They may be of double wall construction with insulation packed between each wall or of triple wall construction designed in such a way that air circulates down one wall cavity and up the other. The inner lining of these chimneys is usually of corrosion-resistant stainless steel. To be used for wood stoves, these chimneys must be listed as UL ALL FUEL chimneys. The necessary clearances from combustible materials are marked on each section of a UL-listed prefabricated chimney. For the insulated double wall-type chimney, the minimum clearance is usually two inches.
Prefabricated chimneys are efficient and easy to install.
Creosote build-up is minimized in the insulated type chimney because of rapid warming of the inner walls. However, the triple wall-type which circulates the air may cause rapid cooling of the flue gases and thus increase creosote formation. To keep rainwater from entering the house, prefabricated chimneys are usually sold with storm collars and flashings as well as caps and spark-arresting screens. On flat roofs, the chimneys must be at least 3 feet above the roof. These precautions should prevent down drafts as well as roof fires from sparks.