All of the trunking from the heater on my boat is single skin flexible aluminium hose that runs through all of the cold areas under bunks and settee's. It was losing heat all of the way from the heater outlet until what was left of the warmed air arrived at the cabin outlets. After purchasing some flexible thermal insulation that the seller referred to as "Thinsulate," I cut it into strips and have started to wrap the aluminium trunking, working from the outlets towards the heater. The job is not yet finished, but the difference it already makes to the temperature of the air at each of the outlets, and therefore the speed at which it warms the cabins, is quite remarkable. There's probably also a knock on effect of the heater not needing to work so hard, thereby making the system more fuel efficient. On the down side I'm a bit worried, if I finished the job, whether the higher working temperatures within the trunking might have some adverse effect that might be detrimental in one way or another. Having said that, some suppliers already sell double skinned and insulated trunking. Anyone got any thoughts on the matter ?