I have an insulated a/c register that has black mold/mildew at the bottom couple inches of the insulation and surrounding metal near the vent opening - if I replace the register, how do I prevent this from reoccurring?
pictures would help and really are necessary for this forum to do its magic.
without pictures, its just a wild guess, but I would say that somehow water is available to the mold. Mold dont grow without water and food. So eliminate those things and your mold goes away. A picture would help the forum members guide you in tracking down the food and water.