I recently bought a 40 year old house that has heating ducts in the
slab. It occasionally has water leak into the ducts after a lot of
rain. It dries up within a couple of days and there is no evidence that
there has ever been any mold problems.

I just finished rerouting the ducts into the attic (MAJOR job), and
removed the slab ducts from the supply feed. Now I need to close off
the slab ducts. I have seen other forum posts suggesting filling them
with concrete. Filling the entire slab duct would be a lot of concrete
and a real pain.

I am looking for other (simpler/cheaper) alternatives. One would be to
seal off a couple of feet each side of each register with conrete. An
even more attractive solution I think would be to use expandable foam
(like "Great Stuff") to fill around each register.

Does the expandable foam solution have any problems?? Are there other
alternatives??

Thanks,
JJ