Insulation, Radiant and Vapor Barriers - Vapor Barriers and Kraft faced insulation

Should I install a vapor barrier over paper faced insulation. Is it necessary and/ or will it cause problems with condesation. My house is in Pennsylvania.

From what I understand, and have read, one has to understand a couple things.

Kraft faced insulation is a vapor barrier. But it is not an air barrier. Meaning, if the wind is blowing, you will feel air passing around the kraft faced insulation... if the air is moist, it has bypassed your vapor barrier.

Visqueen or poly is both an effective vapor barrier AND air barrier. Do you NEED to use it over your kraft faced insulation? Not necessarily. If you were installing 1x6 tongue and groove cedar, you might want the poly to stop air movement. But if you have drywall, the drywall is an effective air barrier (air can only infiltrate along the bottom plate (into the room) and top plate (into the attic or 2nd story floor).

Is it harmful to use poly over kraft faced insulation? Not necessarily. Most professionals realize that in order for a double-vapor barrier problem to develop there has to be a temperature differential. If you install 1 vapor barrier right over another, with no insulation inbetween them, there should normally be no problems associated with it. The exception would be in a bathroom- especially near showers- where high humidity would preclude the use of ANY type of faced insulation, and the potential for problems with a double vapor barrier would exist simply due to the extreme difference in temperature and humidity from one side of the drywall to the other.

I've installed poly over kraft faced paper and have had no resulting problems. (Nebraska- hot, humid summers / cold, arid, sometimes snowy winters)

Funny you should mention air blowing. Its not the paper thats causing the problem, its the fiberglass its self. It it sealed properly, you shouldn't have a infiltration problem. The air will still come through the light socks, plug outlets, etc, but maybe not as bad if you add another vapor barrier.

A contractor is finishing my living room and stated I shouldn't put the poly over the kraft face. he had me wondering since I did the rest of the house that way.

Thanks for the responses.

I guess I'll have him just install the vapor barrier as I did the rest of the house.

There are still many different theories in the insulation and vapor barrier world. One of them being double vapor barriers. Some say you shouldn't because it could trap moisture. Others say it doesn't matter. I personally wouldn't do two as long as the kraft paper overlaps each other.