How to Calculate the Additional R-value of Insulation Needed in an Attic

Insulation slows the movement of heat. During the winter, this slows the movement of warmth in the home to the colder outdoors; while during the summer, it slows the movement of the heat occurring outdoors into the home. Because heat rises, insulation in the attic of the home is particularly important and is often considered an important part of a home renovation.

Ruler or tape measure

Instructions

- 1
  Measure and identify the existing insulation. Use this information to calculate the existing insulation R-factor in the attic. For example, loose filled cellulose insulation has an R-factor of about R-3.5 per inch of depth. Blown fiberglass about R-2.5 per inch. An attic with 4 inches of blown fiberglass insulation has an R-factor of about 10 determined by multiplying the R-2.5 per inch times the 4 inches.
- 2
  Determine the appropriate insulation factor for your home and climate. This varies from about R-30 to R-50 in the most extreme climates. Insulation keeps homes cooler in the summer and warmer in the winter, reducing energy consumption year-around. The highest R-factor insulations are suggested for climates with either cold or warm extreme units.
- 3
  Subtract the existing R-factor from the appropriate insulation for your area. If you plan to insulate the home to R-40 and you currently have an insulation of R-10, you need to add R-30 to your attic.
- 4
  Calculate the inches of additional insulation necessary to reach the goal. If you need to add R-30, for example, and choose blown fiberglass insulation at R-2.5 per inch you need to add 12 inches of insulation to the attic. This is determined by dividing the R-30 by the R-2.5 per inch.

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