How to Determine the R-Value of Insulation

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How can I determine the R-value of my attic insulation? -Anita

Hi Anita,

The R-value of insulation is a measure of how well it reduces the flow of heat and cold into and out of your house. The higher the R-value, per inch of thickness, the better job the insulation will do when you heat or cool your home.

While you can’t have too much insulation, older homes often don’t have enough. To determine the R-value of the insulation in your attic, you need to know the R-value of the type of insulation in your home and the thickness of the insulation.

R-Value of Insulation by Type

Here are the R-values per inch of the most common types of insulation:

  • Fiberglass (blown): 2.2 – 2.9
  • Fiberglass (batts): 2.9 – 3.8
  • Cellulose (blown): 3.1 – 3.8
  • Rock Wool (loose): 2.2 – 3.3
  • Foam (sprayed): 3.6 – 8.2

As you can see, there’s some variation in R-value depending on the particular type and brand of products used. Depending on where you live, it’s recommended that attics have a minimum R-value of R-30 in warm climates to R-60 in cold climates.

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Recommended Insulation by Climate

By dividing the recommended R-value for your area by the R-value of your particular type of insulation, you find that an attic should have on average:

Warm Climates (R-30 to R-49):

  • Fiberglass (blown): 14” – 18”
  • Fiberglass (batts): 11” – 14”
  • Cellulose (blown): 11” – 13”
  • Rock Wool (loose): 12” – 18”
  • Foam (sprayed): 5” – 11”

Moderate Climates (R-38 to R-60):

  • Fiberglass (blown): 17” – 22”
  • Fiberglass (batts): 13” – 17”
  • Cellulose (blown): 13” – 16”
  • Rock Wool (loose): 15” – 22”
  • Foam (sprayed): 6” – 14”

Cold Climates (R-49 to R-60):

  • Fiberglass (blown): 19” – 25”
  • Fiberglass (batts): 14” – 19”
  • Cellulose (blown): 14” – 18”
  • Rock Wool (loose): 17” – 25”
  • Foam (sprayed): 7” – 15”

To find the R-value of the existing insulation in your attic, multiply the number of inches of insulation by the R-value for your particular type.

For more detailed information on insulation in your part of the country and recommended insulation R-value for attics, walls, and floors; go to the Energy Savers Insulation page on the U.S. Department of Energy website.

Good luck with your project,

Danny

Further Information

6 COMMENTS

  1. The door between the kitchen and the garage is a hollow core door that has a large gap at the bottom. Should I replace this with a solid core door that has a tighter seal?

  2. Will Light-Straw Clay pass an insulation inspection? I have an entire conventional house with 4″ and 6″ walls to insulate on a VERY limited budget. One side of the walls are lath and plaster (old house) so the exposed side will be Sheetrock; unless I learn to plaster once I solve my insulation problem.

  3. If I had a 1000 square foot attic and 750 sq/ft is insulated to R30 and 250 sq/ft is insulated to R19. What would be the overall average R- value of the attic.

  4. Is the R value calculated the same in all countries. Australian builders put R 3-4.5 in a ceilingf (using 92mm stud width) and as little as R 1.5 in walls. The differential between Warm climates as indicated by US values seems very strange. Personally I think as an Australian we use far too little insulation. However with 4″ external walls I found it difficult to insert what is labelled as R5 in Australia.

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