We have a one-story house, about 1750 sq ft. The whole house gets good
AC flow except one of the two small bedrooms. They are right next to
each other. Up in the attic, the cool bedroom has 6 inch flex ducting
from the main unit, while the warm room has only 5 inch flex ducting.

The difference in airflow (one seems like a fan is on, the other you
have to kind of concentrate to feel the cold air) is substantial and I
can't believe it's just from an inch difference in the ducting
diameter. I've checked the ducting for kinks and holes, but it seems
pretty solid. My only thought is that it's clogged or blocked
somewhere. The vent in the room is firmly painted to the ceiling and
while I may end up doing so, I'd rather not rip it off just so I can
see a foot into the 10 foot section of ducting.

I'm considering just buying some new flex ducting and replacing it. I
haven't checked cost, but doing it myself would undoubtedly be hundreds
of dollars cheaper than paying an AC technician. My concern is with
sealing it to the metal round ducts. I've heard metal duct tape is
good, but that there are also special sealants to use as well.

Thoughts on this fairly minor AC project?

Lymond