I want renovate my place and would like to drywall the ceiling without taking down the existing ceiling. Currently there is just fibreboard tiles nailed to the rafters for a ceiling and I don't know if there is a vapor barrier above it or not. The attic has some insulation but it's all dirty and I would like to just add to it if I could instead of removing it. My question is can I just put the new drywall over the existing ceiling or will it cause a mold problem between the 2 surfaces. If there is no vapor barrier currently can I put one between the ceiling and the new drywall?
More than likely those tiles were put on furring strips installed on the original ceiling. If the house was built before 1960, the ceiling is probably plaster. It started to crack and the homeowner decided to "fix" it the only way he could, cover it up.
So the answer is no, you shouldn't put another layer over what could be a compromised ceiling structure.
Take down the tiles and screw the wood strips to the ceiling joists with the correct length screws. Then install the new drywall to the furring strips.