Measure the heat loss through an exposed element with an accurate U-Value Calculation.

Calculation methods for the determination of a U value of building elements are based on standards that were developed in the European Committee for Standardisation (CEN) and the International Organisation for Standardisation (ISO) and published as British Standards.

A U value calculation is at its simplest for a building element which consists solely of plane, parallel, uniform layers: the heat flow through such an element is directly from inside to outside in a straight line, and all that is needed to obtain the U value is a simple sum of the thermal resistances of each layer.

Virtually all practical building components, however, have non-uniformities, whether they be joints between masonry units, timber joists with insulation between them, other types of structural member separated by infill panels, glazing within a frame, and so on. The consequence of non-uniformities, or the presence of any layers that are not plane and parallel, is that the heat no longer travels in straight lines. That affects the total heat transfer through the element and needs to be allowed for in the determination of the U-value.

U value calculations must be performed in accordance with BS 6946: 2007 using a method known as the ‘Combined Method’. Calculations should also be carried out in strict accordance with the guidance issued in BR 443 Conventions for U value Calculations (2006) and should be undertaken by competent persons.

R- Value

R-values are more commonly used in other parts of the world, in contrast to the UK’s preference for U-values. The units of measurement for thermal transmittance are m²K/W.

A higher figure indicates better performance.


Psi- Value

The measure of heat loss due to a thermal bridge is termed linear thermal transmittance (as opposed to ‘area’ thermal transmittance that is otherwise termed a U-value), with the units of measurement being W/m²K.

Psi values are used to generate y-values (thermal bridging factor) in Appendix K of the Standard Assessment Procedure.



k- Value

Thermal conductivity is the ability of a material to conduct heat.

A high thermal conductivity means that heat transfer across a material will occur at a higher rate; this is also temperature dependent.

The units of thermal conductivity are W/m⋅K. Unlike U-values and R-values however, k-values are not dependent on the thickness of the material in question.


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