Condensation that forms on a metal surface inside the building or on the interior side of the vapor retardant facing on the insulation is caused by the moisture that is dissolved in the air (water vapor) coming in contact with a surface that is at or below the condensation (dew) point. (The condensation point is dependent on relative humidity.) When water vapor comes in contact with something that is at or below the dew point, the vapor condenses at the point of contact and is seen as water in the liquid state. Condensation on the interior surface of a vapor retarder is evidence that the vapor retarder is relatively effective, in that the water on the inside of the vapor retarder has not gotten through the retarder and in to the fiberglass insulation, thereby reducing the effectiveness of the insulation.
When condensation forms on the interior surface of a roof or wall panel, behind the insulation, it is evident that water vapor has either gotten through or around the vapor retardant facing, and has then condensed on the inside surface of the wall or roof panel. Water vapor can get through a vapor retarder (it is not a barrier) if the vapor pressure between the interior and the exterior of a building is too great or if a vapor retarder with an insufficient perm rating has been used. Water vapor can get around a vapor retardant facing wherever there are holes in the facing, where insulation splices have not been properly made, or where the facing has not been adequately sealed around necessary penetrations such plumbing and/or electrical fixtures.