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Fourier's law gives steady conduction as Q = kA.dT/d. A 15 m2 wall of 100 mm insulation at k = 0.04 loses 120 W across a 20 K difference, or 2.9 kWh a day. Thickness sits on the bottom, so doubling insulation halves the loss - with diminishing returns as other paths dominate.
Fourier's law
Q = k A dT / d
Layer resistance
R = d / k
About 0.03-0.04 for mineral wool and foam, 0.6 for water, 1.0 for brick, 45 for steel and 400 for copper.
Because R combines conductivity with thickness and adds up across a construction, so a wall's total resistance is the sum of its layers.