Tips and instructions

Outdated windows, such as box-type and bonded windows, often lack seals and are not airtight. When sitting near such windows in cold weather, we feel drafts and experience cold discomfort. Additionally, drafts can occur in poorly installed windows and outdated roller shutter casings. The connection between the window and the wall may have gaps and is not properly sealed.

Even windows that have built-in seals and are airtight can still cause discomfort if their glazing has poor thermal insulation. For example, box-type and bonded windows with only two separate glass panes or single windows with “thermopane” glass still have a very high heat transfer coefficient of U=2.9-3.1 W/m²K. On cold days, such windows allow significant heat loss. As a result, the temperature of the inner glass surface remains relatively low, even though the room air temperature is, for instance, 22°C, and the interior walls are relatively warm. This large temperature difference between the warm interior wall and the relatively cold inner glass surface can cause convection, meaning the movement of air from the warm interior wall toward the cold glass. This invisible air circulation can be perceived as a draft and coldness, even when the windows and doors are closed! The greater the temperature difference, the stronger the perceived draft.

A common solution is to increase the room temperature to around 26°C or higher, which prevents us from feeling this circulation as a cold draft. However, this significantly increases heat loss through windows and walls, as well as energy consumption for heating. Studies have shown that for every degree increase in room temperature, energy consumption rises by 5 to 7%!
The only efficient and healthy solution to this invisible draft is modern, airtight, energy-efficient windows and proper thermal insulation of exterior walls. The goal is to keep the inner surface temperature of the glass as high as possible. Measurements have shown that at an outdoor temperature of –10°C and an indoor temperature of +20°C, the inner glass surface temperature strongly depends on the quality of the installed glass.

• With ordinary “thermopane” glass (Ug = 2.9–3.1 W/m²K), the inner glass surface temperature is only +8.4°C.
• With heat-insulating glass (Ug = 1.1–1.6 W/m²K), the temperature rises to +13.8 to +15.5°C.
• With high-performance insulating glass (Ug = 0.4–0.8 W/m²K), it can reach +16.8 to +17.3°C.

Triple-glazed heat-insulating glass typically has a structure of 4-12-4-12-4 mm, with a low-emissivity coating on the inner side of the inner pane and a hermetically sealed argon gas filling between the glass layers. Experts therefore recommend replacing outdated windows and installing energy-efficient windows in new constructions.

A heat balance analysis for an average single-family home shows that replacing old thermopane-glazed windows with modern heat-insulating glazing (e.g., U ≈ 1.1 W/m²K) can halve heat losses through windows. Additional energy savings can be achieved by installing modern roller shutters, which also enhance security, provide blackout options, and protect against excessive sunlight.

Studies also indicate that over 20% of residents in apartment buildings and single-family houses plan to replace worn-out and outdated windows to improve their living environment—ensuring better indoor climate control, noise reduction, increased security, energy savings, and an improved aesthetic appearance of their home.

Decorative grilles on windows are not a technical necessity but serve purely aesthetic purposes. They come in various designs, one of which is integrated between the glass panes in insulating glass units. This design requires careful consideration of technical aspects to prevent damage to the insulating glass.

The thickness of these decorative grilles must not match the exact width of the space between the panes, as this could lead to glass breakage under certain climatic conditions. Depending on environmental climate changes, glass panes can deform—becoming convex or concave—which means the space between the panes may expand or contract, causing the glass to bulge outward or curve inward.

This phenomenon results from the physical properties of insulating glass and is therefore unavoidable. When the space between the panes expands, the decorative grilles inside may lose their support. Any movement of the window or glass can cause the grilles to oscillate. This movement occurs not only when opening or closing the window sash but also due to air vibrations from passing vehicles. Similarly, sudden changes in air pressure—such as when opening or closing doors in adjacent rooms—can cause the glass to vibrate.

When the window or glass pane shakes, the integrated decorative grilles inevitably strike the glass, producing noise. While plastic or felt pads can be placed at the grille intersections to reduce this noise, they cannot eliminate it entirely. The intensity of the noise depends on the degree of glass deformation and the size of the glass pane, with larger panes experiencing more deformation than smaller ones.

Therefore, this noise is not considered a defect in the glass or window, even if plastic or felt pads are installed. However, these pads may be visually unappealing and not always acceptable to customers. For these reasons, buyers should be informed in advance about the potential for noise and the appearance of decorative grilles with pads.

Why Do Windows Fog Up? (Existing Glass)

The appearance of fogging on the windows, or condensation on the interior surface of glass, is a common and often troublesome occurrence. It mainly happens during cold autumn and winter months. It is more frequent in buildings with older, single-glazed windows or outdated multi-glazed windows that lack proper seals or have low thermal insulation properties.

The cause of condensation on windows is rooted in the physical and chemical properties of air. Warm air can hold more moisture than cold air. As the air cools down, its relative humidity (RH) increases. When the air cools and reaches 100% relative humidity, condensation occurs, and water is released in the form of droplets on cold surfaces, leading to fogging.

