Why is underfloor heating not radiant heating?

With underfloor heating, heat transfer takes place mainly by convection (Latin convectum - 'carried along' = carried along by a flow), similar to a radiator. Of course, the entire floor surface also radiates heat, but its effectiveness is very low.

Thus a bubble of warm air forms on the floor due to the higher temperature. Since warm air is lighter than cold air, it rises to the top. Over time, a circulation of warm and cold air forms in the room, as with vectors. The room is perceived as "warm" when all the air in the room has been heated to approx. 20 °C.

Luftzirkulation bei herkömmlichen Heizkörpern