Adding mass to existing exterior wall

Hello, I am building a studio in a walkout basement. I live just south of St. Paul Minnesota. I plan to do double wall construction with 2 layers of drywall in a 2 leaf system as in Rod's book. I have one problem. Since this is a walkout basement, one length of wall is 2X6 frame with fiberboard, Tyvek, and vinyl siding for the exterior leaf. The rest of the walls are cinderblock. I am trying to isolate outside noise from air traffic. The Minneapolis international airport opened a new runway and the flight pattern is now over my house. I was thinking of adding 2 layers of drywall to the inside stud cavities of the 2X6 wall up against the back (inside of house side) of the fiberboard and then caulk airtight. Does anyone have a better idea to add mass here. I am also concerned with mold buildup of the drywall behind the fiberboard. I am thinking of adding a 4mil vapor barrier against the fiberboard before attaching the drywall. Any ideas or suggestions will be greatly appreciated. Thanks Scott

Tyvek (house wrap) lets vapor out but does not let water pass through it. Are you sure it's only Tyvek between fiberboard and vinyl siding (it should be, but you need to be sure :D ) Look in the reference section, Knightfly has posted a few real good links concerning walls and moisture. I think you'd be OK adding the drywall directly against the fiberboard with no vapor barrier. Remember the rest of your wall system will need to "vent" out vapor/moisture somehow. Also pay attention to the correct placement of paper vapor barrier attached to insulation for your climate which I think should be against inner drywall, IF you decide to use it. The point is to stop vapor passing from warm side of wall to cold side which causes condensation. This is a simple concept which can be confusing... at least for me. Also be careful to not install 2 vapor barriers which would rot out what ever is in between. This place is great: http://www.buildingscience.com/ AND just for you: http://www.buildingscience.com/designst ... apolis.htm It would be worth while to look up/ research other common wall systems for your area that are proven to work for reference. Hope this helps Marc