What is a U-Value?
When it comes to measuring the effectiveness of a particular material as an insulator, something called a U-Value is used. With this particular measurement, the lower the U-Value, the better. But why?
Technical Description:
Thermal conductivity, also known as lambda, is a measurement of how easily heat flows through a material, regardless of how thick the particular material is. The lower the thermal conductivity a material has, the better its thermal performance is. This means that heat moves across the material at a slower rate. It is measured in Watts per Metre Kelvin or W/mK.
To use some of the more well-known materials for an example, sheep's wool used as a property insulator would have a W/mK of 0.034. While some wood fibres are around 0.061 W/mK. The very best methods of insulation using vacuum insulated panels have a W/mK of just 0.008.
R-Values are used to measure the resistance to heat flow a material has at a given thickness. In this case, the higher the number, the more thermal resistance the material offers and the better it works as an insulator.
The U-Value is a way of measuring how much heat is lost through a material at a particular thickness and takes into account the three main ways that heat can be lost €“ conduction, convection and radiation. This measure is the one used in Building Regulations and the lower the U-Value, the better the material works as a heat insulator.
So to summarise:
- Thermal conductivity and R-Values are best with high numbers
- U-Values are best with low numbers
- U-Value is the best way to judge the insulating ability of a material across the three main ways that heat is lost.
Why is it U-value important?
When it comes to windows, the better the insulation offered the less heat that is lost. This translates directly into a saving on energy bills for the household so is important. According to Building Regulations, a window in a new property must have a U-Value of 1.6 W/mK at the maximum. A single glazed window only has a U-Value of 4.8 W/mK so this is why all new houses have at least double glazed windows installed.
By upgrading to double glazed windows, the difference is clear. A double glazed window with a 6mm gap between the panes that is filled with air has a U-Value of 3.1 W/mK while this changes to 2.8 W/mK when the gap is widened to 12mm and further to 2.7 W/mK when it is at least 16mm apart. This means that the larger the gap between the panes, the better the insulation the window provides.
The U-Value can be further decreased by the use of argon gas to fill the gap between the panes and also by the thickness of the glass in the window.
Conclusion
Windows will never be as insulating as walls by their very nature but by investing in good quality windows with the right features, the U-Value can be brought right down and those extra pounds kept in the bank, instead of paying out to the utility companies.
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