How Much Attic Insulation Do I Need? | Do It Yourself

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How Much Attic Insulation Do I Need?

Cory R. of Richmond, VA wrote:

Hey Itchy,

My attic needs more insulation added. I'm gonna do it myself but I don't know how much insulation to add to it. How do I figure out how much insulation I need for my attic?

On this website, I mainly cover two types of attic insulation - blown in or fiberglass batts. Before adding either one, you have to measure your existing insulation. In most climates you want to bring your insulation level to a R-38 or R-49.

Measuring Existing Insulation

Say you crawl into your attic and you see you have loose fill fiberglass insulation, you measure it and it's about 8" deep overall. By looking at the below chart, you can see you have an existing R-19 in your attic.

Well, like I mentioned above, most areas of the country require attics to be insulated at R-38 to R-49 so you now know you need to add more insulation. This means you will have to add an additional 8" of blown in fiberglass to achieve an R-38.

Comparison Chart

Insulation Type	R-Value per Inch

Loose-fill:
Cellulose	3.1-3.7
(dense pack)	3.4-3.6
Fiberglass	2.2-2.9
(dense pack)	3.4-4.2
Rock wool	2.2-2.9

Batts:
Fiberglass	2.9-3.8
Cotton (Recycled denim)	3.0-3.7

Courtesy U.S. Department of Energy

Measuring How Much Additional Insulation You'll Need

Measure the length and width of the attic area and multiply to obtain the square footage of the attic. If you plan on using fiberglass batts, you need to also measure the distance between the ceiling joists - usually 15" or 23".

Blown In Insulation

Using the above example, you know you need to add 8" of loose fill blown in insulation. When you purchase the insulation there is a chart on the bag that will tell you how many square feet that bag of insulation will cover, based on the desired R-Value needed. Simply divide the bag coverage into the total square feet of the attic, based on the R-Value needed and round up to the next whole number.

Fiberglass Batt Insulation

Using the above example, you know you need to add an additional R-19. Since you already have existing insulation, don't buy the batts with the kraft paper vapor barrier attached to it. Purchase unfaced insulation. You've measured between the joists so you know it will need to be in widths of 15" or 23". The bag or roll of insulation will have a chart indicating how many square feet it will cover. Same as for blown in, simply divide the bags square foot coverage by the total square feet of your attic. Round up to the next whole number.

Important: If the ceiling joists are completely full of insulation don't worry about measuring the width of the joists as you'll be installing them across the joists anyway.

Related Tips and Advice:

How Much Wall Insulation Do I Need?
How Much Floor Insulation Do I Need?
How To Insulate A Concrete Wall
How To Insulate Ducts

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