System for improving the thermal performance of an existing building, comprising: - a first added glass panel to be positioned in front of a first facade of said building so as to generate a first air gap on the side of said first facade, - a second added glass panel to be positioned in front of a second facade of said building opposite, in terms of solar irradiation, to said first facade, so as to generate a second air gap on the side of said second facade, - each glass panel (11, 15) being intended to cover at least 70% of a surface of the corresponding facade (12, 16), or even more than the total surface of the existing façade (12, 16), wherein the system further comprises: - two control devices capable of selectively operating said system for improving thermal performance notably: in a conservative mode in which said first and/or second air gaps (13, 17) thermally insulate said facades (12, 16), and in a dissipative mode in which calories are dissipated at said facades (12, 16) by generation of a stream of air, and - wherein a first control device comprises, on the side of each facade (12, 16) of said building, - a first and a second switching device (21, 22) respectively positioned at the bottom and the superior part of each facade (12, 16), said system being characterized in that: - each switching device (21, 22) is movable in a graduated manner between a closed minimum position and an open maximum position allowing a passage of air, and in that it comprises - a heat exchanger adapted to ensure, in a conservative mode, an energetic transfer between a fresh air, renewed and heated, circulating inside an internal distribution circuit and a polluted air recovered from said air gaps (13, 17). - a first added glass panel to be positioned in front of a first facade of said building so as to generate a first air gap on the side of said first facade, - a second added glass panel to be positioned in front of a second facade of said building opposite, in terms of solar irradiation, to said first facade, so as to generate a second air gap on the side of said second facade, - each glass panel (11, 15) being intended to cover at least 70% of a surface of the corresponding facade (12, 16), or even more than the total surface of the existing façade (12, 16), - two control devices capable of selectively operating said system for improving thermal performance notably: in a conservative mode in which said first and/or second air gaps (13, 17) thermally insulate said facades (12, 16), and in a dissipative mode in which calories are dissipated at said facades (12, 16) by generation of a stream of air, and in a conservative mode in which said first and/or second air gaps (13, 17) thermally insulate said facades (12, 16), and in a dissipative mode in which calories are dissipated at said facades (12, 16) by generation of a stream of air, and - wherein a first control device comprises, on the side of each facade (12, 16) of said building, - a first and a second switching device (21, 22) respectively positioned at the bottom and the superior part of each facade (12, 16), - a first and a second switching device (21, 22) respectively positioned at the bottom and the superior part of each facade (12, 16), - each switching device (21, 22) is movable in a graduated manner between a closed minimum position and an open maximum position allowing a passage of air, - a heat exchanger adapted to ensure, in a conservative mode, an energetic transfer between a fresh air, renewed and heated, circulating inside an internal distribution circuit and a polluted air recovered from said air gaps (13, 17).