To determine R factor in insulation, first figure out what kind of insulation is installed, such as loose fiber, cellulose or rock wool. Take a measurement of the depth of the insulation installed. With loose fiber, multiply the depth in inches by 3 to obtain the value of the R factor.
With granule insulation, multiply the depth of the material in inches by 2.7 for the R value. The depth in inches of fiberglass batting must be multiplied by 3.2 to find the R value. If the material is foam board or spray-on foam insulation, multiply the thickness in inches by 5 to get the R value.
To find the insulation, remove socket covers from the interior superficial wall after turning off the power to the socket. Use a bright flashlight to illuminate the interior of the wall enough to see the insulation. Next, if you live in a particular climate zone, carefully remove a section of siding to check for insulation sheathing. If there is sheathing, its R value likely ranges between R-2.5 and R-2.6.
Open up any crawl spaces, unheated garage floors and unfinished basements to check for insulation. Many homes have fiberglass batting in these areas. Check the batting for a label containing the R value. If not, use the above formula to obtain it.