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I'm adding additional insulation in my attic. Unfortunately, I tore the kraft paper vapor barrier of the existing fiberglass insulation in many places while air sealing. Is this a problem? Or is this simply what happens when you air seal? Do I need to replace the insulation where the kraft paper is torn?

More Detail:

I live in Maryland. The vapor retarder (kraft paper) is facing toward the conditioned side. The house was built in 1967, so I had to pull up the existing insulation in order to air seal. As the kraft paper tabs on the existing insulation are stapled, I inevitably ended up with a lot of tears around light junction boxes, top plates, etc.

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3 Answers 3

The purpose of a vapor barrier is to prevent moisture from condensing in the insulation, greatly reducing it's insulating value. To do this, it must be placed on the warm side of the insulation. In cold and moderate climates, this is towards the living space. In hot humid climates where keeping the living area cool is the primary concern, it goes towards the weayther side.

It sounds like in your case the paper is on the top, weather side. If you are in a hot humid climate and you are adding more insulation on top, you WANT to have many holes in the old barrier that is now sandwiched inside all the insulation, allowing any moisture that got past the new barrier on top to hopefully get carried out without condensing.

If you are in a cold or moderate climate, and it is on top, it was installed wrong. You're better off having holes in it. Any moisture that got that far should be allowed to escape, lest it condense on the paper and make the insulation wet.

To summarize, in either climate, if the paper is on top of the old, and you are adding more insulation, put all the holes in it you can! It is not helping. In a hot humid climate, insure you place a new barrier on top of your new insulation. If you are in a cold or moderate climate, consider how you may add a vapor barrier to the living space. Several heavy coats of latex paint can do this. It may be something you already have.

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You can use Tyvek tape (or Tuck tape in Canada) to seal any tears in the paper. bcworks's answer is right about not wanting multiple vapor barriers, and that your only vapor barrier should be on the conditioned side of the insulation (closest to the inside of your home). In the winter, humidity levels will be higher inside of your home and without a proper vapor barrier, moisture will travel through the insulation and condense on the cold side of the insulation, to putting a vapor barrier towards the outside does no good.

However, if your existing vapor barrier is in bad shape, I wouldn't rely on paint inside of your home to act as a barrier. You have openings in the wall for outlets and plumbing, interior walls that will have holes drilled in them for wiring, etc. Instead, if your insulation is improperly installed or the vapor barrier is beyond repair, I'd suggest pulling up the old insulation temporarily and putting down a plastic barrier between the joists.

Whenever you are installing insulation where there's an existing vapor barrier, there's no need to try cutting lots of holes in it, just peal it off and throw it away. Also, when layering up rolled fiberglass insulation in an attic, lay the second layer perpendicular to the first layer (and the first layer should be between the joists). This reduces the risk of air gaps on the edges of the insulation.

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up vote 0 down vote accepted

I contacted Johns Manville and this is what they sent me:

"To ensure maximum resistance to vapor migration, the insulation should be installed such that the facing is in substantial contact with the attic floor substrate and butted together well enough to ensure there are no gaps between the batts. Vapor barriers are "area weighted", therefore seams where the batts meet in addition to the joint between the sides of the batt and studs are not expected to let excessive amounts of moisture through the assembly. Batts should be butted as close together to minimize opening where leakage might occur. Be sure to replace any missing kraft paper and lay it down around the boxes, pipe, etc as best as possible."

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