When do I put up plastic barrier sheeting? before or after the insulation?

I'm insulating walls in porch and will be drywalling after insulating.When do I put up plastic barrier sheeting? before or after the insulation?
Plastic sheeting used as vapor barriers should be installed after UNFACED insulation batts, or poly insulation panels are in place and NEVER installed over foil or paper faced insulation.





The concern expressed in another answer about the possibility of moisture damage is, so far as I aware, unfounded. Your wall and insulation will still be able to ';breathe'; to the outside where water vapor is readily absorbed by the fluctuation of temperature differences in the air.





What you want to do is prevent your insulation from absorbing water vapor from the interior or being saturated with water due to the temperature and humidity difference between the interior and exterior. Thus the use of a vapor barrier on the heated side of the insulation.When do I put up plastic barrier sheeting? before or after the insulation?
I believe that practice was stopped years ago. I remember when that was the new thing to do. But after many homes had moisture damage from trapped water the practice was terminated.


Maybe where you live there is a moisture problem. Check with your county dept that issues building permits.



Simply put, the vapor barrier goes to the ';warmest'; side, usually the inside of your house.
Install the insulation then the poly as a vapor barrier.It is code and has to be inspected.
insulation, then sheeting, and tape it, good seal.
after for a water barrier.