# What is the heat transfer coefficient of the window Uw?

Uw coefficient of the window is a parameter that determines the heat transfer through glass panes and profiles. What is the Uw standard for woodwork produced and assembled today? What has changed recently? What to watch out for when choosing a specific model? Read!

## What is the Uw coefficient of a window?

Window joinery must meet specific requirements for thermal and acoustic insulation. The former are determined by the commonly used U-factor, while the latter – R. As far as heat transfer is concerned, three specific parameters are used, which are:

• Ug – glass penetration coefficient (ang. glass)
• Uf – frame penetration coefficient

• Uw – window penetration coefficient

Of the above-mentioned coefficients, the Uw coefficient describes the properties of the window in the fullest way, which is why investors meet this indicator most often.

## How is the Uw of windows calculated?

The Uw coefficient is a parameter that determines how much heat flows through one square meter of the window assuming that there is a constant temperature difference of 1 Kelvin between the interior of the building and its external surroundings (the basic unit in the SI system – International System of Units, equal to 1 degree Celsius). This coefficient is expressed in the unit W/m2K. Of course calculation Uw of the window is a duty that rests with its manufacturer, who is obliged to perform appropriate measurements and calculations and takes responsibility for their correctness.

## The latest Uw standard for a window in the house

What is the norm Uw for a window in the house? From January 1, 2021, it is 0.9 W / m2K, which means that it is very high, similar to the requirements for joinery installed in passive houses. Until the end of 2020, the law stated that heat transfer coefficient Uw it cannot be higher than 1.1 W/m2K – so it was a less restrictive record. The change in this regulation was included in the regulation of the Minister of Infrastructure “on technical conditions to be met by buildings and their location”, which is a response to the progressing climate change and the need to reduce energy consumption and carbon dioxide emissions.

## When does the heat transfer coefficient Uw have to be 0.9?

Importantly, the penetration coefficient Uw at the level of 0.9 is valid for all rooms in which the planned temperature will be at least 16 degrees Celsius. In practice, these are typically residential interiors – rooms, kitchens, bathrooms, etc. In the case of rooms such as garage, utility warehouse, non-usable attic, where the temperature is usually lower, The Uw of windows can be 1.4 W/m2K.

The second important caveat is that the new standards apply to those investors who received a building permit for a house after January 1, 2021. Construction started earlier can be continued with the use of windows with worse indicators (although always, if possible, it is always worth considering increasing the energy efficiency parameters).

## Be careful: Uw windows are not the same as Ug

Although manufacturers and sellers are obliged to reliably inform buyers, as with any other major purchase, also in this case you should be careful: carefully read the information on the parameters regarding heat transfer through the windows, and in case of doubt – ask for details.

Quite a common practice on the market is to tempt customers with a sensational U-factor of 0.5-0.6. You should be aware that this information often concerns the glass itself, i.e. the Ug coefficient, not Uw. It is precisely these types of sizes that are considered optimal if the so-called glazing unit is to ensure the energy efficiency of the entire window. It is worth remembering that Uw window is a kind of resultant of indicators Ug and Uf.

What is important – the size of the reference window, to which the Uw coefficient should refer, is 1230 x 1480 mm. If this window is larger (e.g. 1500 x 1500), this parameter is falsified due to the proportionally larger surface area of the panes themselves, which by their nature have better properties than profiles.