Insulation Values… U-VALUE R-VALUE K-VALUE?


By Dermot Moore

One if not all of these insulation values are mentioned if you look into purchasing insulation. However, confusion can arise when bombarded by this terminology, along with the many symbols and numbers that accompany them on tech spec documents.

What’s the difference between the U-value and R-value

There are high and low values for U and R. Am I meant to look for a low U- value and a high R-value, or is it the other way around?

Does one value exist as part of the other value, and is one more important than the other?

Then there are the K-values, which seem almost the same as U and R. What’s the relevance of K-values if I understand U and R, which appear more often in tech specs, suggesting perhaps that these values are of greater significance?

Some questions we can answer!

Basic Definitions

U- Value – Relates to the thermal transmittance or combined thermal value of all the materials in an insulation system 

  •  Rate of heat transfer through a structure (which can be a single material or a composite), divided by the difference in temperature across that structure
  • A measure of the quantity of heat that will flow through unit area in unit time per unit difference in temperature of the individual environments between which the structure intervenes.
  • Lower U-Value preferable – means insulation retains heat better = less energy use of heating appliances.
  • This value is written as W/m2K (Watts per metre squared Kelvin). The number of Watts per square metre of the construction per degree of the temperature difference between one side and the other (W/m2K).

R-Value – The measure of how well a two-dimensional barrier, such as a layer of insulation, a window or a complete wall or ceiling, resists the conductive flow of heat.

  • Thermal resistance R-value is used in combination with numbers to appoint thermal resistance values.
  • A high R-Value is preferable.
  • Written as m2K/W (metres squared Kelvin per Watt). Thermal resistance is calculated by dividing the thickness of the material by its thermal conductivity, giving an R-value specific to that thickness.

K-Value is used to describe thermal conductivity.

  • Thermal conductivity is the rate at which heat is transferred by conduction through a unit cross-section area of a material when a temperature gradient exits perpendicular to the area.
  • The word Homogeneous might appear when discussing K-value. Simply referring to one material that has a consistent composition throughout. In other words, there is only one type of insulation, as opposed to one layer of one type and a second layer of a second type
  • The measure of a homogeneous material’s ability to conduct heat
  • It is expressed in W/mK, and a lower value is desired. This is calculated by dividing a material’s thickness in metres by the K-value.

Similarities that may confuse: 

  • All relate to insulation’s interaction with heat 
  • R and K values represent measurements but differ on the type
  • R measuring insulations abilities to restrict heat flow
  • K measuring the amount of heat that passes through insulation within a unit of time 
  • U and R values are both dependent on the thickness of the material being measured


Siddhartha Kosti, Assistant Professor (ME Department) at Rajkiya Engineering College, Banda (2018-present) – Conductance and Conductivity


Thermal conductivity


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