If you are basically exhausting basement air into that space and letting it go up through the floor boards in the porch it depressurized the house, and air comes into the house via myriad air leaks.
The second-best is to air-seal it to the outdoors, and ventilate it with basement air, with the return air going back to the basement, which at least localizes the problem.
If you air seal it and insulate it with rigid foam (my other suggestion), you can replace the door with a screen door (or remove it completely), and it'll track the indoor humidity without excessive wintertime heat loss or introducing lots of moisture into the house.
But really, what I'd do? Buy a dehum and put it in there.
This is awkward, but...
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