PURPOSE: To save energy by providing an outer heat insulating layer on an existing residence, connecting a heat exchanger provided in a new garret space to a underfloor space through a pipe and a main duct, and connecting to a living space through a pipe.

CONSTITUTION: Heat insulating material 8 is provided between the sheathing plate 4 of the existing roof 2 of an existing residence and the rafters 5 to form a heat insulating roof, and a new garret ventilation port 10 is formed. Ventilating sheets, heat insulating material 8, furring strips, and dry type outer wall material 18 are provided on an existing wall structure to form a new heat insulating wall 16, and a new floor heating floor construction 35 is formed. Fresh open air flowing in through a suction port 33a and a pipe 31a, is exchanged heat with a heat exchanger 29 provided in a new garret space 9, and fed to an underfloor space 25 through an air feed fan F and a main duct 30. Further, polluted air in the room is fed from a living space 32 to the heat exchanger 29 through an exhaust port 32a, a fan, and a pipe 31.