Solid duct under staircase
I will be installing a cold air return into a room in an unfinished basement. The wall the return will be in is on one side of an interior staircase. The cold air plenum is on the other side of the staircase. I can run the duct up the wall, and round the staircase, but it would be much easier to run the solid duct through the space under the staircase (about 3 feet) and then up and into the plenum. Is running solid duct through the space under a staircase allowed?