A method for producing a thermoset polyester foam, comprising the following successive steps: (a) heating an essentially anhydrous preoligomerization composition, containing - a polyol component comprising at least 70 wt%, preferably at least 80 wt%, of a mixture of at least one hydrogenated sugar and at least one C 3-6 diol, - a polyacid component comprising at least 50 wt% of a porogenic polyacid preferably selected from citric acid and the α-hydroxylated dicarboxylic acids of formula HOOC-(CHR 1) n -CHOH-COOH where n is an integer in the range from 1 to 4, preferably from 1 to 3, in particular 1 or 2, and still more preferably 1, and each R 1 represents independently a hydrogen atom, a hydroxyl group (-OH) or an alkyl group, preferably C 1-4 alkyl, and optionally - a catalyst, at a temperature between 100°C and less than 160°C for a time between 10 and 60 minutes, preferably between 15 and 40 minutes, with mechanical stirring, so as to cause the polyol component to react with the polyacid component and form a composition of oligomeric esters, (b) incorporating a surfactant composition in the composition of oligomeric esters, so as to obtain a thermosetting expandable composition, (c) introduction of the thermosetting expandable composition obtained into a mold or application of the expandable composition and thermosetting on a substrate, (d) heating the thermosetting expandable composition at a temperature equal to at least 165°C, preferably between 175°C and 220°C, ideally between 190°C and 215°C, for a time between 10 minutes and 8 hours, preferably between 20 minutes and 7 hours and in particular between 30 minutes and 6 hours, so as to form a thermoset polyester foam. (a) heating an essentially anhydrous preoligomerization composition, containing - a polyol component comprising at least 70 wt%, preferably at least 80 wt%, of a mixture of at least one hydrogenated sugar and at least one C 3-6 diol, - a polyacid component comprising at least 50 wt% of a porogenic polyacid preferably selected from citric acid and the α-hydroxylated dicarboxylic acids of formula HOOC-(CHR 1) n -CHOH-COOH where n is an integer in the range from 1 to 4, preferably from 1 to 3, in particular 1 or 2, and still more preferably 1, and each R 1 represents independently a hydrogen atom, a hydroxyl group (-OH) or an alkyl group, preferably C 1-4 alkyl, and optionally - a catalyst, at a temperature between 100°C and less than 160°C for a time between 10 and 60 minutes, preferably between 15 and 40 minutes, with mechanical stirring, so as to cause the polyol component to react with the polyacid component and form a composition of oligomeric esters, - a polyol component comprising at least 70 wt%, preferably at least 80 wt%, of a mixture of at least one hydrogenated sugar and at least one C 3-6 diol, - a polyacid component comprising at least 50 wt% of a porogenic polyacid preferably selected from citric acid and the α-hydroxylated dicarboxylic acids of formula HOOC-(CHR 1) n -CHOH-COOH where n is an integer in the range from 1 to 4, preferably from 1 to 3, in particular 1 or 2, and still more preferably 1, and each R 1 represents independently a hydrogen atom, a hydroxyl group (-OH) or an alkyl group, preferably C 1-4 alkyl, and optionally n is an integer in the range from 1 to 4, preferably from 1 to 3, in particular 1 or 2, and still more preferably 1, and each R 1 represents independently a hydrogen atom, a hydroxyl group (-OH) or an alkyl group, preferably C 1-4 alkyl, and optionally - a catalyst, at a temperature between 100°C and less than 160°C for a time between 10 and 60 minutes, preferably between 15 and 40 minutes, with mechanical stirring, so as to cause the polyol component to react with the polyacid component and form a composition of oligomeric esters, (b) incorporating a surfactant composition in the composition of oligomeric esters, so as to obtain a thermosetting expandable composition, (c) introduction of the thermosetting expandable composition obtained into a mold or application of the expandable composition and thermosetting on a substrate, (d) heating the thermosetting expandable composition at a temperature equal to at least 165°C, preferably between 175°C and 220°C, ideally between 190°C and 215°C, for a time between 10 minutes and 8 hours, preferably between 20 minutes and 7 hours and in particular between 30 minutes and 6 hours, so as to form a thermoset polyester foam.