In this video, Ask This Old House general contractor Tom Silva explains R-value and the need for good insulation.
What is R-value?
Tom defines R-value as the resistance of heat transfer through the insulation—meaning, the better the R-value, the less heat is lost. When it comes to R-value and recommendations for your home, a lot depends on where you live.
In southern parts of the United States, the recommended R-value for a roof is R-30; for the middle of the country R-38; and farther north, R-49 is strongly recommended, as the climate becomes colder.
How Do You Calculate R-Value of Insulation?
One way to calculate the R-value of existing insulation is to measure it with a tape measure. Place the tape measure down and touch the ceiling board to get the height. For most insulations, including fiberglass and cellulose, every inch of insulation is roughly equal to an R-value of 3.5.