mike wrote:

DerbyDad03 wrote:

What's a boot?


end of runs of the insulated sheetmetal, in the attic, terminate in a
boot which attaches to the registers

And why are we doing this?


for the purpose of sealing some small leaks where warm air from attic is
mixed into the cooled air flowing from the register. this should improve
performance a little bit and would also stop a couple registers from
dripping condensate (inside temp is 77F but air from register is coming
in at 59-61 and a couple leaky boots are condensing on the outer surface)

In my experience, boots are often installed by cutting a hole
about 1" larger than the boot in the floor / ceiling, then stuffing the
boot in with minimal attachment. Consequently, there if often
a lot of air leaking between the boot and the ceiling / floor.
First, if the boot seem loose, do what you can to secure it.
Then caulk or seal the gap with whatever you can (latex caulk,
foam caulk, duct mastic, etc.).
If you think that the joint between the boot and the duct is also leaking
(I have not seen this to be as big of an issue), seal that too.
If you can get to the outside of the duct do it on the outside.
If you use mastic, I would get a small paint brush -- metal edges can be
quite sharp and would easily cut through a latex glove.
Insulate the outside of the boot if it isn't already.