I am considering bids from three contractors regarding complete replacement of my A/C and heating in Houston, TX townhouse. The inside equipment and ducting will be in the attic, which is typical for this area. My current ducts are 25-yr-old gray plastic-wrapped flex, and I'm including their replacement in my bid requests.
One contractor insists on installing R8 insulated hard duct, with all joints throughout the system sealed with "Uni-Mastic 181".
The other two contractors install flex duct.
Going into this, I had in my head that "hard ducts are better, but you gotta pay big $$ for them". I think I got that from reading threads on this board.
However, one of the flex guys tells me, "Yeah, in a theoretical system where duct size is constant and the system is perfectly sealed, hard ducts offer less resistance and will flow more air than flex ducts. But in practice, you can size a flex duct larger to get the correct airflow, but it's impossible to seal a hard duct system and keep it sealed. If it doesn't leak when it's installed, it's going to develop leaks over time, which will perform worse than old-but-intact flex duct." His logic is that the cold/hot cycles the pipes see (cold when conditioned air flows, then hot when air stops and pipes warm to attic temp, then cold when air flows, etc., etc.) will stress whatever sealant material you use, and eventually crack it. Repairing seals are costly because you have to remove the insulation and inspect to find them.
In contrast, he says, flex ducts are stay sealed better. Ensure they're well-sealed at the supply plenum and at the room register, and you know they're intact in-between. Sure, they'll wear out eventually (although all three contractors agree my ducts are in pretty good shape for being 25 yr old builder-grade cheapy stuff), but he points out that you can replace a good flex duct network three or four times for the initial cost of a good hard-duct network.
I expected my decision to be based solely on cost--could I afford the better hard-duct system, or would I have to settle for flex.
Is that still the decision I should make, or does this guy have a point that properly-installed flex can outperform properly-installed hard duct over time due to leaks?