U-values in construction are the thermal transmittance, which is the rate of transfer of heat throughout a structure divided by any difference of temperature throughout the structure. If you have a building that is very well insulated, then the U-value is going to be quite low. Of course, the same holds true with buildings that are not insulated well, because then the U-value will be much higher.
Calculating the U-Values
Calculating the U-value is quite easy. You basically find the reciprocal of the sum of each of the thermal resistances of each individual material within the building. In addition to the materials though, you must also include both the external and internal faces. Therefore, simply go layer by layer in the building when coming up with you U-value.
When you are measuring U-values, you must keep a few factors in mind. Those factors include:
- The temperature difference – the greater the temperature difference, the more accurate the measurement will be
- The weather conditions – cloudier days are better than sunny days
- Excellent adhesion of thermopiles within the test area
- How long you monitor – the longer you monitor the better
- The more tests you do, the more accurate your measurement will be
There are two factors though that can compromise your measurements. Those two factors are:
- The ambient temperature
- The convection currents can contribute to additional heat flow
We recommend finding measurements of the U-values for at least two weeks for the most accurate results. Of course, if you are measuring near a floor slab, you must take measurements for an entire year, thanks to the storage of heat in the ground.
Since it can be very time consuming to collect U-value measurements and calculations, you might want to use one of the many different U-value calculators available. You must be careful of which one you use though, because some of them are too simplistic and do not 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. A few of the common item measurements are:
- A solid brick wall – 2 watts
- An insulated wall – 0.18 watts
- A cavity 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 (done below) – 1 watt
- A solid timber 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 what are used for insulation, which is why that term may sound more familiar to many. However, most people have no idea how these two values relate to each other 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.
While you always want as low of a U-value as possible, you will always want as high of an R-value as possible.
Most people can go their whole life without knowing the U-value of their building or home, let alone the R-value. However, think of all the money you can save if you can improve the energy efficiency of your space by changing your U-value, as well as your R-value.
So, if you do not know your U-value, figure out what it is and see how you can make it as low as possible. The work that you do will be well worth it when the temperature inside your home or building stays consistent, and your energy bill gets lower and lower in the future.