Attic Insulation Calculator

It depends on the material and how much R-value you are adding. As a worked example, topping up from R-11 to R-38 (a 27-point gain) needs about 15 bags of Owens Corning AttiCat blown fiberglass or about 34 bags of GreenFiber cellulose for 1,000 sq ft, using the manufacturers coverage charts with 5 percent overage. Enter your own numbers in the estimator above for an exact count.

Measure the depth in inches with a ruler at several spots, avoiding compressed areas, then multiply by the R-value per inch of the material. Loose-fill fiberglass is about R-2.5 per inch, cellulose about R-3.5 per inch, and fiberglass batts about R-3.2 per inch. So 4 inches of old blown fiberglass is roughly R-10.

For an uninsulated attic the U.S. Department of Energy recommends R-30 to R-49 in climate zone 1, R-49 to R-60 in zones 2 and 3, and R-60 in zones 4 through 8. If you already have 3 to 4 inches of insulation, the recommended top-up target is lower; the estimator above uses your zone to suggest a target.

It depends on the material. For R-38 you need about 15.2 inches of settled blown fiberglass at R-2.5 per inch, about 10.9 inches of cellulose at R-3.5 per inch, or about 11.9 inches of fiberglass batt at R-3.2 per inch. For R-49 the figures are about 19.6, 14.0, and 15.3 inches. The depth tab above computes this for every common material.

Usually no. If the old insulation is dry, uncompressed, and mold-free, you can blow or lay new insulation right over it; just add the R-values together. Remove it only if it is water-damaged, moldy, or contaminated by pests, because trapped moisture and mold will keep causing problems under the new layer.

Yes, both directions work as long as you do not compress the layer underneath, since compression destroys R-value. A common upgrade is blowing cellulose over old batts. Never sandwich a vapor barrier between two insulation layers in a mixed climate, because moisture can get trapped; faced batts go with the facing toward the warm side.

Cellulose gives more R-value per inch (about 3.5 vs 2.5 for settled fiberglass), seals air gaps slightly better, and uses about 85 percent recycled content, but it settles over time and must stay dry. Fiberglass is lighter, moisture-resistant, and barely settles. On a per-bag basis cellulose usually costs less, which is why the cost tab lets you compare both.

Typical U.S. ranges are 1 to 4 dollars per square foot installed for blown-in insulation and 2 to 5 dollars per square foot for spray foam. As a material-only example, 15 bags of blown fiberglass at about 43 dollars a bag is roughly 645 dollars before blower rental, which is often free with a 10-bag purchase. The cost tab compares DIY material cost against a pro quote for your area and material.

Air sealing comes first and matters more than most people expect: seal gaps around pipes, wires, recessed lights, and the attic hatch with caulk or foam before insulating, because insulation slows heat flow but does not stop air leaks. In most retrofit attics you do not add a new vapor barrier over existing insulation; one misplaced barrier can trap moisture.

The EPA estimates that sealing air leaks and adding insulation can save homeowners an average of 15 percent on heating and cooling costs. Attics are the highest-return location because heat rises in winter and roof heat radiates down in summer. Savings depend on your climate, energy prices, and how under-insulated the attic is now.

An R-value lookup answers what R-value your climate zone calls for. This calculator answers the next three questions: how many bags, rolls, or kits that means for your attic size, how many inches deep each material must be, and what it costs DIY versus professionally installed. If you only need the zone target, our attic size calculator has a dedicated R-value mode.

The estimate follows the manufacturers own coverage charts and the manufacturers own top-up method: subtract your existing R-value from your target, look up the coverage per bag at that difference, and divide your attic area by it. Charts are labeled as estimating guides because blower settings, technique, and framing change real coverage, so the tool adds an adjustable overage percentage and rounds up to whole bags.