By Building Code, As long as the attic space is ventilated, no vapor barrier is required at all.
6 mil plastic sheeting should never be used as a vapor barrier in walls and ceilings and it should be immediately removed.
6 mil plastic is designed for use under concrete floors only and is not the correct product to use in walls in ceilings.
6 mil plastic will only serve to allow for condensation to form and pool and can create some very severe water damage and mold growth.
The R-13 in your walls may be too little depending upon your location.
Your rafter (ceiling) insulation should also include rafter vents which extend from the attic spaces behind the knee walls up to the ridge peak and vent for each rafter space.
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