the building was built. “Fully renovated” says nothing about insulation — cosmetic work can be limited to the floors, the kitchen and a fresh coat of paint. Ask separately whether the windows were replaced, whether the façade or roof was insulated, and when the certificate was issued.
If you can, schedule the viewing for whichever time makes the problem most visible: see a west-facing apartment closer to evening, a north-facing one with the heating off. Ask to close the windows for a few minutes and listen for noise. In summer you’ll need to air out a bedroom without AC at night, so a bar, a road, or trash bins under the window matter just as much as which way the windows face.
Eight things to check at the viewing
- Windows. Single or double glazing, the condition of the frames and seals, how easily the windows close. Run your hand along a closed frame and check the windowsill for water marks.
- Sun and shade. Find out which way the bedroom and workspace windows face. North gets less direct sun in winter; west heats up more in the second half of a summer day. External shutters or blinds are noticeably more effective than interior curtains, because they stop the sun before it reaches the glass.
- Position in the building. A top floor depends more on the roof’s condition, a corner unit has more exterior walls, a ground floor can be colder and damper. None of this is a reason to walk away outright, but it is a reason to look more closely at the certificate and the repair history.
- Damp. Check the corners of exterior walls, the space under the windows, the bathroom ceiling and the wall behind a wardrobe. A musty smell, bubbling paint, and a locally repainted patch all need an explanation.
- Ventilation. Turn on the extractor fan in the bathroom and the kitchen. Ask where it vents to. A grille with no airflow doesn’t solve anything.
- Cooling. Whether there’s AC in the bedroom, whether you can get cross-ventilation, and whether the windows are shaded from direct sun.
- Heating. Check whether the AC can run in heating mode, whether there are any fixed heaters and whether they work. Ask to see winter electricity bills, if they’ve kept them.
- Water and repair history. Ask whether there have been leaks from the roof, the façade, or neighbours, and what exactly was repaired afterwards.
Before signing
Get the full energy certificate and read the sections of the lease on repairs and upkeep. Article 1074 of the Civil Code puts the necessary work to preserve the apartment on the owner, unless the parties have written something different into the contract.
Log every defect you find in a written record or lease attachment, and photograph it before moving in. If you’re planning to install AC, replace an appliance, or attach anything to the façade, get the landlord’s written consent in advance.
If you already live in a problem apartment
When you write to the landlord, don’t describe a general feeling — “it’s cold in here” — describe a specific defect: the roof leaks, the façade lets water in, a window won’t close, the extractor doesn’t work, mould has appeared, or equipment that came with the apartment has broken. That way the landlord knows exactly what to inspect and what repair you’re asking for.
Article 1074 of the Civil Code requires the owner to carry out the necessary routine and major work to preserve the home, unless the lease assigns responsibilities differently. It doesn’t require retrofitting an old building to modern standards automatically. Responsibility for damp and mould depends on the cause: a construction defect, a leak, faulty ventilation, or how the apartment is used. Which is exactly why photos, dates and a description of the circumstances matter.
Report the problem in writing, to the address or email listed in the lease. Attach photos, note when the defect appeared and how it’s changing, ask for an inspection, and propose a reasonable deadline for a response. Keep the correspondence and receipts.
If the apartment’s condition threatens safety or sanitary conditions, the tenant can go to the local municipality — the Câmara Municipal. Under Article 90 of the RJUE the municipality carries out an inspection, and under Article 89 it can order the owner to carry out the necessary work.
Changing windows yourself, drilling into the façade for AC, or doing other work is usually only allowed when the lease permits it or the landlord has agreed to it in writing.
There’s an exception for urgent repairs: Article 1036 of the Civil Code lets a tenant carry out the work and claim reimbursement, if the owner has already missed the deadline to act and there’s no time to wait for a court process, or if the situation simply can’t wait at all. In that last case, the owner has to be notified at the same time. Using this mechanism without documentation and legal advice is risky; automatically deducting the cost of the work from the rent isn’t advisable.
What you can do without a renovation
These steps won’t fix a leak or a façade defect, but they’ll help cut down overheating and condensation.
- In summer, close the external shutters before the sun hits the window. Ventilate at night and early morning when it’s cooler outside, and close the windows in the heated part of the apartment during the day.
- Check your AC’s mode. Many split units can both cool and heat; in heat-pump mode they usually use less electricity for heating than a portable direct-heat heater.
- In winter, ventilate briefly and intensely. A window left ajar for a long time cools the window reveals and the wall around them; a few minutes of cross-ventilation changes out damp air faster.
- Don’t push a wardrobe up against a cold exterior wall. A small gap improves airflow and lets you spot moisture sooner.
- Use a dehumidifier if humidity stays persistently high. It reduces condensation, but it doesn’t replace fixing a leak or the ventilation.
