Metric U-Value of a window into Imperial U-Factor Converter

What is a good U-Factor for a window?

Generally, the lower, the better, but it all depends on the region along with building regulations.

In the United States, the international ENERGY STAR® program recommends that windows have a U-Factor of 0.27 or lower in colder climates and a U-Factor of 0.40 or lower in warmer places.

In Europe, the recommended Uw value for windows depends on the country and the specific regulations in place. For example, in Poland, Warunki Techniczne state that window and balcony doors must have an Uw value of 0.9 (U-Factor 0.16) or lower.

Windows from Poland are well-known for being among the warmest. If you are interested in the topic of energy efficiency, learn more about the Best energy-efficient uPVC windows

What U-Value and U-Factor are?

The Metric U-Value measures the thermal conductivity of a material, such as a window, in watts per square meter per Kelvin (W/m²K). It describes how much heat can pass through a material per second per square meter for every Kelvin difference in temperature between the inside and outside the window.

In the U-Factor’s case, The Imperial U-Factor measures the same thing – the rate of heat transfer through a window, but in British Thermal Units per hour per square foot per Fahrenheit degree (BTU/hr·ft²·°F).

Why does Europe use Uw value?

The difference in terminology between Europe and the United States regarding window thermal performance metrics can be attributed to the different standards and regulations that apply to building codes in each region. It also stems from a more simple matter that we stated earlier — the difference in measurement units.

In Europe, the Energy Performance of Buildings Directive (EPBD) established a common methodology for calculating the energy performance of buildings, which includes the use of Uw.

In contrast, the United States has its own building codes and regulations, which generally use the U-Factor to measure the thermal performance of windows.