If you are planning to have an attic insulation project for your home, you may be wondering whether it is important to have a vapor barrier installed as well. To be able to answer this question correctly for your home, there are a few factors you need to consider before any major attic insulation project.
What role does a vapor barrier play?
Within your attic, there is free movement of air and vapor, and water vapor tends to move from warmer to colder spaces. This essentially means that in the winter, vapor inside your home will want to move into your attic, where it is colder. In the summer, the reverse occurs; warm, humid air will move towards your cooler living space from the attic.
In an ideal situation, your home would have a perfect sealing with drywall and latex to reduce this movement, but most ceilings have patches for access and holes for lights and mechanical vents.
What climate do you live in?
With a vapor barrier, you will be able to significantly reduce this movement of vapor and make your living space more comfortable. A plastic vapor barrier is best installed in a vented attic within climates with over 8,000 heating degree days. If your climate does not meet these conditions, a vapor retarder like latex ceiling paint will work well for your house if you are in all other climates except the hot-dry and hot-humid. For hot-humid climates, attics should not be vented, and you should not install any vapor retarders in the assemblies interiors. In hot-dry climates, your attic can be vented, though, like, in hot-humid climates, you should not install any vapor retarder or barrier. However, all attics, vented or unvented, and in all climates should have an air barrier such as an airtight drywall ceiling.
Before the introduction of attic insulation, attics in cold climates were poorly insulated, and plastic ceiling vapor barriers were omitted. The heat being lost from the house below warmed up the attic so that moisture was lost through the vents as vapor. Now, with attic insulation, heat is no longer lost from the living space, such that moisture may remain trapped in the attic vapor barriers were, therefore, necessary to reduce moisture flow into the attic. To stop the movement of vapor effectively through air leaks, your vapor barrier should also have an air barrier, such as an airtight drywall ceiling. If you opt for plastic or latex, the layer must be continuous to stop the movement of vapor via leaks in air vents. This essentially means taping or caulking all joints and penetrations.
Should you install a vapor barrier over preexisting attic insulation?
No, the best option is to install un-faced or loose-fill fiberglass or cellulose on top of your drywall.
The best way to answer this question, in addition to considering the factors above, would be to talk to a professional. They will analyze the size and architecture of your home as well as location and come up with the best answer as well as materials and solutions you should try.