U-values in construction are thermal transmittance, which is the rate of heat transfer through a structure divided by any temperature difference across the structure. If you have a very well insulated building, the U-value will be quite low. Of course the same applies to buildings that are not well insulated, because then the U-value will be much higher.
Calculating U-values
Calculating the U-value is quite easy. You basically find the inverse of the sum of each of the thermal resistances of each individual material in the building. In addition to the materials, you must also include the external and internal faces. So simply go layer by layer through the building as you arrive at your U-value.
Measuring U-Values
When measuring U-values, you must keep a few factors in mind. These factors include:
- The temperature difference – the greater the temperature difference, the more accurate the measurement will be
- Weather conditions – cloudy days are better than sunny days
- Excellent adhesion of thermopiles within the test area
- How long you monitor – the more you monitor, the better
- The more tests you do, the more accurate your measurement will be
There are two factors that can compromise your measurements. These two factors are:
- Ambient temperature
- Convection currents can contribute to additional heat flow
We recommend measuring U-values for at least two weeks for the most accurate results. Of course, if you're measuring near a slab, you'll have to take measurements for an entire year, thanks to heat storage in the ground.
Because it can be very time-consuming to collect measurements and calculate U-values, you may want to use one of the many U-value calculators available. You should be careful which one you use because some of them are very simplistic and don't offer all the little details that you should have included when doing your calculations.
U-Values of Common Items
U-values are always measured in watts. Some of the common item measurements are:
- A solid brick wall – 2 watts
- An insulated wall – 0.18 watts
- A hollow wall without any insulation – 1.5 watts
- Single glazing – 4.8 to 5.8 watts
- Double glazing – 1.2 to 3.7 watts
- Triple glazing (made below) – 1 watt
- A solid wood door – 3 watts
Converting U-values to R-values
Most people have never heard of U-values, but almost everyone has heard of R-values. R-values are used for insulation, which is why this term may seem more familiar to many. However, most people have no idea how these two values relate or that you can convert U-values to R-values quite easily.
The equation used to convert U values to R values is as follows:
R-Value = 1 / U-Value
Therefore, a U-value of 0.35 would have an R-value of 2.86.
Although you always want as low a U-value as possible, you will always want as high an R-value as possible.
Most people can go their entire lives without knowing the U-value of their building or home, let alone its R-value. However, think about all the money you could save if you can improve the energy efficiency of your space by changing its value. U, as well as its R value.
So if you don't know your U-value, find out what it is and see how you can make it as low as possible. The work you do will be worth it when the temperature inside your home or building stays consistent and your energy bill gets progressively lower in the future.
