It is normal for air returning to the furnace to be in wall spaces.
Sometimes they nail or screw some sheet metal to the floor joists, and use
the bay between two or three floor joists.
The return air is essentially the same temp as the room, so it shouldn't
sweat or be hot or cold. The return air is under slight negative pressure,
so the duct doesn't have to be sealed totally tight. And it's before th e
air filter, so a little dust can get in.
Couple other fellows have adressed the question "is it a big enough duct".
Christopher A. Young
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