When insulating a ceiling, you'll need to assess several factors before determining the appropriate thickness of the insulation material. Depending on the local climate and construction codes, insulation standards may be more stringent or lax. The ability of insulation to retain heat is measured by its "R-value," which varies depending on the type of insulation. Therefore, you may meet a necessary R-value with a thin high-density type of insulation or with a thicker, less dense material. The U.S. Department of Energy (DOE) divides the country into eight zones according to climate and the corresponding recommended R-value for insulation.
8 to 10 Inches
When insulating ceilings in warmer climates, or when using higher density materials, as little as 8 inches of thickness may suffice. According to the DOE, 8 inches of high-density batt insulation will provide around an R-30. Likewise, 9 1/2 inches of regular batt insulation will provide an R-value of approximately 30. Even in the case of cathedral ceilings, notoriously difficult to insulate, 2 by 12 rafters will allow sufficient space for ventilation, plus 10-inch batts of insulation, with at least R-30 value. In much of the southern U.S., R-30 is the minimum recommended insulation level for ceilings or attics. The southernmost DOE zone, which covers southeast Florida, Hawaii, Guam, Puerto Rico and the Virgin Islands, recommends an R-30 to R-48. For the next two warmest zones, extending as far north as North Carolina, northernmost Oklahoma and much of California, the Energy Star ratings recommend between R-30 and R-60.
10 to 12 Inches
In cooler climates, or in cases where you have sufficient space for more insulation material, you may opt for a thickness between 10 and 12 inches. According to the DOE guide, standard 12-inch batt insulation provides an R-factor of 38. R-38 meets minimum standards for climates as far north as New Jersey, Missouri and Kansas, parts of New Mexico and Arizona and much of California, as well as the coastal Pacific Northwest. For greater insulation, opt for denser high-performance batts, which typically offer R-3.7 to R-4.3 per inch of thickness. An extremely high-performing 12-inch batt could offer a value over R-50, making it suitable for anywhere in the U.S.
More Than 12 Inches
It's less likely you'll need to use batt insulation with a thickness exceeding 12 inches. However, you may have ample space for insulation and prefer the lower cost of lower-density insulation instead of the slimmer high-performance types. Based on the DOE recommendations, you can expect regular batt insulation to have an approximate value between R-2.9 and R-3.8 per inch of the material's thickness. Based on this standard, 14-inch insulation of the lowest calibre will provide just over R-40, making it acceptable throughout most of the southern U.S.