Method for the production of a product for building in the form of a semi-liquid or liquid paste adapted to be applied on a wall or to manufacture insulations boards comprising phases of mixing of lime putty and aerogel together with other components, wherein said method includes the following steps of addition of components and mixing phases in a mixer provided with mixing means, said steps of addition and said mixing phases occurring until obtaining said product: a) adding in the mixer lime putty or a mixture of lime putty and gypsum; b) adding in the mixer a first quantity of water; c) adding in the mixer resins of the colloid or thickening type; d) adding in the mixer a material in the form of natural fibres or a material in the form of synthetic fibres or mixtures thereof; M1) first mixing phase for a first time T1 at a first mixing speed V1, in which the first time T1 is between 10 minutes and 60 minutes, preferably between 10 minutes and 30 minutes, in which the first mixing speed V1 between 500 revolutions/minute and 1200 revolutions/minute; M2) second mixing phase for a second time T2 at a second mixing speed V2, in which the second time T2 is between 10 minutes and 60 minutes, preferably between 10 minutes and 30 minutes, in which the second mixing speed V2 is between 1200 and 1800 revolutions/minute; e) adding in the mixer a first quantity of aerogel; f) adding in the mixer a second quantity of aerogel; g) possibly adding in the mixer a second quantity of water; characterised in that said step e) is an addition step of aerogel in a granular form with an average size of the granules between 0.1 and 5 mm, preferably with an average size of the granules between 1.5 mm and 5 mm, said step f) is an addition step of aerogel in the form of a powder with an average size of the particles between 2 and 40 microns, preferably with an average size of the particles between 2 and 24 microns, and further characterized in that a third mixing phase M3) is present after said phase f) in which said third mixing phase occurs for a third time T3 at a third mixing speed V3, the third time T3 being between 20 minutes and 60 minutes, preferably about 30 minutes and the third mixing speed V3 being between 1200 and 1800 revolutions/minute. a) adding in the mixer lime putty or a mixture of lime putty and gypsum; b) adding in the mixer a first quantity of water; c) adding in the mixer resins of the colloid or thickening type; d) adding in the mixer a material in the form of natural fibres or a material in the form of synthetic fibres or mixtures thereof; M1) first mixing phase for a first time T1 at a first mixing speed V1, in which the first time T1 is between 10 minutes and 60 minutes, preferably between 10 minutes and 30 minutes, in which the first mixing speed V1 between 500 revolutions/minute and 1200 revolutions/minute; M2) second mixing phase for a second time T2 at a second mixing speed V2, in which the second time T2 is between 10 minutes and 60 minutes, preferably between 10 minutes and 30 minutes, in which the second mixing speed V2 is between 1200 and 1800 revolutions/minute; M1) first mixing phase for a first time T1 at a first mixing speed V1, in which the first time T1 is between 10 minutes and 60 minutes, preferably between 10 minutes and 30 minutes, in which the first mixing speed V1 between 500 revolutions/minute and 1200 revolutions/minute; M2) second mixing phase for a second time T2 at a second mixing speed V2, in which the second time T2 is between 10 minutes and 60 minutes, preferably between 10 minutes and 30 minutes, in which the second mixing speed V2 is between 1200 and 1800 revolutions/minute; e) adding in the mixer a first quantity of aerogel; f) adding in the mixer a second quantity of aerogel; g) possibly adding in the mixer a second quantity of water;