Is that overkill? Is the kraft face a sufficient vapor barrier? Or should I also use a 4mil polyurethane over the kraft faced batts?

 

The kraft face is a sufficient vapor barrier but a poor air barrier. Vapor barriers primarily deal with the moisture in heat as the heat travels through (conduction) materials. Air barriers deal primarily with moisture inside air. Plastic sheets are usually good air barriers. The probability that a moisture problem could occur with air transported moisture is more than 100 times greater than heat transported moisture. The reason for this is the volume of air within the house remains constant. An example of this is a balloon filled will air and you put it in your freezer. After a few minutes when you take out the balloon, you will notice that the balloon is smaller than when you put it in the freezer. This is because when you cool air it contracts and when you heat it, it expands.

However your house cannot get bigger or smaller as the air in your home is heated and then cools. So when your heat comes on in your home, the air expands. Since the house cannot get bigger, the air is pushed out of the house. If your walls and ceilings have holes in them for outlets and/or fixtures, the air can be pushed behind them. And if there are gaps between the insulation and framing members, the air gets pushed there too.

This is the reason for the plastic sheet over faced insulation. This is considered prudent and not over kill.

 

thanks for the explanation, will use the poly 4 mil afterall

ONe more question, using faced batts and poly, what installation method is recommended, inset or faced stapling?

 

Considering that you intend to install the 4 mil plastic, I would recommend inset.

 

We always staple the paper inside the stud. Then the 4 mil poly over it all as one sheet over windows and all helps keep them clean till trim out time. If you staple the paper on top of the studs edge you end up with a bump some times that shows up in the drywall.

ED