The material which you are talking about is known as "ductboard". The
other poster is correct in that insulated sheet metal is the best way to
go. Around here (San Antonio) sheet metal hasn't been used in
residential for over 30 years. It went from systems entirely of
ductboard starting in the late 60s, to ductboard boxes on the supply and
return hooked up with flex duct in the early 80s. Ductboard can be ok IF
done correctly, which yours must not have been. The company I work does
only replacement of equipment (no installations in houses that did not
already have central) We use ductboard to transition to the existing
ducwork as needed, and usually systems like yours if we replace existing
ductwork. I always use mastic sealer designed for the purpose, and seal
the iniside and outside of all the ductboard joints I do. Once it dries
and hardens, you couldn't get it apart if you wanted to. Bottom line:
try to get yours replaced with sheet metal, but if they will not do so,
insist that ductboard be done properly. If the "national company" you
are talking about is Sears, you have my sympathy. Stories like yours are
commonplace with them around here.