Polyurethane / polyisocyanurate foam block of a thermal insulation block of a tank and its preparation process. The present invention relates to a block of fiber-reinforced polyurethane / polyisocyanurate foam (20) in which the foam block comprises, depending on its thickness E, two peripheral zones, one called upper extending from the upper face of the block and the other, said lower, extending from the lower face of the block, these two zones each representing at most 20% of the thickness E of the block and being separated by a so-called intermediate zone; and the peripheral zones, upper and lower, respectively have a fiber density Tf sup. and Tf inf. so that: 2/3 ≤ Tf sup. / Tf inf. ≤ 3/2; and the fiber density Tf int. of the intermediate zone is lower than the densities of the peripheral zones, ie: Tf int. Tf sup. and Tf int. Tf inf .. Figure for the abstract: Fig. 3
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