Insulation Calculator

How much insulation you need, in R-value

Insulation is measured in R-value, a number that says how well a material blocks heat from moving through it. The higher the R-value, the slower heat escapes in winter and enters in summer. The question "how much do I need" is another way of asking "what R-value should my attic end up at, and how much do I have to add to get there."

Two numbers decide that. The first is your climate zone, which runs from 1 in the hot South to 8 in the far north. Colder zones need more insulation. The second is what you already have. If your attic is bare, you are adding the full target. If you already have a few inches, you are topping it up to the target.

This calculator gives you the R-value to add, the depth to blow in, and the number of bags to buy for the material you pick. It focuses on attics, because that is where most homes lose the most heat and where the fix is cheapest per dollar spent.

Attics are the first place to look because heat rises and an unsealed attic floor is common in older homes. Walls and crawl spaces matter too, but they are harder and more expensive to treat, so most people start overhead. If you can see the ceiling joists when you look across the attic floor, you need more insulation.

The math behind the insulation estimate

The calculator starts with a target R-value for your climate zone, then subtracts the R-value you already have. The difference is what you add. ENERGY STAR publishes the same recommended insulation levels by zone and location in the home.

R-value per inch varies by material. Blown cellulose gives about 3.7 per inch, blown fiberglass about 2.5 per inch, and fiberglass batts about 3.2 per inch. That means the same R-value needs a different depth depending on what you blow in.

Bags are the last step. A bag covers a fixed number of square feet at a fixed R-value, so the number of bags is your area times the R-value you are adding, divided by the coverage of one bag. Round up, because a bag left over is cheaper than a second trip to the store.

The Department of Energy's consumer guide to home insulation explains how to measure what you already have and where the biggest losses usually are.

Worked example: adding R-49 to a 1,000 square foot attic

Take a 1,000 square foot attic in climate zone 5, with about 3.5 inches of old fiberglass in place. That old insulation is roughly R-11. The zone 5 target is R-60, so you need to add R-49.

With blown cellulose at R-3.7 per inch, R-49 divided by 3.7 is 13.2 inches deep. That is the settled depth to mark on your joists, so you can check your work with a ruler as you go.

Bags are next. Multiply 1,000 square feet by 49 to get 49,000 square feet of R-value. A 25 pound bag of cellulose covers about 520 square feet at R-1, so 49,000 divided by 520 is 94.2 bags. Round up to 95 bags. At about $16 a bag that is roughly $1,520.

If you use blown fiberglass instead, the bag covers about 400 square feet at R-1, so you need 123 bags. Fiberglass also needs a deeper layer for the same R-value, about 19.6 inches, because it insulates less per inch.

Whatever you choose, spread it evenly. Thin spots leak heat the way a hole does, so keep the depth consistent across the whole attic floor and check it with a marked stick as you go.

Air sealing comes before insulation

Insulation slows heat, but air leaks move it. A bare attic floor with gaps around wires, pipes and the chimney can lose more heat through air movement than the insulation itself saves. Seal those gaps before you blow in anything, or the insulation just filters the leak.

Use caulk for small cracks and canned spray foam for bigger gaps around pipes, wires and framing. Cover the top plate of the wall and the hatch or stair opening. A well-sealed attic lets the insulation do its job, and it also keeps moist indoor air from condensing in the cold attic.

Never block soffit vents. Insulation that covers the vents at the eaves chokes the airflow that keeps the roof deck dry. Use baffles to keep a clear path for air, and keep the insulation a few inches away from any chimney or hot flue. The Department of Energy puts air sealing first in its guide for a reason, because a leaky attic wastes the insulation you just paid for.

Choosing a material and doing it yourself

Blown cellulose and blown fiberglass are the two common choices for a do it yourself attic top-up. Both are blown with a rented machine, and both are easy to spread. Cellulose settles a little, so the bag lists a settled depth as well as a loose depth. Blow to the settled mark.

Cellulose is denser and blocks air better, and it is often cheaper per R-value. Fiberglass is lighter and does not hold moisture, which some people prefer in a damp climate. Batts are the slow way to cover a large attic, and they are hard to fit around obstacles, but they work for a small space or a garage ceiling.

Wear a respirator, goggles and gloves, and keep the machine outside with the hose run through a window. Check that the attic has enough venting before and after, because a tight attic with no airflow can trap moisture and rot the roof. If your attic has no insulation at all, or you are not comfortable moving around joists and wires, a professional crew is worth the cost.

Check for rebates before you buy. Many utilities and state programs cover part of an insulation upgrade, and a home energy audit often unlocks a larger rebate. The audit also tells you whether air sealing or insulation will pay back faster in your specific house.