How Thick Should My Attic Insulation Be?

Installing the right attic insulation and air-sealing your home improves comfort and energy efficiency, but insulation thickness plays a role. It corresponds with a material’s R-value or ability to re-heat flow. Your climate and home’s age also determine the best type and thickness of insulation Heating & Air, specializes in installing attic insulation and will share our expertise to help you make informed decisions.

Insulation Thickness vs. R-Value

The Greater San Francisco Bay Area lies mainly in Climate Zone 3. An uninsulated attic should have R30 to R60 insulation. If you have an existing layer of insulation of 3 to 4 inches, any additional material should be rated R25 to R38.

Here’s a more general look at insulation thickness and R-value.

• 4 to 5 Inches: Is rated R-15 and found mainly in poorly insulated and older homes.
• 6 to 7 Inches: Is rated R-21 and considered inadequate, even in a filled joist.
• 13 to 14 Inches: Is rated R-38 and suited for most southern locations.
• 16 to 18 Inches: Is rated R-49 and suited for most northern climates.

You can also use the thickness of existing insulation to determine how much to add. For example, if your attic insulation is 9 inches deep (R26) and you want to increase it to R38, which is 12.75 inches thick, you must add 3.75 inches of insulation. Professionals use more precise calculations so consider hiring a contractor to achieve optimal efficiency.

Calculating the R-Value of an Insulating Material

Every material in a house has an R-value. An inch of solid wood has an R-value of 1. An inch of blown fiberglass insulation is rated 3.1 to 3.4. To determine a material’s total R-value, use the following formulas:

• Fiberglass Batts: Multiply the depth of the material by 3.2.
• Vermiculite/Perlite Granules: Multiply the depth by 2.7.
• Cellulose Fibers: Multiply the depth by 3.7.
• Rock Wool: Multiply the depth by 2.8.
• Loose Fiberglass: Multiply the depth by 2.5.

How to Know You Need More Attic Insulation

Whether you need more insulation in your attic depends on the following:

• Your Location: You’ll need thicker, higher R-value insulation if you live in the Northeast or Rockies than in most parts of California. 
• Age of Your Home: Homes that are over 10 years old usually need more insulation. Your home may need to be retrofitted with mineral wool or fiberglass insulation.
• Your Attic Is Well-Insulated: If the insulation is thick enough to hide the floor joists, you probably don’t need more. However, if it’s level with or just below the floor joists, consider adding more insulation.
• Insulation Is Evenly Distributed: Low spots in the insulation can lead to efficiency issues. If your attic insulation is thicker in the middle and there’s less near the eaves, it should be improved.

To check the depth of your attic insulation, first find your attic access. This is often a drop-down ladder or a hole in the ceiling where you need to use a ladder to gain access, while other homes have an access point in a closet. Next, determine your insulation type (fiberglass is often pink, yellow, or white, cellulose often consists of small gray flat pieces, and rock wool tends to be gray or near white with black specs).

Now, you can directly measure your insulation. Use a ruler or tape measure in different spots. Then, average the measurements you get to determine the insulating material’s overall depth and R-value.

A Word on Adding Insulation

Insulation can be added to existing materials. If you have fiberglass batt or blanket insulation, you can put loose fill on top of it. Batts and blankets can also be installed above loose fill if any paper or foil backing has been removed. Adding insulation can be tricky depending on the material. For example, loose fill requires using a blowing machine. Unless you’re handy with this type of equipment, it’s best to hire a professional.

Contact Trio Heating & Air

If you live in Santa Clara County or San Mateo County, we can install attic insulation in your home. We often install fiberglass or cellulose insulation in different forms, including blown-in or rolled, and remove and replace old insulation when necessary. A thorough inspection allows our technicians to determine the best approach to improving energy efficiency so you save money over time. Contact us today to schedule a free estimate and insulation installation services in your home.