If you want to use fiberglass insulation to insulate your exterior two-by-four (2×4) and two-by-six (2×6) framed walls, what thickness should you use? This is an important question because wall assemblies and insulation are permanent, making it difficult to change once they are installed.
Choosing too little insulation can result in a colder house during winter and a warmer house in summer, as insulation reduces heat transfer into the house during warmer seasons. Conversely, over-packing insulation can also result in a colder house.
The key to keeping a home warm is the tiny air pockets created within the insulation. These pockets help maintain the home’s temperature. Finding a balance between too little insulation and too much insulation will keep you and your family comfortable throughout the year.
Basics of Faced Fiberglass Insulation, From R-13 to R-19
R-value is the basic unit of measurement that determines the insulation’s effectiveness. The R refers to thermal resistance. Higher R-value numbers indicate that the material resists cold or heat from the outside better. R-value is determined by thickness, density, and type of materials used.
Insulation for 2×4 Walls
Most wall assemblies, especially those in older homes, are constructed with two-by-four (2×4) studs. These modern two-by-fours are not actually four inches, so the true cavity depth is only 3.5 inches. In most cases, R-13 or R-15 kraft-faced fiberglass insulation rolls are used for these two-by-four stud walls. While rated differently, these two thicknesses can both be used for modern two-by-four wall systems.
For older homes, especially those predating the 1950s, use R-13 or R-15 fiberglass insulation since the two-by-fours are 4 inches deep. Currently, there is no 4-inch thick-faced fiberglass insulation available in batts or rolls in the market.
Insulation for 2×6 Walls
Newer homes may have walls built with 2×6 studs. For these walls, use R-19 or R-21 kraft-faced fiberglass insulation to ensure neither too loose nor too tightly packed insulation within the walls.
The Best Insulation for 2×4 and 2×6 Walls
Choose the appropriate insulation thickness for your wall type based on the table below:
Too much insulation can reduce effectiveness if insulation is too thin to contain air pockets, which maintain warmth. However, properly installed spray foam or rigid foam insulation is effective because they contain millions of tiny air pockets.
If you live in a cold climate where R-value needs exceed available wall space, consider adding thick insulation batts to the attic. You can also seal door and window cracks with caulk, add exterior wall sheathing, or install storm windows. If all else fails, consider removing and replacing old insulation, especially if it has become damp over the years.