Energy requirements for buildings have progressively become more rigorous. Units of resistance to heat flow (R-factor) are now aligned with maps of the U.S. by climate zone, with minimum values of R=20 for warm climates to R= 40 in extremely cold climates. R-values per inch for commonly used roof insulation boards range from 2.7 for wood fiber, perlite, and foamed glass to 5 for extruded polystyrene and 5.6 for isoboards.
Meeting Code-Mandated R values
If, for example, you need an R-value of 25, you would need 9 inches for perlite (!) to 4.46 inches of thickness for isoboards. To find your needed R-value, see the climate zone table and maps above.
Does Thickness Matter?
In commercial roof design, the thickness of the wood nailers used at roof perimeters needs to match the thickness of the thermal insulation (see illustrations above). However, 1 inch doesn’t always mean 1 inch.
Everyone has heard of a 2x4, but few people realize that the actual height and width of a 2x4 is really somewhere close to 1 1/2 x 3- 1/2 inches, depending on the dryness of the material and milling methods. Similarly, a 1x ("one-by") is only about 3/4 inches in thickness, according to Wikipedia. To be safe, multiply the required inches of thickness by 1.5. This means that for insulation with high R-values, such as isoboard, you would actually need 6.7 inches of lumber, not 4.46. That is a lot of wood, plus metal roof edging must extend below the exposed nailer.