To provide an underfloor spacer, a floor heating structure, and a building using it, having superior soundproof performance, walking feeling, and working efficiency.

This underfloor spacer is laid in a peripheral part of a room, adjacently to the peripheral edge of a floor heater composed of a ligneous floor plate material embedded with a heating surface body. The underfloor spacer is composed of a hard foaming body having an average value of the aspect ratio Dz/Dxy of indwelling foam of 1 to 4.0, foaming power of 3 to 20 times, and a compressive elastic modulus or 5 MPa or more. A floor heating structure is used by laminating the underfloor spacer on a floor material composed of a particle board or plywood.

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