What is Vapor Barrier Insulation?

Vapor barrier insulation is meant to prevent mold and mildew.
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  • Written By: Mary McMahon
  • Edited By: O. Wallace
  • Last Modified Date: 26 October 2015
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Vapor barrier insulation is insulation which comes packaged with a vapor barrier, eliminating the need to install one. Typically, this form of insulation comes in rolls which can be unfolded and cut to create strips of insulation for installation in a structure. The advantage to vapor barrier insulation is that it eliminates the additional step of installing a vapor barrier. The disadvantage is that it cannot be used in a structure which already has a vapor barrier, because two vapor barriers can create a vapor trap, which is very undesirable.

In most regions of the world, vapor barriers are strongly recommended. A vapor barrier prevents water from entering the walls, floors, and ceilings of a structure, preventing rot, mold, and mildew. Typically, the vapor barrier faces the living quarters of the house, preventing moisture from leaching from the interior of the house to the walls. In very humid climates, the vapor barrier may be placed on the outside, so that moist air from the outside cannot penetrate the walls.

Moisture in the walls is not just a problem because of the rot, mold, and mildew issues. In cold climates, having wet insulation can actually decrease the efficiency of the insulation, making the house colder and potentially raising heating bills. The water can also freeze and then melt rapidly, creating water stains on the walls and ceilings. Chronic moisture can also cause the wood in a structure to warp, causing structural instability.


When structures are built, a vapor barrier is often installed in the process of building the walls, with the insulation being added later. However, it is possible to use vapor barrier insulation instead, condensing this process into one step. This type of insulation can also be used for things like making a finished basement, insulating an attic, or replacing damaged insulation in a home. If a vapor barrier is already present, the vapor barrier insulation can be perforated to allow the moisture to pass through.

Installing vapor barriers can get tricky. It is a good idea to consult a contractor about the best location for a vapor barrier, because it is dependent on climate, and to get information about whether or not a home already has a vapor barrier. Newer homes tend to be more likely to have vapor barriers built in, while older homes are more likely to have damaged vapor barriers or no barrier at all. Taking the time to assess a building before installing vapor barrier insulation is important, as the last thing one wants is a moisture trap in the walls, floors, or ceilings which could lead to water damage down the line.


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Post 3

@Feryll - If you already have blow insulation in the attic then you don't want to put a layer of barrier in before blowing in the new insulation because there could be moisture already in the old insulation. In fact you don't want to use a vapor barrier anywhere in the attic unless you remove the insulation you have and start fresh.

I have a friend who made the mistake of using a vapor barrier over wet insulation and he ended up with a soggy mess. And since you know you have had leaks previously, then you can be certain you have at least some moisture issues.

Post 2

@Feryll - You should make sure your roof is leak free before you install the blown-in insulation, or any type of insulation for that matter. As I understand the barriers, the idea is to get them between the cold air and the warm air. This is because normally moisture is going to develop and build up where cold air and warm air meet.This means you would need to get the barrier below your insulation, and there would be no protection from water leaking into the attic from the roof and onto the insulation.

Post 1

I'm about to have blown-in insulation put in my attic, and I am concerned about moisture for a couple of reasons. The first reason is that last summer it got really hot in the attic and I'm afraid we might have some issues because of this. And the bigger concern is the roof.

We have had so many issues with leaks in the roof that there has been a good bit of water dripping into the attic. We had a roofer close some of the openings, and I finally managed to seal the ones he couldn't get. I am still worried that I might have missed a leak, or that another spot might pop up. I don't want a leak to open up once the new insulation is put in and have it ruined.

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