How do i keep mold from building up in my insulated barn.

Where is the moisture coming from? unsealed concrete floor, leak in roof, too much humidity? If its too much humidity, run a dehumidifer large enough for the space to reduce moisture level. I would do a moisture test on the concrete floor with a piece of plastic for 48 hours, then check underneath. If its wet, put a sealer on the floor. Then I would go searching or water marks on the roof top plate, soffits, How are you removing the mold you do find. I would get a professional mold remover product to remediate it. If all animals are out, you could also do a fogger too.

Is the floor Insulated? Don't keep damp thing in there or keep doors closed or install a dehumidifier..............or More ventilation...........

Find the source of the dampness and increase ventilation.

Exhaust fan

Dehumidifier

There is too much moisture in your barn that is causing the mold. This is because the barn is too tightly insulated and whatever is in there is causing moisture build up. it could be anything from hay storage or animals. You need to increase the airflow. If it's animals, most animals do not need to be in an insulated barn full time, and their living areas need to be cleaned frequently.