Achieving R-23 Wall Insulation: Options for New Builds and Retrofit Projects
Reaching an R-23 rating in a wall means different things depending on whether the wall is new or already exists. New construction allows for simpler strategies, while older walls often require layering different insulation types to hit that target.
By Avery Evans
Getting a wall assembly up to R-23 is less complicated when you are working with new wall units. For walls that are already in place, the job is tougher and typically calls for blending more than one insulation method to reach an R-23 value. The U.S. Department of Energy suggests exterior wall assemblies in new homes and additions fall between R-11 and R-38.
Key Factors
How much insulation you can fit inside a wall cavity depends on the dimensions of the framing. A 2 x 4 wall cavity gives you 3 ½ inches of depth, while a 2 x 6 cavity provides 5 ½ inches. That means you can place 3 ½ inches of insulation in a 2 x 4 wall, or 5 ½ inches in a 2 x 6 wall.
Many different insulation materials exist for wall assemblies. The most frequently used products for interior wall cavities are fiberglass batt, cellulose, and wool. Each has a different R-value. R-value indicates how well insulation resists heat loss and is typically expressed as R-value per inch of material.
Fiberglass batt insulation is placed between the studs before drywall or plaster goes up. Its R-value ranges from 3.1 to 3.5 per inch. Cellulose or wool must be blown into existing walls. Cellulose offers an R-value of 3.2 to 3.8 per inch, while wool provides 3.0 to 3.3 per inch. Spray foam insulation is another option for walls. Rigid foam board can be applied on the exterior. Both of these products have R-values between 3.6 and 7.0 per inch.
Installation Advice
Before installing insulation, check the walls for signs of moisture or air leaks. Closed-cell foam products can deliver high insulating performance while also acting as a water and air barrier. Fiberglass batt, cellulose, and wool insulation do not stop water or air from getting through. When water soaks into insulation, it adds weight and creates gaps and unprotected spots.
Any breaks in the thermal envelope—cracks, gaps, or holes—need to be fixed. Spray foam in a can is one of the most effective ways to make these repairs.
For 2 x 4 walls, you can install R-13 or R-15 batt insulation. Spray foam delivers about R-7 per inch. Some slow-expanding foam products are made for existing walls, but they are easier to apply in new wall systems. Still, if you achieve R-7 per inch with spray foam, you could reach R-23 in a 2 x 4 wall, which has a 3 ½ inch cavity. In wall systems framed with 2 x 6 members, installation can accommodate R-19 or R-21 batt insulation. To get an R-23 wall assembly, you need to combine batt or cellulose insulation for the existing wall with rigid foam board on the exterior.
In new construction or remodeling projects that involve new walls, builders and homeowners are turning to a combination of materials: a first layer of closed-cell spray foam followed by un-faced fiberglass batt insulation. The foam also serves as a water and air barrier. Additionally, it has an insulating value of 7.0 per inch. The fiberglass batt insulation can be applied over the foam once it has cured.
