The “Ask a Builder” series is dedicated to answering some of the many questions Fairbanks residents have about building, energy and the many other parts of home life.
Q: Can I use spray foam as a vapor barrier in a wall in Fairbanks?
A: 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 this 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 SPF. It cannot function as a vapor barrier because water vapor can travel into the foam’s open spaces.
SPF is installed using a gun that mixes two chemicals and sprays them onto a building assembly. Spray foam expands as it is installed 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 the right equipment and perform the installation in the right environmental conditions (such as temperature).
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, the City of Fairbanks building code requires an even greater level of diffusion control, equivalent to 6-mil thick polyethylene sheeting (commonly called poly or Visqueen), so approval by a building official will be necessary in the City to use closed cell spray foam as the vapor barrier.
If you plan on applying to an energy rebate program, make sure to check with your inspector or energy rater about using CC SPF as a vapor barrier as early as possible in the design phase to avoid any potential complications.
It is common in Fairbanks to expect a vapor barrier to also perform 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.
Properly applied closed cell spray foam 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.
For more information on this topic, Building Science Press published a research report in 2009 on the need for a vapor barrier in above-grade walls with spray polyurethane foam, which is located here: www.buildingscience.com/documents/reports/rr-0912-spray-polyurethane-foam-need-for-vapor-retarders-in-above-grade-walls
Ask a Builder articles promote home awareness for the Cold Climate Housing Research Center (CCHRC). If you have a question, contact us at email@example.com or 457-3454.