For a diathermal wall having a stepped "constant heat" boundary condition imposed abruptly onto one side, thermal effusivity e performs nearly the same role in limiting the initial dynamic thermal response (rigorously, during times less than the heat diffusion time to transit the wall) as the insulation U-factor U plays in defining the static temperature obtained by the side after a long time. A dynamic U-factor U d y n and a diffusion time t L for the wall of thickness L , thermal diffusivity α α and thermal conductivity λ λ are specified by: