Mold inside the house: possible causes, how to eliminate and prevent it

From condensation to mold: how the problem starts

Mold inside the house never appears by chance: it’s the result of a process that begins with something seemingly harmless, that is condensation. When warm  and humid air inside the home comes into contact with colder surfaces like external walls, window glazings or poorly ventilated corners, temperature quickly drops below the dew point.

The water vapor then turns into tiny droplets that settle on surfaces: this moisture buildup is the true trigger for mold spores to grow. The ideal conditions for mold formation include a combination of:

• Relative humidity above 75–80%;
• Temperatures between 18°C and 32°C;
• Poor ventilation and porous materials where mold can take hold.

The most common areas affected are hidden or poorly ventilated spots: behind wardrobes, in bathrooms, in the corners of rooms, around window frames or inside roller shutter boxes. Ceilings, especially with a thermal bridge, are also frequently affected.

It’s important to understand that mold almost always results from a combination of structural factors (linked to the building) and behavioral factors (linked to daily habits).

For example, a poorly insulated external wall may get cold enough to cause window condensation, and if, at the same time, a room isn’t ventilated after a shower or if clothes are left to dry indoors, indoor humidity will rise beyond critical levels.

Consequently, mold is a clear symptom that something is off in the home’s microclimate: thermal imbalance, excess humidity, insufficient air exchange. This is why it’s essential to approach the issue with a broad perspective that considers both the physical characteristics of the home and the behavior of its occupants.

Structural causes of mold in the house

Mold problems can arise from humidity issues that are built into the structure of the home itself. However it’s important to note that mold in new houses, built to modern energy and insulation standards, rarely stems from structural defects: the following problems typically affect older buildings or partially renovated homes.

Mold by interstitial condensation

Beyond visible condensation there’s interstitial condensation, which forms inside building elements like walls, ceilings or suspended ceilings. It occurs when water vapor penetrates materials and meets colder surfaces deep within, causing it to condense. This type of internal moisture is hard to detect initially but can compromise the wall structure over time and lead to mold on interior walls, even far from obvious moisture sources.

Mold by thermal bridges

Thermal bridges on windows and structural junctions, like wall and floor connections or window frames, are areas where heat escapes more easily. These surfaces cool down faster than the surrounding space, encouraging condensation and mold growth. That’s why you often see mold on the walls, in upper corners, around radiators or near windows.

Mold by water infiltration

Mold can also result from leaks and water infiltration coming from damaged roofs, broken pipes, clogged gutters or poorly waterproofed terraces. Water penetrates the structure, collects behind walls and creates local moisture that leads to mold growth even without condensation. The earliest signs include stains or peeling paint on ceilings or walls.

Mold by rising damp

As a common cause of mold issues in older homes and outdated constructions, rising damp happens when ground moisture travels upward through walls by capillary action, in the absence of a proper moisture barrier. Ground-floor and basement rooms are the most affected, with mold typically appearing along the lower parts of the walls.

Poor construction and lack of maintenance

In older buildings, construction flaws like poor insulation or lack of ventilation, and poor upkeep such as uncleaned gutters, deteriorated window frames or uninspected systems, create ideal conditions for chronic moisture. Materials like drywall, untreated wood and porous plasters absorb humidity easily and become breeding grounds for types of mold on walls. Over time, this leads to recurring and widespread mold all over the house.

Behavioral causes of mold in the house

An international research confirms just how much daily habits can influence humidity, condensation and mold inside the house: in roughly 35% of analyzed cases, the cause of mold in the house was directly linked to inadequate behaviors or lifestyle choices.

Water vapor from everyday activities

Common household activities like cooking, showering, drying clothes indoors or even breathing, generate several liters of water vapor each day. A family of four can release up to 12 liters of moisture into the air daily: without proper management, this excess humidity raises indoor moisture levels beyond the critical threshold, encouraging mold to form.

Poor ventilation in rooms

Without adequate ventilation, moisture gets trapped in rooms, making them highly vulnerable to mold: improper or inconsistent air exchange is one of the most frequent behavioral mold causes. Some typical examples: windows left in tilt-and-turn mode for hours during winter (instead of being fully opened for a few minutes); bathrooms not aired out after a shower; kitchen range hoods never used; rooms left permanently closed off.

Unbalanced indoor temperatures

Temperature management also plays a key role: heating only part of the home while leaving other rooms cold, as well as drastically lowering the heat at night or when away, can create cold surfaces on walls and windows. This leads to condensation build-up and, in turn, mold near windows, in corners and along cold spots.

Neglected cleaning and maintenance

Poor hygiene and neglected maintenance allow moisture to stagnate and create an ideal environment for mold spores. Absorbent materials such as carpets, wallpaper, curtains and fabric furniture worsen the issue. In addition, worn-out window seals can lead to drafts and cold zones, increasing the risk of window condensation and mold on windows.

New windows and unexpected mold

One striking example is mold forming after installing new windows. While older frames allowed some unintentional air exchange through gaps, new airtight windows offer excellent insulation but require responsible use. If windows remain shut and rooms aren’t aired regularly, water vapor from everyday life builds up, condenses on cold surfaces and leads to mold all over the house, despite the home being recently upgraded.

How to prevent condensation and mold in house

How to prevent mold in the house? It all starts with consciously managing your home’s indoor climate. Ironically it’s often newer, well-insulated buildings that are more prone to mold problems, especially when certain ventilation habits are overlooked.

Modern windows are designed to create an airtight seal, improving energy efficiency and thermal insulation: but for this very reason, they don’t allow humidity to escape naturally. Without proper ventilation water vapor becomes trapped, leading to window condensation and eventually mold inside the house, even in new or recently renovated homes.

Using new windows effectively requires active control of indoor air quality. Here’s how:

• Ventilate at least 2-3 times a day for a few minutes, especially in kitchens and bathrooms;
• Use shock ventilation: open opposite windows at the same time for 5-10 minutes to refresh the air without chilling the walls;
• Monitor humidity levels with a hygrometer, keeping it between 40% and 60%;
• Avoid drying clothes indoors or use a dehumidifier.

To make humidity control easier, modern windows can include integrated microventilation systems, like DQG ones: these discreet but effective features allow for a continuous airflow even with the window closed, helping to prevent condensation on windows and reducing the risk of mold without compromising energy efficiency, comfort or security.

DQG doors and windows with integrated micro-ventilation

DQG doors and windows, proudly made in Italy with high-quality materials and advanced technology, are designed to meet the real needs of everyday living. Among our features is the integrated microventilation system, available as standard on all DQG frames.

This function allows for a constant air exchange even when windows are closed, helping to prevent humidity buildup, window condensation and the formation of mold on windows – all without heat loss or drafts, nor compromising comfort and safety.

In modern homes where insulation is increasingly efficient, microventilation provides a practical solution. DQG combines Italian design with some of the most effective features on the market, truly improving the quality of life inside the house.

Want to eliminate mold in the house and improve comfort? Contact DQG to discover windows with micro-ventilation, designed for a healthier, tailor-made indoor environment.