How to Calculate Thermal Transmittance in Windows and What to Pay Attention To

The thermal transmittance of windows is one of the most important parameters that determines how much heat passes through the window. Understanding how to calculate it and the key factors affecting its value allows for an informed choice when purchasing energy-efficient windows.

What is the U-value?

The thermal transmittance coefficient U (expressed in W/(m²·K)) is the basic parameter that determines the thermal insulation of a window. The lower the U-value, the better the insulation and, consequently, the lower the heat loss.

The U-value is calculated for the entire window, which includes:

1. Glazing unit (Ug) - the impact of the glazing on thermal transmittance.
2. Window frames (Uf) - the insulation of the window profiles.
3. Thermal bridges (Uw) - e.g., at the junctions between the frame and the glazing or between the window and the wall.

The total U-value (Uw) can be calculated using the following formula:

Uw = [(Ag · Ug) + (Af · Uf) + (ψ · l)] / A

Where:

• Ag – area of the glazing unit,
• Af – area of the frame,
• Ug – thermal transmittance coefficient of the glazing,
• Uf – thermal transmittance coefficient of the frame,
• ψ (psi) – thermal bridge coefficient at the glazing-frame junction,
• l – length of the thermal bridge,
• A – total area of the window.

What to Pay Attention to When Choosing Windows?

1. Glazing unit
   • Choose windows with triple glazing, which have a lower Ug (0.5-0.6 W/(m²·K)) than double glazing.
   • Pay attention to the filling with noble gases (argon, krypton) and low-emissivity coatings on the glass.
2. Window frames
   • High-quality profiles made of PVC, wood, or aluminum with thermal insulation inserts have a lower Uf value.
3. Thermal bridges
   • Ensure that the windows have warm edge spacers between the panes, which minimize heat loss at the glazing-frame junction.
4. Declaration of the U-value of the entire window (Uw)
   • Manufacturers often provide the U-value for the glazing (Ug), but the key value is the Uw, which includes the entire window.

Additional Factors Affecting Thermal Transmittance

• Window size: Larger windows have a higher proportion of glazing in the total area, which usually lowers the Uw value.
• Installation: Proper installation eliminates leaks and thermal bridges between the window frame and the wall.
• Exposure: Windows facing north lose more heat than those facing south, which can additionally benefit from solar heat gains.

Summary

For a window to be energy-efficient, its thermal transmittance coefficient Uw should be as low as possible, ideally below 0.9 W/(m²·K) for facade windows. It is crucial to consider the parameters of both the glazing and the frames, as well as to ensure proper installation. This way, heat loss can be minimized and thermal comfort in the building can be increased.

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