R-values are used as a method of comparing the effectiveness of insulation, typically by the construction industry, but sometimes by in the field of thermal packaging. A high R-value is a sign of an effective insulating material. As you can see in the formula below, it simply shows that the less conductive a material, and the thicker it is, the higher the R-value will be.
The formula for calculating the R-value is:
R = d/k
The problem with using R-value to measure the performance of thermal pallet covers is it only represents heat transfer through conduction. Some of the most commonly used thermal pallet covers are made from a thin metallised polymer which is most effective against radiant heat. This kind of material will fare badly in an R-value test because it is thin and the metallic surface is highly conductive.
Thermal pallet covers made with a metallised polymer outer are, however, highly effective against radiant heat. They are typically able to reflect over 90% of infrared radiation. This is important because heat from the sun comes in the form of infrared radiation, and for many users this is the biggest danger to their product. This is a particular issue in the airfreight industry where product can be left exposed on the tarmac of an airport while it is waiting to be loaded onto an aircraft or transferred to another flight.
Insulated pallet covers are also available, but these are usually designed to defend against a mixture of radiant, convected and conducted heat which R-value tests don’t accurately measure as a whole.
It is typically measured in a lab by placing two plates with different temperatures on opposite sides of a material and measuring how long it takes for heat energy to pass from one plate to the other. This measures the conductivity of the material (k in the formula above).
Sometimes quoted is a measure called an Apparent R-value. This is a test that is designed to simulate real-world conditions including radiant and convective heat. However, there is no standardised method of testing thermal pallet covers in this way. If an apparent R-value is given, it’s important to ask about how the test was done, and consider whether it reflects your own usage. At Cargowise we always recommend our customers do their own tests and we are happy to provide samples for this purpose.
R-value is not a reliable measure of thermal pallet cover performance, as it only accounts for conductive heat transfer. Metallised polymer covers, for example, excel at reflecting radiant heat despite having a low R-value. Since real-world conditions vary, we recommend testing covers in practical scenarios. At Cargowise, we’re happy to provide samples for this purpose.