<p>A solution to the Stefan problem for a one-dimensional layer of thermal phase-changing material insulation of the walls of a room is proposed under the assumption that the temperature of the outer surface of the wall is equal to the ambient temperature and its fluctuations are harmonic and quasi-stationary. The indicated problem was solved depending on the relative amplitude of fluctuations of the ambient temperature ∆ϑ, the Biot number, which characterizes heat emission from the room, and a set of physical properties of the thermal insulation material. An analytical solution to the problem for relatively large amplitude of ambient temperature fluctuations is observed — ∆ϑ ≥ 1. It is shown that in using a phase-change material with a phase transition temperature equal to the room temperature, there are no fluctuations of the temperature of the inner surface of the room wall, the temperature of which is equal to the room temperature during the entire period of ambient temperature fluctuations.</p>