Insulation, Radiant and Vapor Barriers - garage ceiling insulation

I installed R-15 Kraft insulation in the gable roof of my garage several years ago with the paper facing the inside but with no finished sheetrock or other finish material over it. I live in a very dry climate, however, in the spring after the snow is about all melted, the humidity from the concrete slab, from being frozen all winter underneath, the humidity from it gravitates to the paper insulation and becomes moldy for a few weeks just in the highest peeks. Does this mean I need a gable vent for this problem? If so, how do I close it off in winter when I need the garage to be warm? I don't heat the garage regularly, and it's only in the spring I have the problem. Thanks.

It is unlikely that there is water vapour rising from the concrete slab in your garage, unless it is wet/damp and the sun is shining on it, as long as its cold in comparison to the air in your garage then any water in the concrete will stay there.
It is more likely that the humidity is in the air.
What happens is the insulation is cold compared to the air temperature and the water in the air is condensing on it.
Once the surface temperature of the insulation reaches the same temperature as the air, the damp will move away and the (problem?) for another year.