Last year we had cellulose insulation blown into our 1920s house. Now I am working on the bathroom on the 2nd floor and have taken down lots of original drywall for various reasons in different spots. The insulation in the exterior wall stays nicely packed into its stud bays without falling out! I started reinstalling greenboard on this exterior wall yesterday but then read about something called a "vapor barrier." Do I have to rip out what I already did and install this? The greenboard will be primed and sealed with Kilz Premium and will have beadboard on the lower half.