I am installing a LPG furnace in my 1973 50 foot Burns Craft. I want to use insulated ducting from the furnace to the individual rooms of at least R-3 rating, 4 inch ID, and need to locate and purchase maring grade or heavy duty industrial grade ducting. The Home Depot stuff is not durable enough on the outer jacket being only about 1 mil thick. This ducting will be run in the high level bilge under the floor. Once above the floor, I will use 4 inch dia flex hose that I used with my engine blowers. Any information on this subject willo be helpful. Thanks in advance. Richard in Stockton, CA.

For your main trunk you might consider insulated board ducting. It is foil covered, comes in different lengths and can be formed into any profile that fits your application. It is held together with foil tape and is durable and damp resistant.