A process for preparing a block of fiberized polyurethane/polyisocyanurate foam that is suitable for a heat-insulating slab of a leaktight and heat-insulating tank, the block of fiberized polyurethane/polyisocyanurate foam being composed of closed cells stockpiling a gas and being in the form of a parallelepiped with a thickness E and a width L, the preparation process including the following steps: a) a step of mixing chemical components required for obtaining a polyurethane/polyisocyanurate foam, said components including reagents for obtaining polyurethane/polyisocyanurate, at least one swelling agent consisting of a physical and/or chemical expansion agent and optionally at least one reaction catalyst and/or one emulsifier, b) a step of impregnating fibers with the abovementioned mixture of chemical components via a controlled distribution of liquid over the width L on a conveyor belt, the fibers having been placed beforehand on the conveyor belt and representing between 1% and 60% by mass, c) a step of forming and expanding the fiberized polyurethane/polyisocyanurate foam, said expansion of the fiberized polyurethane/polyisocyanurate foam being physically constrained by the walls of a double-belt laminator forming a tunnel, of rectangular cross section with a distance between the walls arranged laterally equal to L and a distance between the walls arranged horizontally equal to E, thus enclosing the fiberized foam undergoing expansion so as to obtain the abovementioned block of fiberized polyurethane/polyisocyanurate foam, in which the positioning of the walls of the tunnel of the double-belt laminator is defined from a law of evolution of the expansion volume, so that the expansion constraint on the fiberized polyurethane/polyisocyanurate foam leads to a volume of fiberized polyurethane/polyisocyanurate foam, exiting the double-belt laminator, representing between 92% and 99% of the expansion volume of this same fiberized polyurethane/polyisocyanurate foam in the case of free expansion, without the constraint of the walls of such a double-belt laminator. a) a step of mixing chemical components required for obtaining a polyurethane/polyisocyanurate foam, said components including reagents for obtaining polyurethane/polyisocyanurate, at least one swelling agent consisting of a physical and/or chemical expansion agent and optionally at least one reaction catalyst and/or one emulsifier, b) a step of impregnating fibers with the abovementioned mixture of chemical components via a controlled distribution of liquid over the width L on a conveyor belt, the fibers having been placed beforehand on the conveyor belt and representing between 1% and 60% by mass, c) a step of forming and expanding the fiberized polyurethane/polyisocyanurate foam, said expansion of the fiberized polyurethane/polyisocyanurate foam being physically constrained by the walls of a double-belt laminator forming a tunnel, of rectangular cross section with a distance between the walls arranged laterally equal to L and a distance between the walls arranged horizontally equal to E, thus enclosing the fiberized foam undergoing expansion so as to obtain the abovementioned block of fiberized polyurethane/polyisocyanurate foam,