Minimum Rafter Size for Insulation

Rafter Sizes

• Insulation can pile up beyond the depth of the rafter if placed horizontally between the rafters, such as in a ceiling. In these cases, the minimum size of a rafter doesn’t matter, as the insulation often exceeds the depth of the rafter anyway, whether the rafter is a 2-by-6, 2-by-8, 2-by-10 inch or higher dimensions. How far apart the rafters are set -- usually 16, 20 or 24 inches on center -- determines the required width of the insulation.

Insulation Depth to Rafter Depth

• Insulation comes in multiple widths based on construction framing, but has various thicknesses based on its R-value. Since attic insulation thickness will surpass the depth of the rafter, use the right amount of insulation for your region. For instance R-11, R-13 and high-density R-15 are all 3-1/2 inches thick and are usually only used atop a thicker insulation to reach the needed R-value in an attic. If your region requires R-30 in the attic, you can choose to use R-30 insulation or two R-15 insulation batts.

R-Value, Compression and Rafter Sizes

• Insulation doesn’t get its R-value from the fiberglass threads, as glass essentially has no R-value. Rather, the trapped air during the manufacturing process acts as the resistance to heat flow through the insulation. The trapped air only stays inside the insulation as long as it is not compressed. Once the insulation is compressed, it significantly loses its R-value. This makes choosing the right insulation to fit the rafter cavity of the utmost importance to its R-value. Regardless of the rafter size, choose the insulation to fit in the space between the rafters, 16, 20 or 24 inches on center.

Analyzing Existing Insulation

• To ascertain the depth of loose-fill insulation already in your attic, the total R-value is a calculation of 2.5 times its depth for fiberglass loose fill. For rock-wool, the calculation is 2.8 times its depth for total R-value. Cellulose insulation is 3.7 times its depth, while vermiculite or perlite insulation calculates R-value at 2.7 times the depth. For batt insulation, use 3.2 times its depth to obtain R-value. Use these calculations to measure the amount of insulation in your attic to determine if you need more. Visit the U.S. Department of Energy's website to obtain the recommended R-value for your region.