I posted this in the ventilation section also

I have a 2 story colonial, built in the 1960's (I'm at least the 3rd owner of the house)
The house runs north/south.

I am in the process of refinishing my hardwood floors up stairs.
I was sanding the floors in the master bedroom and removed the vent covers for the heat and cold air return.

I noticed that there was moisture in the cold air return.

In this room the cold air return is on the outside wall (east side of the house).

The hole for the cold air return drops below the floor and into the floor joist that run east west.

There is insulation in this space.

I check the other bedrooms and all their cold air returns are on interior walls and their heat vents are on exterior walls.

The Master bedroom had the heat vent on the outside wall also.

I went in to the basement and can locate the cold air return lines coming down from the 2nd story and they correspond to the rooms with cold air return on the interior walls. I can find nothing for the master bedroom cold air return.

I believe I need to seal the cold air return on the outside wall.

What would be the best way to do this?

secondly how would I add a cold air return to this room, the area directly across (west of) the present cold air return is the door from the hall way.

I really do not want to put a vent into the floor.