Our house was built with a flat roof. A gable roof was later added over the flat roof. It even still has gravel on what now would be the attic floor. The only insulation above about 70% of the our ceilings is in the joist bays. I am wanting to add R-38 in these areas. I am wondering if this would be effective and worth the time and expense since the insulation will actually be above and on top of the original flat roof? Am I going to have problems with condensation since the old roof is effectively a vapor barrier?