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# What is the heat transfer coefficient (U) and the thermal conductivity coefficient (λ)? Which of these factors should be considered when selecting insulation material?

The heat transfer coefficient is a parameter denoted by the symbol U, given in units W / (m2.K), which determines the amount of heat flow through the surface of 1m2 of the building partition if in both sides the temperature difference are 1K.

On its basis it is possible to determine heat losses separately for a given partition such as walls, roofs, foundations or windows. The lower the U factor, the lower the heat loss of the given partition. The value of this factor depends on the type and thickness of the material from which the walls are made, but also on the nature of the barrier.

To determine the U factor, you need to know the thermal conductivity coefficient λ (lambda)  for wall materials and insulation layers as well as the thicknesses of all masonry, insulation and finishing layers from which the bulkhead is assembled. When determining the value of U factor, possible thermal bridges and insulation leaks are also taken into account – if there are any, the value of the coefficient increases.

The heat conductivity  coefficient is the parameter marked with the symbol λ and given in W / (m.K) units. It determines the amount of heat flow through the unit surface of the material of a given thickness, if the temperature difference between its two sides is 1K.

As with the U factor: the lower the value of the thermal conductivity  coefficient (λ), the better the thermal insulation can be. Heat conductivity coefficient λ is a constant material property characterizing it in stationary heat transfer conditions. The most important feature of insulating materials is its declared heat conductivity coefficient λ [W / mK].