Everything you need to know about U-values

When you are considering new glazing for your home, you will no doubt come across the term U-Values in your research. But what are U-values and why should you be concerned with them? Here’s an overview of everything you need to know about U-values and glazing.

What are U-Values?


Simply put, a U-Value is the rate of heat loss through a window system. The lower the U-Value, the better the window’s resistance to heat loss and the higher its insulating properties.

“A U-value is the measure of heat loss through a structural element. It is calculated on the rate at which heat transfers through 1 square metre of a structure, where the temperature difference between the inner and outer face is 1 degree Celsius.
U-Value (w/m²K) = the measure of structural heat loss per unit of surface area”



Because the U-value rates the entire window unit and not only the glass, there are many factors that influence a window’s rating: the type of the glass, amount the layers, the fill between the panels, location of the window and insulation of the frame all contribute.

Type of the Glass

Low-E glass or low-emissivity glass has a microscopically thin, clear coating that reflects long-wave or infrared energy. The central and inner panes of a triple glazed unit are made up of low-E glass. That means that when the interior heat tries to escape, it is being deflected from the window back into the room, thereby reducing the radiant heat loss through the glass.

Amount of Layers

The quantity of layers of glass that the heat needs to escape through, directly influences the amount of heat that escapes. If a single layer of glass has a U-value of 5, a double glazing unit 1.4 and a triple glazing unit has a U-value of 0.7. The addition of a third layer of glass thereby significantly reduces the heat loss.

Gas-fill between panels

A very important factor in the efficiency of a triple glazed window, is the size and fill of the gaps between panels. Where most double-glazed units have a gap of 20mm between the 2 panes, a triple-glazed unit has 2 gaps of 16mm each. These narrower gaps are filled with Krypton [Kr] gas instead of the usual Argon [Ar] gas which has an even lower conductivity. The gas layers then further increase the heat retention properties of a triple-glazed window unit.

Location of the Window

A triple glazed window on the south side of a home will perform differently from a window on the north. Allowing direct sunlight to heat up a home plays as important a part as the right glass preventing that heat from escaping. This is where the energy balance of a window comes in, which measures not only heat loss but solar heat gain as well.

Frame Insulation


The efficiency of the window unit will only be as good as the insulation of the frame. If there are gaps between the frame and the surrounding wall, then the heat will be allowed escape, regardless of the glass insulation. It is therefore essential to use a FENSA or CERTASS registered glazing installer to ensure the optimum efficiency of your new windows.

The window with the lowest U-value will have the highest impact on your energy bill and will have the biggest benefit to you and your family in the long run. Triple glazing typically have U-values of 0.8, making them the most efficient insulated window systems available.

Published on : 3rd September 2016

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