Your enclosed porch probably has some warm air from the house entering the porch area and the moisture in this air will condense on the closest cold surface, which appears to be the roof. You don't say whether your aluminum roof is shingles or long panels, but it is possible that there is an ice dam forming allowing some melt-water to backup into whatever opening it finds. Adding insulation will minimize the transfer of heat to the roof of the porch (thereby minimizing the heat which creates the ice dam), but you will also need to install some kind of vapour barrier to prevent the moist warm air from the house from causing condensation inside the newly installed insulation.