A process for the preparation of a block of fiber-reinforced polyurethane/polyisocyanurate foam of a thermal insulation slab of a sealed and thermally insulating tank, the block of fiber-reinforced polyurethane/polyisocyanurate foam being composed of cells storing a gas and exhibiting a density of less than 50 kg.m -3 with a content of fibers C f representing at least 4% by weight of the block of fiber-reinforced foam, the preparation process comprising the following stages: a) a stage of mixing chemical components necessary for obtaining a polyurethane/polyisocyanurate foam, said components comprising reactants for obtaining polyurethane/polyisocyanurate, optionally at least one reaction catalyst, optionally at least one emulsifier, and at least one blowing agent, b) a stage of impregnation, by gravitational flow of the abovesaid mixture of chemical components, of a plurality of fiber reinforcements chosen from fabrics of fibers and mats of fibers, said fibers having a length of at least five centimeters (cm), arranged in superimposed layers, in which the fiber reinforcements extend essentially along a direction perpendicular to the direction of said gravitational flow, these fiber reinforcements exhibiting a permeability K c to the abovesaid mixture of chemical components, expressed in m 2, equal to: K c = r f 2 × p 3 / k × τ 2 × 4 × V f 2, with r f = radius of the fibers, expressed in meters (m), p = porosity of the fibers (dimensionless), value between 0 and 1, k = form factor (dimensionless), dependent on the nature of the fiber reinforcement, with k = 1 for a fabric of fibers, k = 6 for a mat of fibers, τ = tortuosity (dimensionless), dependent on the arrangement of the fibers, V f = volume fraction of the fibers (proportion of fibers in a volume of the reinforcement), value between 0 and 1, c) a stage of formation and of expansion of the fiber-reinforced polyurethane/polyisocyanurate foam, characterized in that the abovesaid mixture of chemical components exhibits a dynamic viscosity η, during the impregnation stage b), such that an impregnation time of the fibers t i is less than the cream time t c of the polyurethane/polyisocyanurate foam, the impregnation time of the fibers t i being equal to: t i = η × e m 2 / K c × Δ P, Δ P = M sd × g T × k p, with η = the dynamic viscosity, expressed in pascal.seconds (Pa.s), e m = sum of the mean thicknesses of the fiber reinforcements, expressed in meters (m), each fiber reinforcement exhibiting a mean thickness corresponding to the mean of the distances between a plurality of pairs of local extremums of said fiber reinforcement spaced out from one another along a direction of thickness of said fiber reinforcement, ΔP = pressure gradient, expressed in pascals per meter (Pa), M sd = surface density of the abovesaid mixture of chemical components, expressed as weight per unit area (kg.m -2), g T = terrestrial gravitational force, regarded here as equal to 9.8 N.kg -1, k p = mean pressure factor, constant equal to 0.5. a) a stage of mixing chemical components necessary for obtaining a polyurethane/polyisocyanurate foam, said components comprising reactants for obtaining polyurethane/polyisocyanurate, optionally at least one reaction catalyst, optionally at least one emulsifier, and at least one blowing agent, b) a stage of impregnation, by gravitational flow of the abovesaid mixture of chemical components, of a plurality of fiber reinforcements chosen from fabrics of fibers and mats of fibers, said fibers having a length of at least five centimeters (cm), arranged in superimposed layers, in which the fiber reinforcements extend essentially along a direction perpendicular to the direction of said gravitational flow, these fiber reinforcements exhibiting a permeability K c to the abovesaid mixture of chemical components, expressed in m 2, equal to: K c = r f 2 × p 3 / k × τ 2 × 4 × V f 2, with r f = radius of the fibers, expressed in meters (m), p = porosity of the fibers (dimensionless), value between 0 and 1, k = form factor (dimensionless), dependent on the nature of the fiber reinforcement, with k = 1 for a fabric of fibers, k = 6 for a mat of fibers, τ = tortuosity (dimensionless), dependent on the arrangement of the fibers, V f = volume fraction of the fibers (proportion of fibers in a volume of the reinforcement), value between 0 and 1, r f = radius of the fibers, expressed in meters (m), p = porosity of the fibers (dimensionless), value between 0 and 1, k = form factor (dimensionless), dependent on the nature of the fiber reinforcement, with k = 1 for a fabric of fibers, k = 6 for a mat of fibers, τ = tortuosity (dimensionless), dependent on the arrangement of the fibers, V f = volume fraction of the fibers (proportion of fibers in a volume of the reinforcement), value between 0 and 1, c) a stage of formation and of expansion of the fiber-reinforced polyurethane/polyisocyanurate foam, η = the dynamic viscosity, expressed in pascal.seconds (Pa.s), e m = sum of the mean thicknesses of the fiber reinforcements, expressed in meters (m), each fiber reinforcement exhibiting a mean thickness corresponding to the mean of the distances between a plurality of pairs of local extremums of said fiber reinforcement spaced out from one another along a direction of thickness of said fiber reinforcement, ΔP = pressure gradient, expressed in pascals per meter (Pa), M sd = surface density of the abovesaid mixture of chemical components, expressed as weight per unit area (kg.m -2), g T = terrestrial gravitational force, regarded here as equal to 9.8 N.kg -1, k p = mean pressure factor, constant equal to 0.5.