Answer: casing the entire length of the pipe, with a casing angle not exceeding 90 °.
The installation of casing in a chimney flue is not compulsory, but this operation is recommended in order to reinforce the insulation and to avoid fouling of the flue. According to the standards in force, the casing must be done along the entire length of the duct, impossible to tub only one part (it's all or nothing). In the connection area, the angle of the casing must not exceed 90 °. The law also provides for two cases where chimney casing is prohibited: collective smoke pipes "shunt" and "Alsace". Finally, follow all the instructions given by the casing manufacturer, particularly with regard to the connection elements such as collars which must be made of the same metal as the casing. You too, send us your brico question