I am renovating a kitchen and have taken down some of the interior walls and plan on taking the drywall off the ceiling above which is attic space. There is avapour barrier but it is not 6mil which is proper vapour barrier. Can I apply another layer of 6mil plastic vapour barrier directly against this existing plastic or would this be classed as a double vapour barrier. I am afraid to take down the existing vapour barrier because the insulation is liable to fall out and cause a lot more work!

You can put a vapor barrier 6ml or heaver and as long as its barrier to barrier and you Tuck Tape it good. The 6ml is supposed to completely keep the moisture out to begin with. As long as its on the WARM side of the ceiling Remember TUCK TAPE COMPLETELY. If there is a problem at the end of the joist or side wall to joist then Spray foam that Tightly in that joist bay(s) and seal it against the Vapor barrier joist. The no no In vapor barrier is pouting it on the cold side or in the insulation or on top of the cold side. Again Seal all seams and edges so NO moisture gets through. Condensation is formed where the cold and hot are to close to each other IE summer iced glass sweats.
The barrier is to keep moisture from getting up into the actual insul so when the transition of heat and cold slowly meet that there isn't any moisture to create a rain storm. If we had no moisture in our air and a cold front hits a warm front there would be no rain fall

Signed 21 Boat

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I am just a little curious, how do you know you don't have 6mils? Most folks have difficulty telling the thickness of plastic.

The plastic that is presently on the ceiling can be stretched and torn by hand. The 6ml plastic vapour barrier is much more rigid than this and cannot be easily torn. What is on the ceiling is no more than heavy weight dropsheet plastic. I also believe that the 6ml vapour barrier we get here in Canada actually is stamped saying it is CSA approved, but don't quote me on this.