THE MEANINGS OF R – VALUE AND U – VALUE

Unless University trained it is often difficult to understand the meanings of R and U Values. More importantly, an understanding of presented figures is imperative to ensure that you are installing the correct insulation at the correct depth within a property.

One Layer of insulation will have an R-value but not a U – value as it is just a single layer. 

R-Value is the measure of thermal resistance from a single material

R-values only represent resistance to heat transfer. Insulation material is a means of providing a structure with resistance to conductive heat flow or thermal resistance, so the higher the R-value the better its insulation effectiveness. R-value ratings are dependent upon the type of insulation, it’s thickness and its density; therefore, the R-values of a multi-layered installation are added together.

 

 

 

 

 

 

The 3 basic mechanisms of heat or energy flow are conduction, convection and radiation:

Conduction: The way heat or electricity is transferred or flows through a material or substance. An example is placing a spoon in a hot cup of coffee. The heat from the coffee is transferred to the spoon.

Convection: The way heat circulates through liquids and gases. This is why warm air rises and cool air sinks. Warm air is less dense, so it rises, and cool air is denser, so it sinks.

Radiation: Heat or energy that travels in a straight line and heats anything that can absorb the energy—the way the sun heats the earth. Heat moves in and out of structures in different ways. R-value, or Resistance Value, only refers to conduction. This way, two types of insulating materials can be compared if you know the thermal conductivity for each one. This also allows an understanding of the impact of adding layers of insulating material.

Combined layers of different materials, including insulation allow us to create a U-value to assess the effectiveness of a structure build up against heat loss. 

 

U-Value is a thermal transmittance, or the heat loss through a structural element

U-value refers to all of the different materials that comprise a layer and all of the different mechanisms of heat loss. U-value is more of an engineering term used to describe thermal performance and is often used to rate windows and doors. The lower the U-value, the more energy efficient. U-value is a measure of how much heat energy is gained or lost; therefore, the higher the R-value the lower the U-value.