The problem you are describing is common with metal roof buildings in climates with high or mixed humidity with temperatures low enough to reach dew point.
As the air inside the building is heated it rises, carrying moisture with it. When the warm moist air reaches the ceiling it easily passes through the fibrous insulation material allowing it to reach the underside of the metal roof. If it is cold outside the metal roofing is also cold so the moisture in the warm air condenses on the underside of the metal roofing. The higher the moisture content the greater the condensation potential.
You have several options to stop/reduce this problem in the future.
- First you could reduce the moisture content in the air. You didn't indicate how you used the building so this may not be practical.
- Second, you could increase the insulation and add a moisture barrier to prevent warm moist air from passing through the insulation.
- Third, you could move the insulation from contact with the metal roof, create a roof ventilation system and reinsulate with material that is a vapor barrier (closed cell or foam board foam).
- Another option might be to add insulation to the top side of the metal roofing. Placing foam insulation on the roof would prevent the metal roofing from reaching dew point on the interior.
This option only makes sense if the roof is older or if the condensation problem is causing water damage to the structure or personal storage items in the building.
If you want to leave the roof in its current condition you could opt to have the old insulation removed and replaced with closed-cell, spray -applied foam insulation 3 - 4 inches thick. Closed cell insulation prevents moisture from reaching the underside of the roof deck and (at) 4-inches thick should prevent the insulation from reaching dew point.