Vapor Barrier


Old 03-23-11, 12:24 PM
Thread Starter
Join Date: Dec 2009
Location: VA
Posts: 35
Vapor Barrier

Does the Kraft paper on batt insulation function as a vapor barrier or just a vapor retarder? Basicaly do I need to use plastic sheeting over the kraft paper to get a true vapor barrier?


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Old 03-23-11, 01:29 PM
Join Date: Oct 2008
Location: New England
Posts: 9,994
Hi Leslie, vapor diffusion retarders range from extremely effective to minimal effect. Your first decision needs to be, what do you need in VA? To make your decision more difficult, the answer you are looking for has been changing, so the answers you get will vary. Search Martin Holladay and vapor barriers for some related opinions. here is a dot gov on defining vapor diffusion retarders:
Energy Savers: Vapor Barriers or Vapor Diffusion Retarders

In my opinion, along the same line as Mr Holladay, if you reduce the air leakage properly, then the paint would probably be sufficient in your climate. If you heat only, the Kraft will be sufficient. If you heat and air condition, then you may not even want the Kraft. Here is the dilemma. In general, in the north, the VB goes to the inside. In the deep south, the VB goes to the outside. And since you can have only one, where does it go for everyone who lives in the middle?

One guiding authority, regardless of all scientific advice, will be your local code authority. They rule.

I'll let you read,
Old 03-26-11, 01:24 PM
Join Date: Mar 2009
Location: WA
Posts: 1,167
It appears you are in Zone 4 ?; Chapter 11 - Energy Efficiency

No vapor retarder required for Zone 4, unless Marine, where I live; Chapter 6 - Wall Construction

Like Bud said, check locally.


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