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What is the heat transfer coefficient and how to calculate it for windows?

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Introduction

The heat transfer coefficient, denoted as Uw, is one of the key parameters affecting the energy efficiency of a building. Its value determines how much heat penetrates a window, and thus affects indoor thermal comfort and heating costs. In this article, we will discuss what the Uw coefficient is, its significance and typical values for different types of windows.

What is the heat transfer coefficient?

Definition of the heat transfer coefficient (Uw)

The Uw heat transfer coefficient is a measure of the amount of heat that penetrates a square meter of window surface per unit of time with a temperature difference between inside and outside of one degree Kelvin. The lower the Uw value, the better the thermal insulation of the window.

The importance of Uw for the energy efficiency of a building

The Uw value has a direct impact on heat loss in a building. Windows with a lower Uw coefficient contribute to less heat loss, which leads to lower heating costs and improved indoor thermal comfort. That is why it is so important to pay attention to this parameter when choosing windows.

Heat transfer coefficients for different types of windows

Double-glazed windows: Heat transfer coefficient double-glazed windows

Characteristics of double-pane windows

Double-glazed windows consist of two layers of glass separated by a space filled with air or a noble gas such as argon. This space acts as a thermal barrier, reducing heat transfer.

Typical Uw values

For double-glazed windows, typical Uw values range from 1.1 to 1.4 W/(m²-K). These values can vary depending on the material of the window frame and the type of filling of the space between the panes.

Triple-glazed windows: What coefficient do triple-glazed windows have?

Description of triple-pane windows

Triple-pane windows consist of three layers of glass, between which there are two spaces filled with air or noble gas. This design provides better thermal insulation compared to double-pane windows.

Advantage of triple-pane windows over double-pane windows in terms of thermal insulation

Triple-pane windows are characterized by a lower Uw coefficient, usually between 0.6 and 0.9 W/(m²-K). As a result, they better protect against heat loss, which is especially important in climates with large temperature fluctuations.

How to calculate the heat transfer coefficient (Uw) for a window?

Window heat transfer coefficient – formula

Uw coefficient is calculated using the formula:

Uw = 1(Rf+Rg)

Where:

  • Rf – thermal resistance of the frame
  • Rg – thermal resistance of the glass

Explanation of the formula for calculating Uw coefficient

This formula takes into account the thermal properties of both the window frame and the glass itself. For an accurate calculation, other factors, such as the effect of gaskets and the shape of the window, should also be taken into account.

Heat transfer coefficient standards for windows for 2024

What is the heat transfer coefficient for windows 2024?

As of 2024, new standards for the heat transfer coefficient for windows are in force in Poland. According to them, the maximum Uw value for windows in residential buildings is 0.9 W/(m²-K).

Current regulations and requirements for the heat transfer coefficient

The regulations are aimed at making buildings more energy efficient and reducing CO2 emissions. The Uw value requirements are part of broader climate and energy policy measures.

Thermal transmittance of old windows

Thermal transmittance characteristics of older window models

Older window models, especially those with single glazing, have much higher Uw values, often exceeding 2.0 W/(m²-K). This means that they are much less efficient in terms of thermal insulation.

Which windows to choose in terms of heat transfer coefficient?

Recommendations for different building types and locations

The choice of windows depends on many factors, such as the type of building, its location and the individual needs of the users. In single-family homes in areas with harsh climates, triple-glazed windows will work best. In multi-family residential buildings and in milder climate zones, double-pane windows with a low Uw coefficient can be considered.

Examples of low Uw windows available on the market

Numerous energy-efficient windows from reputable manufacturers are available on the market, such as PVC windows with triple glazing, wooden windows with additional insulation, and modern aluminum windows with thermal breaks.

Summary

In summary, the Uw heat transfer coefficient is a key parameter affecting the thermal insulation of windows and the energy efficiency of buildings. Choosing the right windows with a low Uw coefficient allows for significant energy savings and improved thermal comfort indoors. When choosing windows, it is advisable to be guided by current standards and individual needs to ensure optimal thermal protection and comfort.