A waste thermolysis installation includes a first, drying, enclosure able to vacuum dry the incoming waste and a second, calcining, enclosure, able to perform a vacuum calcination treatment on the dried waste coming from the first enclosure, each enclosure including an external-heating system including a combustion chamber and a vacuum pump which makes it possible to maintain the vacuum in the enclosure and is connected to the enclosure by an extraction pipe, the installation being characterized in that it includes a pipe circulating gas coming from the second enclosure to the second enclosure through the system for the external heating of the second enclosure. Thermolysis method implementing the installation.