While remodeling our 1986 2 story we encountered a situation where we would need to re-insulate parts of our ceiling. Because we have the perception that open cell spray foam would be better than the blow-in pink fiberglass that was original to the house, we decided to foam under the roof deck in the attic. This would leave insulation below between the ceiling joists and foam above. Our vents on each ends would be covered and sealed creating an "envelope" in the attic. It was suggested we remove the original insulaiton. the reason given was " it will mold". It seems to me that if moisture were to be able to reach the attic space, it would or could mold whatever it happened to gather upon. Should I remove the old insulation and for what reason.