1. Energy-saving renovation with vacuum glass

When using vacuum glass to carry out energy-saving renovation of the envelope structure of existing buildings, some customers think that vacuum glass has very good thermal insulation performance. To save costs, using ordinary window frames can also meet the thermal insulation performance requirements of the whole window in theory. For example, using composite vacuum glass with a heat transfer coefficient K value of 0.45 W/(m²·K) and combined with thermal insulation aluminum alloy profiles with a K value of 2.1 W/(m²·K), the heat transfer coefficient of the whole window can reach 1.1 W/(m²·K), meeting the building energy-saving requirements in Beijing.

2.Existing problems
What problems exist in the above configuration? A customer in Beijing used the above configuration when replacing vacuum glass windows. Without heating in winter, the indoor temperature can reach around 20°C, indicating that the thermal insulation performance of the whole window is still very excellent. Since the customer often dries clothes in the room, making the indoor humidity relatively high, condensation appears on the window frame, and even a flowing state occurs, as shown in Figure 1.

Why does this happen?

This is because compared with vacuum glass, the window frame has a much poorer thermal insulation performance. Heat continuously flows away from the window frame area, making the surface temperature of the window frame relatively low, and condensation occurs under high relative humidity.

3.different conditions
Under the conditions of outdoor temperatures of 0°C, -10°C, -20°C, and indoor temperature of 20°C, the relative humidity at condensation for different window frame K values is shown in Table 1.

At a certain temperature, if the indoor relative humidity exceeds the saturated moisture content in the air, the moisture in the air will condense and precipitate on objects with lower temperatures, resulting in condensation. Therefore, even if the doors and windows are sufficiently insulated, condensation will still occur when the indoor humidity is too high. Even for energy-saving windows with the smallest possibility of condensation, their condensation resistance rate also depends on the indoor relative humidity. For example, even if a window frame with a K value of 1.0 is used, if the relative humidity exceeds 72%, condensation will occur in outdoor temperatures of -20°C or colder.

window frame materials for vacuum glass windows

1. Vacuum glass itself has good thermal insulation performance. Even when the outdoor temperature drops to -35°C, there will be no condensation at 80% relative humidity. The anti-condensation performance of the whole vacuum glass window is like the well-known "barrel effect". The window frame is the short board that affects the overall performance. Therefore, it is not that vacuum glass windows will never have condensation. Its anti-condensation performance depends on the performance of the window frame. If you are used to a higher indoor air humidity and want the window frame not to condense, then you should choose a window frame with a low K value (for specific selection, refer to the data in Table 1). Common window frame types and K values are shown in Table 2.

So, when all property owners replace vacuum glass, if they want to achieve the effect of "1+1>2", they must choose a window frame that matches the vacuum glass. That is, as the saying goes, "a good horse deserves a good saddle. Good things should be matched with better things" to achieve the ultimate effect.

Under normal circumstances, the comfortable humidity range for the human body is between 40% and 60%. If you want the window frame not to condense within this humidity range, you can refer to the following suggestions:

In Northeast China: Since the outdoor temperature is relatively low and can reach below -20°C, you can choose a window frame with a K value of less than 1.3. For example, 90 series thermal insulation aluminum alloy, 93 series or above aluminum-clad wood, etc.

In North China: The lowest outdoor temperature is generally around -10°C. You can choose a window frame with a K value between 1.3 and 1.6. For example, 80 series thermal insulation aluminum alloy, 65 series plastic windows, etc.

In southern regions: Mainly referring to areas with hot summers and cold winters, where the outdoor temperature is around 0°C. You can choose a window frame with a K value between 1.6 and 2.0. For example, 75 series aluminum alloy, etc.