Window U Value Explained

Window performance is measured by its U value.

So what is a u value of a window? In simple terms, if its colder outside than inside, the U value is a measurement of how quickly the heat from the inside leaks through the window. This is where German engineering comes in and why, for good U Values, tilt and turn windows make so much sense.

U Values

The U value of a window or door is a measure of thermal transmittance. It’s used to convey how heat is transmitted through the door or window. A good performing window has a low U Value. U Values for tilt and turn windows, by design, are better. As is their airtightness. Airtightness is so often ignored in the UK, and insulation is viewed on its own. Buildings in Germany are generally designed with engineering and construction in mind. This mindset ensures that the building looks good, is energy efficient and performs for a very long time.

The U value is measured in W/m^2K, which indicates how much heat transfers across the window through 1 m^2 when there is a temperature difference of 1 degree centigrade between inside and outside.

window u value

Uf and Ug Values

Both the frame (Uf) and the glass (Ug) are taken into account when calculating the Uw Value (ie. the overall performance of your window). It is the Uw value that is important, and how you should properly assess the product you are looking to buy. Don’t be fooled by companies stating impressive U values that seem too good to be true. They probably are not quoting the all important Uw value. Instead, many companies simply quote the value for the glass alone.

In the UK, Building regulations generally require you to have a Uw value of 1.6 W/m^2K

A well engineered German window often delivers a Uw value of 0.8 W/m^2K and we can go right down to 0.56 W/m^2k.

Example Energy Loss

Now that we know what a U value is, to put this all into context, a brand new typical single glazed window which is in good order will deliver a Uw value of around 4.8 W/m^2K. So when it’s zero degrees outside and 20 degrees inside, the window will be losing 96W of energy. In a typical room with 4m^2 of windows, this represents a constant energy loss of 384W.

Sadly, these figures are for a new window. But, with draughts coming from failed seals and cracked panes, the actual performance can be dramatically worse. Add to this the heatloss through ceilings, walls and floors and you can quickly find that a 3000w heater is needed to just maintain a room at temperature. Floors, walls and ceilings can be expensive and disruptive to improve. Windows, however, often provide big potential improvements. The jump from a poor performance failed double glazing unit to a modern, airtight, German window will deliver instantly recognizable improvements.

Choosing the Right U value – Performance Windows explained

We can help you choose the most effective product for your home.

By understanding the age and likely energy efficiency of your home; discussing walls,floors, doors, roofing and if the building has insulation, we can provide tailored advice to suit your home.

In new Buildings, the UK Building regulations require new properties to have a Uw value of 1.4W/m^2K.

In existing buildings requiring replacement glazing, the value is lower at 1.6W/m^2K. But again, the bar is likely to be raised.

Enlightened Windows pride ourselves on making sure we help customers get the right product for their home.

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