How thick should attic insulation be installed?

The answer depends on where your house is located and the type of insulation in your attic. Building codes now prescribe how much energy new houses should use and therefore call for specific performance criteria for insulation. The effectiveness of the various insulation options is quantified in terms of the resistance of the material to transmission of heat. The resistance is designated as an “R” factor; the higher the number the greater the resistance and therefore the more effective the material is at blocking the flow of energy in the form of heat. Different materials have different “R” factors for a given thickness of the respective material.

Here is a list of some common attic insulation materials and their respective “R” values, per inch of thickness:

  • Blown Fiberglass R 2.5 per inch
  • Blown Mineral Wood R 3.5 per inch
  • Fiberglass Batts R 3.2 per inch
  • Blown Cellulose R 3.5 per inch
  • Closed Cell Spray Foam R 6.3 per inch

These values are for attic insulation. The same materials used within wall cavities or under floors will often provide different performance.

So given an insulation material, it is basic math to determine how thick your attic insulation must be to achieve the total “R” value required in your geographic region. 

Something else to keep in mind is that “R” values only tell part of the story. Heat is transferred through three different mechanisms – conduction, convection and radiation. “R” values only address heat transmission through conduction. And the heat that is transmitted by convection and radiation can make a tremendous difference on the real-world performance of some of the materials listed above. For instance, fiberglass batts are almost as effective against conduction as blown cellulose and generally less expensive. But batts will not fill hard to reach voids, so they are not as effective against heat transmitted by air movement (convection). The closed cell spray foam is proven to be superior in almost all respects, but at quite a price premium as well.

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