WINDOWS INSULATION EXPLAINED

The overall thermal efficiency of a window, represented by the Uw value, is not a simple average of the U-values of the glass (Ug) and the frame (Uf). Instead, it's calculated by taking into account the proportion of the window that is glass versus frame, as well as the linear thermal transmittance of the edge of the glass (also known as the Psi value, Ψ).
Here's a simplified formula to give you an idea of how the Uw value is calculated:



Where:

• Uw is the overall U-value of the window.
• Ag is the area of the glazing (glass).
• Ug is the U-value of the glazing.
• Af is the area of the frame.
• Uf is the U value of the frame.
• Lg is the length of the glass edge.
• Ψg (Psi value) is the linear thermal transmittance of the glass edge.

To calculate the overall U-value of a window, you would need the exact measurements of the glass and frame areas, the U-values for both, and the linear thermal transmittance of the glass edge. This calculation is typically done using specialized software that takes into account the complex interactions between these elements.

For example, if you have a window with:

• A glass area (Ag) of 1.2 m² with a Ug value of 1.1 W/(m²K),
• A frame area (Af) of 0.3 m² with a Uf value of 1.5 W/(m²K),
• A glass edge length (Lg) of 6 m with a Psi value (Ψg) of 0.04 W/(mK),

The calculation for the overall U-value (Uw) would be:



The overall U-value (Uw) for the window, calculated using the given formula, is approximately Uw=1.34 W/(m²K).
This value represents the heat transfer coefficient of the entire window, including the frame and the glazing.

This example is a simplification and does not take into account all the possible complexities of a real-world scenario, but it illustrates the general approach to calculating the overall U-value of a window.