Blown-In Insulation Calculator

What your result means

The bag count is the minimum number of bags the manufacturer says you must spread over your attic to get the R-value you picked. It’s printed on every bag as “bags per 1,000 sq ft”. The depth is theminimum thickness the insulation must reach when it’s blown. Both matter: if you hit the depth with fewer bags, the insulation is fluffed too loosely and won’t reach its R-value.

If you already have insulation, the calculator credits it and only counts the R-value you’re adding. Want prices too? The attic insulation cost calculator turns the same bag counts into DIY and contractor costs.

How to use this calculator

1. Pick the product you’ll buy. The four options are the big-box and supply-house products with published coverage charts. If yours isn’t listed, use the closest one, then check its bag label.
2. Measure the attic floor. For a rectangular house it’s length × width of the top floor ceiling. Skip garages and porches that have their own roof, and don’t count the area over a finished bonus room.
3. Choose your target. The climate zone sets ENERGY STAR’s recommended total R-value. Pick “Custom” if your local code or utility rebate asks for something else.
4. Measure what’s there. Push a ruler down to the drywall in three or four places and average. Old insulation is credited at conservative R-values per inch because it settles and compresses.
5. Add extra for joists, trusses, ducts and low eaves that make it hard to blow evenly.

How it’s calculated

Existing R   = Existing depth × R per inch (batts 3, loose fiberglass 2.5, cellulose 3.4, rock wool 3.3)
R to add     = Target R − Existing R
Bags         = label bags per 1,000 sq ft at that R × Area ÷ 1,000 × (1 + extra %), rounded up
Depth        = label minimum installed thickness at that R
Added weight = label bags per 1,000 sq ft × bag weight ÷ 1,000   (lb per sq ft)
Markers      = Area ÷ 300, rounded up

Between label rows (for example, adding R-41.5 on top of old insulation) we interpolate in a straight line. Labels list the net area, so ceiling joists are already accounted for by the manufacturer, and the extra percentage covers what’s lost to obstructions and uneven blowing.

Bags per 1,000 sq ft, from the labels

Bags per 1,000 sq ft with the minimum installed thickness in brackets. Cellulose thickness is the depth when blown; it settles to about 7% less (to 13.3" at R-49). The fiberglass products settle very little, so their labels list the same installed and settled depth. We aren’t affiliated with any of these manufacturers.

Worked example: topping up an existing attic

A 1,400 sq ft attic in climate zone 4 has 4" of old loose fiberglass. The owner plans to buy Knauf Jet Stream Ultra:

• Existing: 4" × 2.5 = R-10. Zone 4 target: R-60. So add R-50.
• The label needs 24.1 bags per 1,000 sq ft at R-50. × 1.4 × 1.05 = 36 bags.
• Blow it at least 16.7" deep, for about 20.7" in total once it sits on the old layer.
• It adds about 0.77 lb per sq ft, 1,152 lb in all, and needs 5 depth markers.

The same job would take 38 bags of AttiCat or 103 bags of cellulose. Bigger bags aren’t automatically a better deal, so compare price per bag × bags needed.

How many bags for my attic?

Bags needed for an attic with no insulation, blown to R-49, before any extra for obstructions:

Fiberglass or cellulose?

• Bag count: cellulose bags are cheaper but you need about 2½–3 times as many. Plan on a longer day feeding the hopper.
• Depth: cellulose reaches R-49 at about 14.3" versus 17" for AttiCat. That matters at the eaves, where the roof is low.
• Weight: cellulose at R-49 weighs about 1.71 lb per sq ft; fiberglass about 0.69. See the ceiling limits below.
• Settling: cellulose settles and its label shows a lower settled depth. Fiberglass loose fill settles very little.

Can my ceiling hold it?

Drywall makers publish how much unsupported insulation a new ceiling can carry before it starts to sag between joists:

Thick cellulose on ½" drywall with 24" truss spacing, common in newer homes, can exceed the 1.3 lb limit at R-49 and above. Blown fiberglass stays well under it. Remember the weight of any insulation already there, and plaster ceilings in old houses are a different story. If cracks or sags appear, ask a contractor before adding more.

Tips for blowing it yourself

• Get the machine free. Home Depot lends a blower for 24 hours when you buy 10 or more bags. Other stores and rental yards have their own terms, so ask before you buy.
• Staple up depth markers (attic rulers) before you start, about one per 300 sq ft, facing the hatch. Building codes require them for new installations, and they’re the easiest way to blow evenly.
• Air seal and install baffles first. Seal gaps around top plates, pipes and light fixtures. Fit baffles at every vented rafter bay so the insulation can’t block the soffit vents.
• Keep clear of heat sources. Leave a gap around chimneys, flues and recessed lights that aren’t rated IC (insulation contact), using a metal or fire-safe dam.
• Count bags, not just inches. Spread the bag count evenly by area, for example one bag per 40 sq ft for AttiCat at R-49.
• Watch for vermiculite. Gray-brown, pebbly insulation may contain asbestos. Don’t disturb it. Have it tested first.

Frequently asked questions

How many bags of blown-in insulation do I need for 1,000 sq ft?

For an empty attic at R-38, about 19 bags of AttiCat, 18 of Jet Stream Ultra or 50 of Greenfiber cellulose. At R-49 it’s 25, 24 and 69. Add 5–10% for obstructions.

How deep should blown-in insulation be?

For R-49, AttiCat needs at least 17", Jet Stream Ultra 16.4" and cellulose 14.3". For R-60, about 20" of fiberglass or 17" of cellulose.

Why does my label say a different number of bags?

Bag labels change between production runs and regions, and some brands publish separate charts for Minnesota’s cold-weather rules. Always follow the label on the bags you buy. The calculator is for planning and budgeting.

Can I blow insulation over old batts?

Yes, as long as the batts are dry and free of mold, pests and smells. Blown-in fills the gaps around and over batts, which improves the whole layer. If the old batts were laid with the paper or foil facing up, slash the facing first so it can’t trap moisture in the middle of the insulation.

How long does it take to blow in insulation?

Blowing itself is usually a one-day job for a typical attic, with two people: one feeding the machine and one in the attic with the hose. Preparation (air sealing, baffles, depth markers) often takes as long as the blowing, so plan a second day for it.

Does blown-in insulation settle?

Cellulose settles, and its label shows both the installed and the settled thickness. The R-value on the label is for the settled depth. Modern fiberglass loose fill settles very little; Knauf, for example, reports about 1% over 20 years in a third-party study.

Sources

• Owens Corning AttiCat manufacturer fact sheet, Pub. No. 10011287-E (Sept 2019)
• Knauf Performance+ Jet Stream Ultra open attic coverage chart
• Johns Manville Climate Pro B7600 attic coverage card, BIC-260 (June 2018)
• Greenfiber Sanctuary attic (open blow) coverage chart, Rev-K
• ENERGY STAR — Recommended home insulation R-values
• GreenBuildingAdvisor — Is ⅝" drywall sufficient to support R-60 cellulose? (quotes USG ceiling load limits)
• International Energy Conservation Code, R303.1.1.1 — Blown or sprayed roof/ceiling insulation (depth markers)