Abstract: A process for preparing light-weight, insulating and semi-rigid or rigid elements, essentially comprises impregnating a porous supple material having an internal structure with preferentially open cells, with the desired quantity of a solution of a compound selected from the group comprising isocyanates, and having consequently in its formula at least a --NCO group, the material optionally having the final shape which is to be obtained, then proceeding with reticulation of the compound after reacting it with water giving a derivative of urea or biuret.