U Values Explained

Are U Values an accurate way of calulation heat loss or gain to a building?

For decades we have been ‘calculating’ heat loss using numbers to 2 or 3 significant decimal places to give the impression of great accuracy. But is mathematical calculation based on science? Is it correct?

Materials are laboratory tested as sample sized components oven dried and the data interpolated as representation for larger components.

Back in the 1970s, the ‘accepted’ reference books had to be re-written when it was proven that U Value calculation could be over 200% inaccurate. The issue then was the ‘accepted’ method for calculating the passage of thermal energy through slotted concrete blocks.  

Physically testing a sample panel using the Canadian Hot Box Method found that U Value calculations were very inaccurate. The Official Handbook was re-written showing a different method for calculation.

Re-visiting the ‘accuracy’ of U Values based on observation, logic and proven science once more theoretical mathematics is put into question.

The units of ‘Resistance’ (R Value) are: m²K/W.

• Area (m²)
• Kelvins (temperature comparison to absolute zero. Degrees Celsius can also be used °C)
• Energy movement in Joules per second (J/s) AKA; the ‘Watt’.

To calculate an ‘R Value’, the sample material thickness is expressed as part of a metre. A brick for example 103mm would become 0.103m. The conductivity of thermal energy passing through a material used to be indicated as the ‘k’ Value. Lower case ‘k’ so it isn’t confused with the upper case ‘K’ used for Kelvins.

For the past several decades industry has been using the ancient Greek letter ‘lambda’ to represent thermal conductivity.

The units for thermal conductivity are W/mK. ‘W’ (Watts), ‘m’ indicates the thickness is measured in metres and ‘K’ (Kelvins). 

All the data so far has been based on the testing of a small oven dry material tested in isolation in a laboratory.

Why then are the calculations being put into doubt? 

The units for the measurement of ‘heat loss’ through the structure or component is stated as W/m²K. Temperature is the difference in air temperature between the two surface areas.

The energy contained in the air when in contact with a solid surface has a surface resistance. The surface resistance can be one of several figures.

• The air movement in a room will be different to outside, therefore the calculation will contain a Surface Resistance inner (Sri) (inside the room) and SRo indicating outer / outside the structure.

There are many variances that include whether the sample is vertical, horizontal, in an exposed, sheltered or normal area. Whether the sample is at ground, or near ground level or high up such as high rise flats. Other variances include emissivity of the surface whether it is ‘high’ or ‘Low’. How radiation is absorbed or reflected. The data is usually shown to the third decimal place.

The method of calculation is to sum the results of the Resistances. The reciprocal of the sum of the R values equals the U Value.

In this example a typical section through a cavity brick / block wall with full cavity thermal insulation:

As the tables show, the U Value is the same whatever the order of the materials.

What actually is heat?

Heat is a product of electron collisions. When an atom or molecule collides or impacts another atom or molecule the electrons will collide. In solid materials the atoms and molecules have restricted movement therefore, only vibrate. The vibrations also cause collisions that produce thermal energy as ‘heat’.

Therefore, when considering conduction through a solid material it is the  energy being converted by impacts that gives off heat.

The electrons require energy to enable movement. The energy is provided by electromagnetic radiation.