Insulation restricts the flow of heat, acting as a thermal barrier between interior spaces and the exterior environment. This thermal resistance is quantified by the R-value, a standardized measurement indicating insulating effectiveness. The R-38 designation is a common requirement for ceilings, attics, and floors in many moderate to cold climate zones across the United States. Because R-value measures performance rather than physical size, the thickness required to achieve R-38 changes significantly depending on the material chosen.
Understanding the R-38 Rating
The number 38 in the R-38 rating refers to the insulation’s thermal resistance, or its capacity to impede heat transfer. This value is derived from the material’s inherent properties, such as density and composition, along with its installed thickness. A higher R-value indicates a greater ability to slow the movement of heat, which translates into improved energy efficiency.
Building codes, often guided by the International Energy Conservation Code, specify R-38 as the minimum performance level for attics in colder regions, generally correlating to Department of Energy Climate Zones 3 and 4. While the goal is to meet this specific performance target, the underlying materials achieve the rating through different physical dimensions. Therefore, R-38 should be understood as a required thermal performance benchmark, not a uniform physical measurement that applies to all insulation products.
Physical Dimensions by Insulation Type
Fiberglass Batts
The thickness required to reach an R-38 rating varies widely across the three primary types of insulation materials. For traditional fiberglass batts or rolls, which are designed to friction-fit between framing members, the thickness typically ranges from 10.25 to 12 inches. Some manufacturers produce a high-density, 10.25-inch thick batt that is specifically designed to fit within a 2×12 rafter bay. This allows for a necessary ventilation gap while still delivering the full R-38 performance.
Blown-In Insulation
Blown-in insulation, such as loose-fill fiberglass or cellulose, must be installed to a greater depth because of its lower density and tendency to settle over time. To achieve R-38, blown-in fiberglass, which has an R-value of R-2.2 to R-2.7 per inch, requires an installed depth of approximately 13 to 17 inches. Cellulose possesses a slightly higher R-value per inch (R-3.2 to R-3.8), generally needing a final settled depth of 10 to 12 inches to meet the R-38 target.
Spray Foam Insulation
Spray foam insulation offers the highest R-value per inch, resulting in the thinnest required layer for R-38. Closed-cell spray foam, with an R-value of R-6 to R-7 per inch, requires a thickness of about 5.4 to 6.3 inches to achieve R-38. Open-cell foam is less dense, with an R-value closer to R-3.8 per inch, requiring a layer approximately 10 inches thick to reach the same thermal performance. The higher density of closed-cell foam allows it to meet the performance standard with a significantly smaller physical footprint.
Installation and Compression Concerns
A major factor in achieving the full R-38 rating is ensuring the insulation maintains its intended loft and density during installation. Compressing fibrous insulation materials like fiberglass batts or rolls reduces their ability to trap air, which is the mechanism by which they resist heat flow. When a batt is compressed, its R-value is diminished, meaning a 12-inch batt stuffed into a 9-inch space will not deliver R-38 performance.
This compression issue is particularly relevant when attempting to insulate sloped ceilings or cathedral ceilings framed with standard 2×10 rafters, which only provide about 9.25 inches of depth. In these scenarios, installers must avoid forcing a thick R-38 batt into the shallow cavity, as this compromises the insulation’s effectiveness. Alternative strategies are often necessary to meet the code requirement.
One common solution involves installing insulation in two layers, such as laying a second layer perpendicular to the ceiling joists to minimize compression and cover the framing members, which act as thermal bridges. Another method utilizes materials with a higher R-value per inch, like closed-cell spray foam, which can achieve R-38 within the limited depth of the framing cavity. Maintaining a proper air space, particularly between insulation and the roof deck in a cathedral ceiling, is also necessary to prevent moisture issues and ensure the longevity of the roof structure.