Why Do Plastic Windows Fog Up?

First of all, are plastic emissions harmful to health when outdoor temperatures rise? Then, is there any point in installing new windows when wooden ones are still in good condition, and won’t plastic deform over time? But as time passes, we increasingly realize that plastic windows are the optimal choice for modern life—they are practical, warm, and beautiful.

The Nature of Window Condensation

The only drawback of plastic windows is that, as people say in folk, they 'cry' or fog up. However, people have faced similar issues even before plastic windows appeared, although less frequently. In such cases, we are dealing with a widely known phenomenon—condensation.

Those who remember their school physics course should know the term 'dew point.' The nature of this phenomenon is condensation, or the transition of water from vapor to liquid form. For instance, morning dew forms when air temperature rises faster than it can increase near the grass. At that moment, the temperature near the grass reaches the dew point, and vapor condenses on the surface, forming droplets. The same principle applies to fogging windows in homes.

Air has a certain humidity level, which varies depending on weather conditions. Additionally, condensation occurs when there is a sharp contrast between the internal and external surface temperatures of the window unit. Normal indoor humidity levels, according to hygrometer readings, range from 35% to 50%. When indoor humidity rises to a level that cannot be normalized through air exchange with the outdoors, excess moisture accumulates on the window. Usually, this problem can be resolved simply by ventilating the room regularly during the day.

Why Do New Windows Fog When Old Ones Didn’t?

This happens because traditional wooden windows had a large gap between the panes within the frame. That’s why the indoor glass had a higher temperature than the same glass in a standard single-chamber glazing unit. To avoid such issues, when choosing plastic windows, do not settle for the cheapest option. This is especially important for kitchens, where humidity is naturally higher due to the nature of the space. A double-chamber glazing unit is strongly recommended for kitchen windows.

• One possible cause of window fogging is a radiator blocked by a windowsill. Think about it—radiators are always placed under windows for a reason. This setup allows warm air to rise from the radiator toward the window. If airflow is disrupted for any reason, moisture can accumulate on the glass. Warm air from the radiator normally blows over and warms the glass. But if the radiator is blocked by the sill, heat circulation is impaired, and condensation forms. To resolve this, you can install a decorative grille with air vents on the radiator, provide an additional heat source to the glass, or drill ventilation holes in the sill. Alternatively, replace a wide sill with a narrower one.
• Inadequate ventilation contributes to increased indoor humidity. Clogged ventilation grilles that cannot draw in moist air may result in moisture buildup on windows. To prevent this, regularly inspect and clean or replace old, damaged grilles.
• Indoor plants on the windowsill as a cause of fogging. Many homeowners now enjoy growing decorative indoor plants, which help create a cozier and more comfortable atmosphere. However, keep in mind that in large quantities, plants placed on the sill release significant moisture, which may cause fogging. Distributing plants evenly throughout the room benefits not only plastic windows but also the overall room appearance.
• Window fogging after renovation. This issue often arises in recently renovated apartments. The reason is that fresh finishing materials—plaster, tiles, and paint—continue to release moisture for up to two years after construction. To help mitigate this, ensure good ventilation and regular airing. However, if owners previously treated the window frames without proper knowledge, they may have compromised the seal, leading to lower surface temperatures. In such cases, professional assistance is essential to fix fogging.
• Failure to follow proper installation guidelines for plastic windows. Rarely, fogging results from installation errors, such as incorrect window or frame fitting. In such cases, a reputable company should correct the issue at no cost, eliminating cold air infiltration and preventing moisture buildup on glass. Nearly all plastic windows have a seasonal mode switch (summer/winter). Switching to summer mode reduces insulation slightly, allowing the interior surface to cool more. If your windows lack this feature, you can use micro-ventilation settings or open the window several times a day for 15 minutes.