The U-value is one of the key concepts in the construction industry, and especially in the context of assessing the energy efficiency of building materials, including windows, doors, walls or roofs. After all, it is a parameter that tells us how well a building element insulates heat, which is extremely important for the energy efficiency of buildings. Understanding, therefore, how U-factor works, how to measure it and what types of U-factor there are, is of great importance both to window manufacturers and to people who are planning to build or retrofit their homes or flats to make them more energy efficient.
 

What is the heat transfer coefficient U?

The U-value (previously referred to as the K-factor) is nothing more than a value describing the amount of heat that passes through a material with a surface area of 1 square metre in 1 hour with a temperature difference of 1 degree Kelvin between the sides of that material. In short, the U-value tells us how well a material or partition (such as a window) insulates heat. The lower the U-value, the better the insulating properties, of course.
For windows, the U-value (pay particular attention to the Uw parameter, i.e. the overall heat transfer coefficient of the window) plays a particularly important role. The window is one of the main elements of a building through which significant heat loss often occurs, which is why window manufacturers strive to achieve the lowest possible U-value, which ultimately translates into higher energy efficiency. For windows that meet modern energy-saving standards, the U-value must already be below 0.9 W/(m²K) from 2021.
 

Why the U-value is so important?

The importance of the U-value is a result of increasing energy efficiency requirements for buildings and the drive to reduce energy consumption for heating and cooling. In the context of climate change and sustainability policies, buildings must therefore meet increasingly stringent thermal insulation standards and thus – must have low U-values.
Heat loss through windows can account for up to 30-40% of a building’s total energy loss, so choosing the right, well-insulated windows is of paramount importance. The lower the U-value of a window, the lower the heat loss will be and therefore the heating costs.
High-quality windows with a low U-value not only contribute to energy savings, but above all provide better thermal comfort indoors, thus eliminating the problem of cold zones near the windows and condensation on the glazing, especially in autumn and winter.
 

How to measure U?

The measurement of the U-value is carried out in accordance with European standards, in particular EN ISO 10077-1. However, this process requires the use of specialised equipment and appropriate laboratory conditions.
For windows, the Uw thermal transmittance is made up of three main components:

• The heat transfer coefficient for the glazing (Ug) – this is the value describing the thermal insulation of the glazing package itself. Modern windows are most often fitted with double or triple glazing units filled with noble gases (e.g. argon), which allows very low Ug values to be achieved – even below 0.5 W/(m²K).
• Frame heat transfer coefficient (Uf) – the window frame also affects the overall thermal insulation of the window. The material from which the frame is made is of key importance. High-quality aluminium, timber or plastic window frames are able to significantly reduce the Uf value.
• Frame edge heat transfer coefficient (Psi) – the warm distance frames, which separate the panes of glass in a glazing package, are designed to minimise thermal bridges in the window. They therefore affect the overall Uw value.

The Uw coefficient, which describes the entire window, is calculated as a combination of Ug, Uf and Psi. It is also worth noting that window manufacturers specify the Uw value for the entire window and not just the glass, which is crucial when choosing the right products.
 

Types of U-value in the context of different materials

It is worth bearing in mind that the U-value does not only apply to windows, but also to other building elements such as walls, roofs and doors. Here are some examples that show how the U-factor varies depending on the type of material:

• External walls – for well-insulated external walls, the U-value is around 0.20 W/(m²K), which means that heat loss through such partitions is minimal.
• Roofs – the roofs of buildings can also be a source of significant heat loss. The U-value for a well-insulated roof is typically 0.15 W/(m²K).
• External doors – for external doors, especially those installed in energy efficient buildings, the U-value is around 0.60-1.10 W/(m²K), depending on the materials used and the door design.
• Floors – floors on the ground, like other building elements, must meet thermal insulation standards. Well-insulated floors have a U-value of 0.20-0.25 W/(m²K).

