Thermal Conductivity or 'k' values of typical building materials. The lower the k vlaue the better the material is at resisting heat energy transfer.

 
The Building Regulations provide minimum standards of thermal insulation, typically expressed as a U value.

A U value of a material can be calculated  as U = k/d where k is the thermal conductivity of a material, d the materials depth, the units of U are W/(K•m²). A U value is simply a guide to the amount of energy lost in Watts per square metre of material for a given temperature difference of 1 degree C or Kelvin from one side of the material to the other

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An R value is the reciprocal of the U value, i.e. R = 1/U with units K•m²/W in SI units. Also, it should be seen that an R value can be calculated as the depth of a material divided by its thermal conductance, i.e. R = d/k or U = k/d . R values are referred to as the thermal resistance of a material.

Some countries (e.g. United States) use the units of R value as 1 ft²•°F•h/Btu. 1 R value US is equivalent to 0.1761 R value SI units, or 1 R value SI is equivalent to 5.67446 R value US.

Because of the relative low performance of timber as an insulator, the Building Regulations now regard timber not as a thermal insulator and timber cold bridges need to be taken account of when designing a thermal insulation system. Timber cold bridges need to minimised in the building design and steps taken to insulate the buildings air from potential timber cold bridges that will suck the heat away from the inside of the building.