This phenomenon is similar to what happens when you open a refrigerator. The cold surfaces of the fridge door and walls quickly fog up because the normally humid room air cools down rapidly upon contact with the cold surfaces, reaching the dew point and condensing. In extreme cases, ice can even form on the freezer walls.

The same happens with windows. When you open a window on a cold day, warm indoor air comes into contact with the cold surface of the glass, and condensation forms. Once the window is closed, the condensation usually disappears quickly and is no longer bothersome.

A similar phenomenon occurs in vehicles. On a cold day, when you get into the car, condensation forms on the interior of the windshield because the warm, moist air from your breath cools down and condenses on the cold glass surface. Turning on the car’s heater quickly eliminates the fogging.

The same happens with windows in a closed room, especially during cold days when the indoor relative humidity is high. The surface temperature of the glass drops to the dew point, leading to condensation. Poorly insulated window frames can also cause condensation on their surfaces, as can cold exterior walls of rooms.

Where Does Condensation on Window Glass Occur?
This mainly happens in areas with high humidity, such as:

• Bathrooms
• Kitchens
• Laundry rooms And even bedrooms can experience condensation if they are not adequately heated or ventilated. A person exhales approximately 1 liter of water during 8 hours of sleep, and a household of four produces about 10 liters of water in the form of water vapor daily. Without proper ventilation, this moisture condenses on cold surfaces, often on windows.

Condensation usually starts at the edges of the glass, particularly at the bottom edge. This is because the edges of the glass are typically colder than the center of the window, a characteristic of insulating glass due to its construction.

How Can We Reduce Window Condensation?

The most effective way to reduce condensation is to opt for high-quality, thermally insulated glass or windows. The likelihood of condensation is significantly higher in single-glazed windows (Ug = 5.8 W/m²K) or standard “thermopan” glass (Ug = 2.8 W/m²K) than in windows with energy-efficient insulating glass (Ug = around 0.5 to 1.1 W/m²K).

It is essential that the indoor glass surface temperature is high enough to exceed the dew point. The temperature of the glass surface is higher the lower the U-value of the glass or window. The optimal U-value for windows should be calculated based on regulations, building orientation, design, and desired energy consumption.

However, good sealing is also crucial for minimizing condensation. Older windows that lack proper seals often lead to drafts, energy loss, and noise intrusion, which exacerbate condensation problems.

Do Thermal Insulating Glasses Eliminate Window Fogging?

Unfortunately, the answer is no. Even windows with thermal insulating glass (Ug = around 1.1 W/m²K) can experience minimal condensation, especially in cases of extreme cold and high relative humidity (60-80% or more), when the room is poorly heated. In these instances, condensation can form primarily at the bottom edges of the glass, where the surface temperature drops below the dew point.
If the room temperature is too low, especially in the morning, the glass might still fog up even with thermal insulation. Maintaining an optimal indoor relative humidity between 40-60% and a temperature of 18-22°C is important for minimizing condensation.

A quick solution to condensation is to increase the room temperature, which raises the glass surface temperature above the dew point. However, this leads to higher heating costs—a 1°C increase in room temperature can raise heating energy consumption by 5-7%. Therefore, proper insulation of walls and installation of energy-efficient windows are long-term and economical solutions.

How to Prevent Window Fogging?

The most practical solution is to ensure proper ventilation. Proper ventilation prevents moisture buildup and the growth of harmful mold. Efficient ventilation involves ventilating multiple times a day (e.g., 10-15 minutes) with wide-open windows or using cross-ventilation. The indoor temperature should not drop below 15°C to ensure that the air remains capable of absorbing moisture without cooling down the walls too much.

In winter, dry outside air can significantly lower indoor humidity when ventilating. To counteract this, humidifiers should be used in winter to prevent excessively dry indoor air, which is uncomfortable and potentially harmful to health.

It is essential to pay attention to ventilation, especially in newly built homes or renovated buildings that have highly airtight windows. These modern windows prevent natural airflow, causing moisture buildup and potential condensation issues.

When Do Exterior Surfaces of Energy-Efficient Glass Fog Up?

Even with energy-efficient insulating glass, fogging can occur on the exterior surfaces. This happens rarely, typically only after very cold nights or during extremely cold mornings. Insulating glass reduces heat transfer from the inside out, meaning the outer glass cannot warm up from the internal warmth. As a result, during cold nights, the outer glass can cool to the point where its temperature falls below the dew point, causing condensation.

This external fogging disappears naturally once the glass warms up in the morning or when the sun heats it up. This phenomenon occurs only in extreme conditions, and it does not constitute a defect in the window.
To prevent this external condensation, you can install external window blinds or shutters to reduce the cooling of the outer surface.

Conclusion
Condensation on windows can be minimized with proper ventilation, high-quality, energy-efficient windows, and insulation. Although energy-efficient windows can reduce the occurrence of fogging, perfect elimination is not always possible under extreme conditions.