U-value
The
term ‘U-value’is talked about a lot when discussing the heat insulating performance
of various building components, especially windows but also walls,
roofs and floors. The U-value provides an estimate of how much heat
flows through a one metre square area of
the component, from its hot side to its cold side, when a 1oC
temperature differenceis maintained across it. So the units of a U-value are watts/m2/oC.
It is a useful number because, for almost all situations, the heat flow (measured in watts) is directly proportional to the temperature difference between the hot and cold side so provided you know this temperature difference between the outside and inside and also you know the area of the component you can calculate how fast heat flows out through that component. Note that the higher the U-value the more heat leaks out.
Here are two simple examples:
1. Suppose you have a double glazed window with a quoted U-value of 2.0 watts/m2/oC and you know that the inside of your house is at 19oC whereas the outside temperature is only 2oC. How much heat is flowing out of the window if it has an area of 4m2?
The temperature difference is dT = 19-2 = 17oC
The area A is 4m2so the heat flowout through the window is Q = U x A x dT = 2.0 x 4 x 17 = 136 watts.
2. If you had just a single glazed window of the same area, with a U-value of 5.0, the heat flow out through the window is Q = U x A x dT = 5.0 x 4 x 17 = 340 watts.
Some typical U-values for frequently-encountered building components are shown in the table below:
| Component Material | Thickness | U – value (watts/m2/oC) |
| Solid brick wall | 230 mm | 2.2 |
| Brick & block cavity wall (unfilled) | 270 mm | 1.0 |
| Brick & block cavity wall (insulated) | 270 mm | 0.6 |
| Single glazed window | 5 mm | 5.0 |
| Double glazed window | 20 mm gap | 2.9 |
| Double glazed window, low emissivity glass | 20 mm gap | 1.7 |
| Solid concrete floor | 150 mm | 0.8 |
Note: these values, taken from http://diydata.com/information/u_values/u_values.php are for general guidance and should not be relied on for accurate estimation.