Thermal conductivity () or Lambda value is expressend in W/mK.
It is the heat flow in Watts/m² for a temperature gradient of 1K per metre thickness.
The lower the value, the better the insulation.
Thermal resistance (R) expressed in m²K/W is the measure of resistance to the passage of heat for a given thickness t.
R = t/
The higher the R-value, the better the insulation
U-value (or K-value) expressed as W/m²K is the coefficient of thermal transmittance and is the reciprocal thermal resistance R so: U = 1/R
This is the amount of energy passing through a material or structure in one hour expressed in Watts per Metre² for a temperature difference between the two surfaces of 1C (=1 Kelvin).
Hence, the lower the U-value, the better the insulation.
The U-value of a system or structure is the reciprocal of the total thermal resistance value of each component element of the structure:
i.e. U = 1/R1 + R2 + R3 ... etc.
NOTE: since R = t () and U = 1/R
U = 1/t/( l ) which = 1 X ( l )
U = ( )/t therefore when t = 1 metre, U = value.
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