Definition and Importance of U Value

U value (unit: W/m²·K) is the core indicator to measure the heat transfer capacity of materials, indicating the amount of heat transferred per unit area of glass under unit temperature difference. The lower the U value, the better the thermal insulation performance. For example, the U value of ordinary single-layer glass is about 6.0 W/m²·K, while the U value of Low-E insulating glass can be reduced to 1.1-2.0 W/m²·K, with significant energy-saving effect.

U value is directly related to the emissivity (e value) of glass. The emissivity of ordinary glass is 0.84, while Low-E glass can reduce the emissivity to 0.02-0.15 through coating technology (such as silver layer), thereby greatly reducing the transmission of far-infrared thermal radiation and achieving a lower U value.

Four major factors affecting the U value of Low-E glass

1. Coating process and number of layers

Offline sputtering method: Multiple layers of metal films (such as single silver, double silver, and triple silver) are coated by vacuum magnetron sputtering technology, with lower emissivity (e value 0.08-0.15) and better U value. For example, the U value of double silver Low-E glass is about 15% lower than that of single silver.

Online chemical deposition method: The coating layer is thicker but the emissivity is higher (e value 0.25-0.5), and the U value is slightly inferior to the offline process.

2. Hollow structure design

Filling the hollow layer with inert gas (such as argon) can reduce the U value by 10%-15%. For example, the U value of double-glass hollow Low-E glass is 1.7 W/m²·K, while the triple-glass double-cavity structure can be further reduced to 0.7 W/m²·K.

3. Coating surface position

In hollow glass, the coating surface position affects the U value and the shading coefficient (SC). For example, placing the Low-E film on the inner side of the hollow layer (the third side) can reflect indoor heat in winter, while in summer, the film surface position needs to be adjusted to balance the insulation and shading requirements.

4. Window frame matching

The U value difference between the window frame and the glass must be controlled within 1.5, otherwise heat will be lost through the window frame. For example, the thermal insulation aluminum frame (U value 2.9-3.6) and Low-E glass (U value 1.6) can achieve the optimization of the U value of the entire window.

U-value selection strategies for different climate zones

1. Cold regions: Double silver/triple silver Low-E hollow or vacuum glass is preferred, with a U value of ≤0.8 W/(m²·K).

2. Hot regions: Focus on the balance between shading coefficient (Sc value ≤0.35) and U value, and high-transmittance Low-E hollow is recommended.

The U value indicates the amount of heat that passes through a unit area of glass per unit time under steady-state conditions (unit: W/(m²·K)). The lower the value, the better the thermal insulation performance. Low-E glass reduces heat radiation loss through low-emissivity coating, and its U value is usually lower than that of ordinary glass (the U value of ordinary insulating glass is about 2.7 W/(m²·K), while the U value of Low-E insulating glass can be as low as 1.1~1.6 W/(m²·K)).