Insulation: Understanding R and U values

The more I get into looking at insulating our house, the more I need to learn! At the moment I’m fighting off a swarm of mismatched R, U, and k values. All I want to know is what type of insulation we need to come close to the PassivHaus standards but everything I see is quoting differing units. So what are all those units? and what’s the relationship between them?

What are R,U and k

  • k is the Thermal Conductivity of a material, so for Insulation LOWER is better. Units are W/mK.
  • R is Thermal Resistance of a specific piece of insulation, we want this to be as high as possible. Units are Km²/W. It is calculated as d/k where d is the thickness of the insulation and k is the Thermal Conductivity.
  • U is the Thermal Conductivity of a specific piece of insulation or building element and, as with k, lower is better.  Its units are W/m²K. It is calculated as 1/R or k/d. 

One response to this post.

  1. […] to work out what will fit into our budget while giving us the best possible insulation effect (U-Value). The PassivHaus standards state that our windows should have a U value of <= 0.8 W/m2K, so […]


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