Does anyone know the expected life of flexible ducts in an attic? My upstairs system is 12 years old and we can't balance the temperature between the rooms. The contractor wants to replace our flexible spider system with a trunk and branch system to allow him to balance the air flow. This a costly upgrade. If the current ducts are nearing the end of life, I can justify the upgrade. Of course, my friends have never heard of replacing ducts. The internet hasn't given me much information on the subject.

It's not a question of life expectancy. Flexible ducts are less efficient than metal ducts, and usually there are no balancing dampers built into the branch duct connections to the main trunk. I would think you could justify the upgrade because it would be replacing what doesn't work now, with something that will work.

Their life isn't unlimited but it is certainly longer than 12 years. I'd expect at least 20 years out of flex duct. I've seen it much older with no problems. I'm not a huge fan of flex ducts but they are extremely common around here.

Are there no adjustable dampers at the outlets? I have flex duct in my house much longer than that without any trouble. The shorter runs, closer to the air handler are throttled back, to push more air to the longer runs. Be sure the air returns in the system aren't blocked by furniture, drapes, etc. before you try balancing. Another thing you may want to evaluate is how the heating/cooling load compares in the out of balance rooms with others in the system. A large window with a southern exposure in one room will require much more cooling than say, a room on the north side with a small window. The thermostat may be in the wrong place as well, causing the system to run shorter than it should to sufficiently cool all the space. I used cool, because you really didn't say whether this was a heating or cooling system or both.

To upgrade your duct system sounds like a good idea. I have no problem with flex if its sized and installed properly, but too often this is not the case. A well designed and installed duct system will not only improve the comfort level it will probably reduce your operating costs as well. Best wishes!