Can I use spray foam as a vapor barrier in a wall?
The short answer is yes, as long as you have the right type of polyurethane spray foam and any approval that may be necessary from building inspectors. However, while using spray foam as a vapor barrier is technically possible, it can be difficult to make it work in real-world applications.
First, let’s specify which type of spray foam can work. Closed-cell spray foam consists of gas bubbles that are trapped within a polyurethane matrix when the foam expands and cures. The trapped gas increases the insulation value of the cured foam while the solid matrix around the cells of gas functions as a vapor barrier. It is also referred to as medium- or high-density foam because the foam enclosing the air pockets must be strong and rigid enough to trap the gas bubbles.
Open-cell foam, on the other hand, does not trap gas when it expands but instead consists of an interconnecting foam web with spaces filled with atmospheric air (like a sponge). This type of foam is less dense than closed-cell spray foam and cannot function as a vapor barrier because water vapor can travel into the foam’s open spaces.
Spray foam is installed using a gun that mixes two chemicals and sprays them onto a building assembly. The foam expands when sprayed to fill stud bays and rim joists, even reaching into cracks that are hard to fill with other types of insulation. But to perform as advertised, it must be applied correctly by trained installers that have specialized equipment and work in the right environmental conditions (the right temperature range, for example).
Research conducted by the University of Waterloo has shown that closed cell spray foam in a minimum thickness of 2 inches adequately resists water vapor diffusion for cold climates such as Fairbanks. A thicker installation will provide more protection against vapor diffusion.
However, some building codes require an even greater level of diffusion control, equivalent to 6-mil thick polyethylene sheeting (the standard vapor barrier used in cold climate construction), so approval by a building official may be required in some places to use closed cell spray foam as the vapor barrier.
Most builders in Fairbanks expect a vapor barrier to double as an air barrier, which requires careful attention to air sealing at joints and around penetrations. This is a very important second function, as water vapor can also move into walls through air leakage.
Closed-cell spray foam, when properly applied, makes for an effective air barrier; however, it can be undermined by framing connections that aren’t covered by the insulation. One example is between king and trimmer studs framing a window opening.
Also, air leaks can appear in the wall after construction due to movement in the structure when it settles, when construction materials dry, or by other events. This complicates using closed-cell spray foam as the air barrier system for walls, as it requires transitions to other air sealing methods around areas not covered by the spray foam.
With proper planning and an experienced installer, closed-cell spray foam has been used successfully as a vapor and air barrier. On the other hand, a layer of 6-mil polyethylene is inexpensive and is often used as both a vapor and air barrier, providing insurance against moisture problems in walls.