Garage moisture when running heater furnace

I'll start by saying my house is very humid. I am pretty sure at least 1 of 2 floors is completely uninsulated, including the attic. Its winter and rainy and our indoor humidity matches outside ~80% which is an issue of its own I want to take care of. The ground level and garage where the furnace is is insulated because of recent renovations. I want to get insulation blown in everywhere else in the near future but back to the subject-

When we run our furnace for 15-20 minutes at least, the garage where the furnace is gets very moist. You can see moisture on the walls, and feel it when you walk in. It is a 1 car garage with insulation. It is attached so it has a living space above it rather than an attic.

Is this normal? Should the heater furnace be producing so much humidity? I tested the carbon monoxide detectors and they work and have not gone off on their own. The return vent is upstairs in the uninsulated living area if that helps. No supply vent in the garage.

If not, what could be causing this concentrated area of moisture?

Thanks