What exactly causes energy loss in doors and windows?
Release date:
2025-09-01
China builds nearly 2 billion square meters of housing annually, yet over 97% of these buildings are highly energy-intensive. And since doors and windows are one of the main culprits behind energy losses in buildings, what exactly causes this inefficiency?
1. Thermal transmittance of doors and windows
The heat transfer coefficient of doors and windows refers to the amount of heat transferred through a unit area per unit of time. The higher the heat transfer coefficient, the greater the heat loss through doors and windows during winter. Additionally, the heat transfer coefficient of doors and windows depends on the materials and types used in their construction.
The air tightness of doors and windows refers to their ability to prevent air infiltration when they are closed. The level of a door or window's air-tightness significantly affects heat loss—fluctuations in outdoor wind conditions can negatively impact indoor temperatures. The higher the air-tightness rating, the less heat is lost, and the smaller the effect on indoor temperature.
2. Window-to-Wall Ratio and Orientation
The window-to-wall ratio refers to the proportion of exterior window area to the total exterior wall area. Typically, the thermal resistance of windows and doors is significantly lower than that of walls, which means a building’s heating and cooling energy consumption increases as the window-to-wall ratio rises. As part of building energy-saving measures, it is essential to determine an optimal window-to-wall ratio while still ensuring adequate daylighting and ventilation.
Generally speaking, solar radiation intensity and sunshine duration vary depending on the orientation of a building, which also means that windows receive different amounts of solar heat. The primary approach to improving energy efficiency in doors and windows is through thermal insulation. Key measures include selecting energy-efficient window designs, enhancing the thermal performance of doors and windows, boosting their air-tightness, and optimizing the appropriate window-to-wall ratio and orientation.
3. Choose energy-efficient window types
The window type is the primary factor influencing energy efficiency performance.
Sliding windows have poor energy efficiency, while casement and fixed windows offer superior energy-saving performance. This is because sliding windows move back and forth along the frame's lower track, leaving a large gap at the top and a space between the wheels at the bottom. As a result, significant convection occurs between the upper and lower parts of the window, leading to substantial heat loss as warm and cool air circulate. Consequently, no matter what type of insulated material is used for the window frame, energy efficiency cannot be achieved.
Typically, flat-opening windows feature rubber sealing strips between the sash and the frame. When the sash is closed, these seals are tightly compressed, leaving almost no gaps—making it difficult for convection currents to form. As a result, heat loss primarily occurs through thermal conduction via the glass, sash, and frame profiles themselves, as well as radiative heat dissipation. Additionally, some heat escapes due to air leakage at the points where the sash contacts the frame and around the perimeter where the frame meets the wall—though these pathways contribute to relatively reduced overall heat loss.
Fixed windows are installed by embedding the window frame directly into the wall, with the glass fitted securely into the frame. Both the glass and the frame are sealed using weatherstripping or sealant, which effectively prevents air from circulating through the gaps—making it extremely difficult for heat to escape via convection. As a result, heat loss in fixed windows primarily occurs through thermal conduction between the glass and the window frame.
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Contact Address: No. 7, Industrial Avenue East, Songxia Industrial Park, Shishan Town, Nanhai District, Foshan City
Warren Doors and Windows, south of Fangshan Road and west of Dongtai Road, Dongcheng Subdistrict, Linqu County, Weifang City, Shandong Province
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