We have a hallway, pantry and mudroom area approximately 12′ x 15′  on the back of our house in what used to be an old porch that then had a bedroom built above it. It has two levels, hall pantry, then a few steps down to the mudroom, In the mudroom there’s about 11-1/2 feet to the bottom of the joists above, less in the hallway pantry area.  There is existing framing that supported a dropped ceiling, 18″ below the bottom of the joists above.  I removed the drywall to access the space above.  I’ve furred out and reinsulated the room walls up to and between the joists on the outside walls so all is within the conditioned envelope of the building. I plan to hang new drywall to the existing ceiling framing restoring the original ceiling heights. So absent any vents, this will be a 180 square feet x 18″ dead cavity with conditioned space above and below and insulated exterior walls. Previously there was R-30 insulation between the joists above (now removed), spotty wall insulation, and no heat in the mudroom. I found and treated some mold on the ceiling framing.  I’ve thought about putting a 9″ x 12″ grill in diagonally opposite corners of the ceiling just to allow some convective airflow.  The room is heated with a hydronic fan coil as is the rest of the first and second floors. Any thoughts on venting this cavity?
p.s. I have an inlet line for our HRV passing through the space so could negatively pressurize it and put inlet grills in the ceiling of the pantry and mudroom. I had planned to have inlets in both spaces anyway.