There are many factors that determine how thick spray foam insulation needs to be. All insulation is rated by R-value which is a rating of thermal resistance. The higher the R-value, the higher the thermal resistance. When installing fiberglass insulation, cellulose insulation or mineral wool insulation R-value is clear. Understanding the R-value of spray foam insulation is a bit different. Spray foam insulation R-value depends on the type of spray foam insulation being applied.
There are two types of spray foam insulation: open cell and closed cell. Open-cell spray foam insulation has an R-value of approximately 3.5 per inch (R-value can vary based on the product). It is a low-density product that is often used in interior walls and other areas that require some moisture permeability. Closed-cell spray foam has an R-value ranging from 5.0 to 7.0 per inch (R-value can vary based on the product). Closed-cell spray foam is more rigid than open-cell spray foam and is used in areas such as basement box sills or similar areas where moisture permeability and a higher R-value are necessary.
So, how thick does spray foam need to be? That depends. Let’s start with your climate zone. The U.S. Department of Energy sets recommended insulation levels based on climate zone and the area being insulated (attic, walls, or floors). Next, it is important to consider the level of moisture permeability required. That determines the type of spray foam insulation to install (open cell or closed cell).
How thick spray foam insulation needs to depend upon many factors. To be sure your home has the right amount of insulation — not too much or not too little — consult a qualified insulation contractor. A professional insulation company has the training and expertise to properly advise you on the amount of insulation your home needs.