A U value tells us how much heat is lost from a building through different elements, such as windows, walls and floors.
It is a measurement of heat transference, also known as the “overall heat transfer co-efficient”, meaning the higher the number value, the more heat is being lost, and the less environmentally friendly the building is. A lower value therefore indicates greater insulating properties and is more energy efficient.
Calculating the U Value
The U value of a building element is calculated by working out heat loss for each component of the building. This is determined by the material used, its thickness, and its positioning.
Rooflights are made up of a number of different component materials that each have different properties when it comes to heat transference. Therefore, to find out the U value of this type of element, a number of factors need to be determined.
Firstly, the ability of each component material to conduct heat must be identified. This is shown as a ‘K value’. This value is used along with the thickness of the given material (d) in the following equation to give the resistance of the component material:
R = (1/k) x d
A resistance value (R) must then be found for each component material.
Next, the internal and external faces of the building element must be taken into account. These are known as external and internal resistances and are fixed values. This information is then combined in the following equation to discover the U value (U):
U = 1 / (Rso + Rsi + R1 + R2 …)
Rso is the fixed external resistance and Rsi is the fixed internal resistance, whilst R1, R2… give the sum of the resistances previously worked out for the different building materials in the building element.
Current building regulations in England and Wales allow a maximum U value of 2.2 W/m2K for skylights, covering 20% of the roof surface.
Green Energy Rooflights
All rooflights from EOS are energy efficient and feature U values as low as 0.97 W/m2K.
The EOS Eco roof lights, which are triple-glazed and injected with Krypton Gas, have an even lower U value of 0.65 W/m2K and include a centre pane U value of 0.6 W/m2K.
The U values of these skylights are so low that official building standards permit them to make up an additional 10% of the roof area than other, less energy efficient designs – taking total potential coverage to 30%.
So, as well as flooding a building with daylight to naturally enhance the indoor environment, EOS rooflights ensure green energy efficiency by reducing the need for CO2-emitting artificial lighting, whilst guaranteeing that heat isn’t lost unnecessarily.
For green efficient skylights with low U Values, speak to the EOS Rooflights team today.
Green house courtesy of Pixabay via CreativeCommons