TOPIC: Attic Insulation Vapor Barrier

Probably been asked a million times before, but here goes again--

Say a homeowner adds additional insulation, in the form of batting, to the attic as an energy upgrade, over the top of batting which already has an air/vapor barrier on the attic floor side where it should be.  If I find an addional air/vapor barrier on the bottom side of the new stuff, I know some guys will call it out for removal of the additional barrier as it increases the possibility of condensation. The additional barrier is technically no longer on the warm side.  Anyone know of a reference which specifically deals with this issue?

Thanks for the help--
DrBob

There is data available that deals with this.  I don't have time to look it up now but if I don't see other replies I may look it up later.  If my memory serves me, I think the article I read says that the vapour barrier can be 2/3 of the way toward the cold side before it would cause problems with moisture.  This is what makes it possible to have a vapour barrier on the exterior of walls in the form of rigid insulation.  In the case of an attic, I doubt the second vapour barrier would be very tight fitting, so it may not be much of an issue.  However that is something you may be able to tell once you're in the attic.

Bob, in my energy classes they said that if the vapor barrier is not touching something (in the attic the top of the drywall and under a fireplace the bottom of the sub floor, etc) then it is not effective.  I expect it would trap moisture, which makes the rest of the insulation less effective.

When an existing layer of batt insulation has a kraft faced paper on it on attic floors, the second layer should be a friction fit or unfaced layer and installed at right angles to the framing members.
Assumming that the first layer is 6" or r-19, adding another layer would increase the insulation value to an r-38.

Thanks for the info, Marcel--Do you have a reference like mfgr. data or some article I can point to as a source?  I would apprciate it

DrBob

Here you go Bob.
www.owenscorning.com/foryourhome/feature_detail.asp?feature=134