Insulation Calculator

How to measure for insulation

For an attic, you need the floor area of the space you’re insulating. If the attic matches the house footprint, measure the house: length times width, in feet. For an attic over part of the house, measure that section. Irregular footprints break into rectangles you add together.

While you’re up there, do three more things. Measure the depth of any existing insulation with a tape pushed to the drywall, and note what it is (fluffy pink or yellow is fiberglass, gray newsprint-looking material is cellulose). Loose fiberglass runs roughly R-2.5 per inch and cellulose about R-3.2 to R-3.7 per inch, which tells you what you’re starting from and how much you’re adding, not replacing. Find and flag anything that can’t be covered: recessed lights not rated for insulation contact need clearance or covers, and bath fans should duct outside, not into the insulation. And check that soffit vents have baffles so the new, deeper layer won’t choke attic airflow.

For walls, measure each wall’s length times height and subtract windows and doors, then match batts to the cavity: 2x4 walls take R-13 or R-15 batts, 2x6 walls take R-19 or R-21.

The formula and what it assumes

The formula is deliberately simple: bags = ceiling of (area ÷ coverage per bag), where coverage per bag comes from the chart printed on the package for your target R-value. Every loose-fill product carries a federally required coverage chart listing, for each R-value, the required depth, the maximum square feet one bag covers, and the minimum bags per 1,000 square feet. The calculator divides your area by that coverage figure and rounds up to whole bags.

So if your attic is 1,000 square feet and the chart says one bag covers 40 square feet at your target R-value, you need 1,000 ÷ 40 = 25 bags.

The reason the calculator asks you for the package figure instead of assuming one: coverage varies a lot between products and falls as R-value rises, since higher R means deeper material from the same bag. A bag that covers a wide area at R-19 covers a fraction of that at R-49.

What to target depends on where you live. The Department of Energy divides the US into climate zones, and the 2021 IECC specifies R-49 ceilings for zones 2 and 3 and R-60 for zones 4 and up, though many states enforce older editions with lower numbers. For an existing home, anything that gets a thin attic up to current recommendations pays back fastest. Check your zone on the DOE map before you pick an R-value.

The formula assumes an open, flat attic. Cathedral ceilings, kneewalls, and finished attic rooms are different assemblies with their own methods, and walls use batts sized to the framing rather than coverage-chart math.

Buying tips

Loose-fill comes in compressed bags, fiberglass typically in the 25 to 35 pound range and cellulose around 19 to 25 pounds per bag, stacked on pallets at every big-box store. Batts and rolls sell by the bundle with the coverage in square feet printed on the plastic, in widths matched to 16 and 24 inch framing.

The blower machine is the key logistics item for loose-fill. Home Depot and other rental counters carry them, and Home Depot has long offered the first day of the blower rental free when you buy 10 or more bags of the matching insulation. Since most attics need well over 10 bags, the machine often costs you nothing but the truck space. It’s bulky, so plan a pickup vehicle or delivery.

Buy the full calculated count, and don’t be tempted to stretch bags by fluffing. The coverage chart’s minimum bag count exists because installed weight per square foot, not just depth, determines the real R-value, especially for fiberglass. Pros verify by counting empty bags against area, and you should too.

Unopened bags return easily, so round up. And buy attic accessories in the same trip: rulers to staple to joists so you can see depth as you blow, baffles for the soffit bays, and a cover or dam for the attic hatch, which is otherwise a bare hole in your brand-new thermal blanket.

Frequently asked questions

What R-value do I need in my attic?

It depends on your DOE climate zone. The 2021 IECC calls for R-49 ceilings in zones 2 and 3 (most of the South) and R-60 in zones 4 through 8 (roughly the middle of the country on north). Many states still enforce older codes with lower minimums, so check what your jurisdiction has adopted. Going above the minimum on an open attic is cheap while the blower is running.

Why does bag coverage change with R-value?

The same bag of loose-fill insulation spreads thin or piles deep. At a higher R-value you blow it deeper, so each bag covers fewer square feet. That's why the calculator divides your area by the coverage figure printed on the package for your specific target R-value rather than using one fixed number per bag.

Can I add new insulation on top of old?

Usually yes, and it's standard practice in attics. Blow loose-fill right over existing material, or lay unfaced batts perpendicular to the joists on top. Don't put a second vapor barrier in the stack: any new batts over old insulation should be unfaced. If the existing layer is wet, moldy, or vermiculite (which can contain asbestos), stop and deal with that first.

Is blowing insulation a DIY job?

Attic loose-fill is one of the more approachable energy upgrades. Home improvement stores rent the blower machine, and Home Depot has offered the first day's rental free with a 10 bag insulation purchase. You need two people, one feeding the hopper and one on the hose. Wear a respirator, long sleeves, and eye protection, and work early in the day before the attic heats up.