Why Mold Appears in Montenegrin Apartments — and How to Actually Prevent It

On forums for foreigners who have moved to Montenegro, this topic comes up year after year: black stains on window frames that need to be wiped periodically, mold in the corner of the bedroom by November, a damp smell that doesn't go away even with the windows closed. This is not rare bad luck with one specific apartment. It is a systemic, measured problem — and it turns out to have precise numbers behind it, not just general guesses.

What the measurements actually showed

Engineers at the University of Montenegro carried out real airtightness measurements of the housing stock using the blower door test method (a pressure difference is created inside the apartment and the amount of air leaking through gaps is measured). The results were published in the peer-reviewed journal Energies in 2023.

The numbers turned out to be more telling than any assumption. In a typical apartment from the 1970s–1990s housing stock (a significant share of housing in the country), about 80% of exterior walls are built from monolithic reinforced concrete with a thermal conductivity above 2 W/m²K — even though Montenegro's own building code (the "Rules on Minimum Energy Efficiency Requirements for Buildings," Official Gazette No. 75/15) allows no more than 0.6 W/m²K for the coastal climate zone. That is more than three times worse than the norm the country set for itself.

Old wooden windows in the same housing stock have no rubber seals at all, with a thermal conductivity of 2.5 W/m²K. And an airtightness measurement before window replacement showed a value of 6.25, against a legal limit of 3 for buildings without mechanical ventilation — more than twice as bad as the country's own standard.

Source: Tombarević, Vušanović, Šekularac. "The Impact of Windows Replacement on Airtightness and Energy Consumption...", Energies, 2023, 16, 2208 (University of Montenegro).

An unexpected finding: airtightness without insulation is a trap

The most valuable part of the study is not the numbers themselves, but one specific conclusion the authors draw. Simply replacing old windows with new energy-efficient ones sharply increases an apartment's airtightness — air stops leaking outward. That sounds like an unambiguous benefit, but the authors explicitly warn: if the walls remain uninsulated, increased airtightness can lead to higher relative humidity indoors and the appearance of condensation and mold on walls exactly where the surface temperature is below the dew point.

In other words: installing expensive new windows in an apartment with old cold walls is not a solution — it's a way of airtightly sealing the problem inside, unless insulation and ventilation are addressed at the same time. The researchers separately note that after window replacement, residents rarely change their ventilation habits — meaning this is exactly the risk group that would be least protected without mechanical ventilation.

Why this is a systemic problem, not individual bad luck

A significant share of residential construction on the Montenegrin coast was originally built not for year-round living but for summer rentals to tourists — a format where the building only needs to perform well for three or four months a year, not twelve. The study's own authors note that airtightness as a problem in Montenegro is still fairly poorly accounted for — partly because of the mild climate, partly because of historically low electricity prices.

Why autumn and winter, not summer

The rainy season on the Montenegrin coast runs roughly from September to April, while July and August are the driest months of the year. The area near the Bay of Kotor, close to Mount Orjen, records one of the highest annual rainfall totals in all of Europe. The difference between the humidity outside and the warm air inside the apartment is exactly the dew point — that's where condensation settles on the cold wall surface. This is precisely why mold complaints appear en masse with the first autumn rains, not at the height of the tourist summer.

Three engineering causes that most often coincide

1. Saving on wall insulation. As the measurements above showed, this is not rare — it's the statistical norm for a significant share of the housing stock: 2 W/m²K instead of the required 0.6.
2. Lack of mechanical ventilation. Especially critical after installing airtight windows — according to the study, it's exactly this combination that increases, rather than reduces, the risk of condensation without insulation and ventilation.
3. Thermal bridges. Balcony slabs, window junctions, and the corners of exterior walls remain colder spots even where the main wall is insulated.

How this is solved at Zen Gardens

We built protection against mold into the project from the very start, knowing the real numbers for the country's housing stock — not marketing promises. Every apartment has 10 cm of wall insulation (many times better than the typical 1970s–1990s housing stock), double glazing with a protective coating, a supply-and-exhaust ventilation system with the option to install anti-allergen filters, and warm-water underfloor heating from an air-to-water heat pump that keeps the temperature steady year-round. That is exactly the combination of insulation plus ventilation whose absence researchers call the main cause of risk.

Frequently asked questions

Where do these figures about 80% of walls come from?
From a real study by University of Montenegro engineers with blower door test measurements, published in the peer-reviewed journal Energies in 2023 — the source is linked in this article.

If an apartment already has new windows, is mold nothing to worry about?
No — according to the same study, airtight windows without wall insulation and ventilation can actually increase the risk of condensation rather than reduce it.

What time of year is best to view an apartment to spot the problem?
In autumn and winter your chances of seeing the real picture are higher — that's exactly when insulation and ventilation shortcomings become visually apparent.

Why we're talking about this openly

Most developers don't like discussing mold publicly. We decided otherwise: if a buyer reads this article, learns the history behind the "summer" housing stock, and comes to us with specific questions about insulation and ventilation — that's exactly the conversation that matters to us.