Up to now our basement was up to 6 degrees Celsius colder then the rest of the house.
It also had the supply and return air at the ceiling only and there was no air movement at the floor.
The first part of the solution was to move the return air inlet from the ceiling to the floor.
The fastest and cheapest way was to use an existing wall as a duct for the return air. I routed the air in a register need the base of the wall and out the top of the wall and into the existing return air path.
Here’s how it looks before the vent and drywall are put on.
After this was done and a few weeks have passed, the basement is now within 2 degrees of the rest of the house and the dampness is gone. I want to put one more return air vent on the other side of the basement later which should improve things more. For now this is much better than before.