It is also worth remembering that for each of these building elements, aiming for the lowest possible U-value reduces energy losses and increases the energy efficiency of the building.

U-value and legislation

Over the years, increasingly stringent regulations on the energy efficiency of buildings have been introduced in Europe, including, of course, Poland. These requirements are primarily aimed at limiting CO2 emissions and reducing energy consumption, which is part of European policy to combat climate change.
From 2021, all newly constructed buildings in Poland must meet the standards of Nearly Zero Energy Buildings (nZEB), which means that they must be equipped with high-quality, energy-efficient components, including windows with a low heat transfer coefficient. For windows, the Uw coefficient must not exceed 0.9 W/(m²K) and for doors 1.3 W/(m²K).
 

Energy certificate – what is worth knowing about it?

As of 28 April 2023, new regulations are in force in Poland that require property owners to carry out an energy audit in certain situations. Its effect is to obtain an energy certificate, also known as an energy certificate. This is a document that assesses the energy efficiency of a building and is an important tool in the fight to reduce energy consumption and improve the thermal efficiency of buildings. Moreover, such a certificate is now mandatory for several key cases.
First of all, every newly constructed building from the date mentioned above must have an energy certificate. This means that an audit must be carried out before the building is put into use, which will assess how well the building meets the requirements related to thermal insulation and energy efficiency. However, the new regulations do not only apply to new construction. If someone intends to sell or rent a house or flat built before these regulations came into force, they must also have an energy audit carried out and obtain a certificate.
The basis for an energy certificate is the thermal performance of the materials used to construct the building. The thermal conductivity of the walls, roofs, floors and ceilings plays a key role. The results of the audit indicate how effectively the building envelope reduces heat loss, which has a direct impact on operating costs and the thermal comfort of the occupants. An energy certificate therefore tells future owners or tenants how energy-efficient a building is, which in the long term translates into lower heating bills and a reduced burden on the environment.
Ignoring the new regulations can have serious financial consequences. Failure to obtain a certificate or carrying out a sale or rental without this document may result in a financial penalty of up to PLN 10 000. This is an important factor which should motivate property owners to comply with the regulations. It is worth remembering, however, that the introduction of an energy performance certificate is not only a legal burden, but above all an action to improve the quality of life. Well-insulated buildings not only protect against heat loss, but also help in the fight against rising energy costs and climate change.
One of the main objectives of the introduction of energy certificates is to raise awareness among investors, architects and construction companies about the essence of energy efficiency. Modern technologies and building materials that meet the highest thermal insulation standards therefore play a key role in creating sustainable and comfortable buildings. Striving for the lowest possible thermal transmittance values is not only a legal requirement, but also a concern for the environment and the health of future building users.
 

Technology and U-value

With advances in technology, manufacturers of windows and building materials are constantly working to improve the thermal insulation of their products. The use of modern technologies, such as low-emission coatings on the glazing or filling the space between the panes with noble gases, therefore allows for increasingly better insulation parameters.
Below are some of the modern solutions that effectively reduce the U-value of windows:

• Triple glazing packages – instead of traditional double glazing, triple glazing packages are increasingly being used to significantly improve the thermal insulation of the window. In combination with warm distance frames, the Ug coefficient for such packages can be as low as 0.5 W/(m²K).
• Low-E coatings – the application of special coatings to the surface of the glazing makes it possible to reduce the emission of heat to the outside of the building, which in turn contributes to lowering the U-value.
• Noble gas in the glazing cavities – filling the glazing cavities with noble gases, such as argon or krypton, significantly improves the thermal insulation of the window.

As we can therefore see, the U-value is undoubtedly one of the key parameters in the final assessment of the energy efficiency of buildings and their components, including windows, doors, walls and roofs. Its understanding is also essential for both manufacturers and customers who want to improve the energy efficiency of their homes. Therefore, by choosing high quality Fintecnic windows with a low thermal transmittance, you are not only investing in thermal comfort, but more importantly in energy savings and environmental care.