The thermal transmittance of a building component may be defined as the amount of heat flux which flows when steady state conditions are maintained across the component. The U-value of the component is a steady state quantity whilst the thermal transmission matrix represents the dynamic response of the component. This work defines the thermal transmission matrix elements in terms of Volterra kernel, or impulse response, functions. The Volterra kernel functions are considered to represent dynamic thermal transmission and the area under each response function may be considered to be equivalent to the steady state value. Linear vector and non-linear univariate thermal systems are considered and discussed with reference to their electrical analogues. The Volterra kernel values are extracted via the time series statistical central moments estimates from the data. (author).