CORRECTION: The original version of this article contained errors in referring to the thickness of vapor barriers, and the article has been updated. A "mil" is a unit of measurement equivalent to one-thousandth of an inch.

Moisture is something we all need to have in order to survive, and it’s surrounding us all the time. Unfortunately, it’s also the enemy of a lot of our building materials, and if it gets into the wrong places in our homes and is allowed to remain, it can do a lot of damage.

To keep moisture from getting where it doesn’t belong, builders use what are known as vapor barriers. The more you understand about what vapor barriers are and how they work in conjunction with the insulation in your home — especially when you’re doing remodeling and repair work — the more you can do to help prevent moisture problems, like dry rot and mold, from occurring.

Moisture on the move

First of all, understand that moisture in your home’s air is a fact of life. Some of it is there naturally, as a product of the humidity that’s in the air, and the more humid the climate you live in, the higher the moisture level that may be inside your home.

Then there’s the moisture that you generate yourself: That can come from a wide variety of sources — anything from showers and cooking to house plants and even breathing.

During the winter months, you keep the air inside your home at a higher temperature than the air outside. Air has a natural tendency to move from a warm area to a cold area, so the heated air in your home is always trying to move toward the ceiling, the floor and the outside walls, carrying moisture vapor with it.

Also, our homes tend to be at a slightly higher air pressure than outside, and that slight overpressure is again pushing the air and moisture toward the ceiling and the exterior walls.

So what is a vapor barrier?

In simple terms, a vapor barrier is a material that won’t allow moisture to pass through it, such as plastic sheeting. A very simple experiment to show how a vapor barrier works is to lay a plastic garbage bag down on some damp soil.

Pick the bag up a little while later, and you’ll see that the underside of the bag is covered with moisture. The damp soil was trying to give off its moisture to the surrounding air, but the bag — the vapor barrier — prevented that from happening.

Once again, remember that the warm air in your home is trying to escape through the exterior walls, carrying moisture vapor with it. If it gets into the exterior walls, some of it will remain in the walls and condense back into a liquid, creating all kinds of problems.

So one of your home’s most common vapor barriers — and one of the most important — is the one used over the insulation in your exterior walls. It’s designed to stop the moisture before it can enter the wall cavities.

There are two basic types of vapor barriers used with exterior wall insulation. The most common is paper-faced insulation. This type of insulation has a Kraft paper face with two flanges. The insulation is installed into the wall cavity with the paper facing into the house. This is very important — the paper, which is the vapor barrier, always faces the warm side of the house.

That’s because that’s where the moisture is coming from. After the insulation is pushed into the wall cavities, the paper flanges are unfolded, then they’re stapled to the face of the studs. Done correctly, that creates a continuous vapor barrier across the face of the entire wall.

The second method is to fill the cavities with unfaced insulation, then cover the face of the wall with 4-mil clear plastic sheathing. The plastic sheathing is the vapor barrier, and has the advantage of having fewer gaps and openings than the paper-face method, and it’s also easier for the drywallers to see the studs during installation.

For the ceiling, if you’re using batt insulation it’s important that the insulation be installed with the vapor barrier facing down — again toward the heated space. If you’re upgrading old batt insulation by adding a second layer of batts on top of the first, never use faced batts for the second layer. If you do, you run the risk of creating a double vapor barrier; any moisture that passes through the first layer of insulation can get trapped by the vapor barrier on the second layer.

For the most part, attics are insulated with blown-in insulation. So you might be wondering where the vapor barrier is. Actually, there isn’t one in that case, other than the drywall and paint on the ceiling. The difference between the attic and the exterior walls is that the attic isn’t a closed cavity. It’s open to the outside, and has ventilation to allow the moisture to escape. That’s why it’s critical that attics be properly ventilated, and that exhaust fans not be vented into attic spaces.

The last area to consider is your crawl space, which actually has two vapor barriers to be concerned with. In the typical crawl space with a dirt floor, a 6-mil plastic vapor barrier is used to prevent moisture from the soil from coming up into the crawl space area. That vapor barrier is laid directly on the dirt, and the seams are overlapped at least 12 inches.

The other vapor barrier is created with your floor insulation. One common mistake people make when insulating a floor is to install faced batts between the floor joists with the Kraft paper facing down, so they can staple the paper to the joists to hold the batts in place. Remember, the paper is the vapor barrier, and it has to face the heated part of the house, which means it has to face up. Always install batt insulation between the joists with the paper facing up against the underside of the subfloor, then hold the insulation in place with lath, wire or other means.

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