You could install the ductwork inside the envelope and get rid of that problem along with a few other issue as well.
Oppps..I better say that is if the envelope is seal as well.
Personally I dont see it happening with radiant floor heat.
one its not depended on convection alone,convection is more
of a mute issue for me being most of your heat from infloor
comes from radiant..hence the name radiant infloor.
Also I know it happen more so with emitter having surface temp of 180 and above which give the air more opportunity
to create more of a stack effect if you will.
Highest emitter temps you will see radiant will, or should
only be 90 degree so that in itself should help, I would think.
Maybe the guys up north in colder climate could answer the
question better,for me personally in my parts I have never experience that problem in the winter.
[Edited by simpleman on 04-01-2005 at 03:31 PM]