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A new parameter e (0≤e≤1), called coefficient of performance, is introduced, whose value quantifies the suitability of the distribution of resistive and capacitive layers in a multi-layer wall, within problems concerning thermal building-plant interaction in case of external sinusoidal or impulsive thermal variations. After giving an integral expression for the calculation of e, the coefficient’s of performance properties are discussed; simple relations allowing to evaluate the value of e in case of walls made of two materials only, i.e. of a capacitive layer followed by a resistive one (two-component walls), are also deduced. The developed analysis shows that the optimal ideal wall, to which the maximum value of e corresponds (e=1), is the lumped-parameter, three-layer, symmetrical one, obtained by lumping all capacity in the mid- plane of the wall and by distributing the resistance, split up into two equal slabs, on the faces. Finally, the coefficient’s of performance values for different two- component walls, realized with materials commonly used in building, have been calculated.