Adding a second vapor barrier could cause condensation to become trapped in the insulation between the two vapor barriers.
Polyethylene vapor barrier attic.
This acts as a barrier to keep heated moist air from rising up into the attic during cold weather.
You can forego the plastic and use a vapor retarder kraft faced insulation or latex ceiling paint in all other climates except hot humid or hot dry climates.
This same process occurs inside the walls of your home.
Condensation occurs whenever warm air meets cold air.
Vapor barriers are sheets of plastic or other material placed on one side of insulation sheets.
You could be creating the kind of problems i described in scenario 1 above.
If your climate does not meet these conditions a vapor retarder like latex ceiling paint will work well for your house if you are in all other climates except the hot dry and hot humid.
In any case the vapor barrier must point to the warm side.
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Tips for installing vapor barriers.
Plastic vapor barriers should only be installed in vented attics in climates with more than 8 000 heating degree days.
Because it s nearly impossible to seal penetrations in the ceiling the reality is water vapor will get into the attic.
Attic vapor barrier detail for cold climates.
If you re in a cold climate however and do use air conditioning you need to be careful with interior vapor barriers like polyethylene.
Crawl spaces benefit from a polyethylene moisture barrier placed directly over the exposed earth.
Plastic vapor barrier in attic prior to blown insulation i am going to blow insulation into my attic and noticed one video where the people put down plastic over the floor studs as a vapor barrier prior to blowing the insulation.
This barrier is meant to keep moisture from getting to the insulation in the walls and ceilings and it is required by building codes when insulating most houses.
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A plastic vapor barrier is best installed in a vented attic within climates with over 8 000 heating degree days.
Attic insulation should always be installed with the paper backing facing toward the living space the ceiling in this case.
Even though we apply vapor barriers to exterior insulated walls in cold climates it s no longer recommended for ceilings floor of the attic.