I am adding an attached garage to our house and am confused about whether or not I need to have an interior vapor barrier (6 mil plastic) installed under the sheetrock. I have been getting mixed information about the need for this layer? What do you think?
Seth - Salt Lake City, UT
Well Seth, there is no easy answer when deciding if you should install an interior vapor barrier on your wall assembly, so let me add another ambiguous opinion to your construction research! Although the use of interior vapor barriers was once standard in the home building industry (and often mandated by local building codes), nowadays you can find many reasonable arguments that favor not installing a plastic vapor barrier on the inside of your home's walls.
The idea behind this recent change in building technique is the reality that water vapor needs to be able to migrate out of the wall assembly, either to the interior or exterior side of the framing. For many good reasons, the exterior side of most newly-built walls is usually sealed pretty tight, either by OSB sheathing, XPS insulation board, house wrap, seam tape, or some combination thereof. A well-sealed exterior means that any water that does find its way into the wall cavity, can only escape towards the interior of the building. Yet, if there is plastic sheathing sealing the interior side of the wall framing, then the water is literally stuck inside the wall cavity and left to create mold or other obvious problems.
Some home builders solve this problem by allowing the exterior layers of a wall assembly to breath (i.e. diffuse water vapor) by installing house wraps, such as Typar, which claim to allow water vapor to escape, or by ensuring that there is a small air gap left between the exterior siding and the underlying structural sheathing. So if you are using "breathable" materials on the exterior side of the new wall, then an interior vapor barrier might be a viable option. The important take away is that you need to make sure that at least one side of your garage wall is designed to allow water vapor to escape.
Like so many building techniques, the "right" way to do things is not always cut and dry, and you should always consider the properties of the specific materials you are working with, as well as the overall goal of the design. While you are researching what materials to ultimately install on the interior and exterior sides of your new garage walls, just remember that water should be kept out of the wall cavity at all costs, but you also need to allow for water to escape if it eventually finds it way in.