How it works
The calculator finds the attic floor area and reads the number of bags per 1,000 sq ft and the minimum thickness from the maker’s chart for the R-value you choose.
Area = Length × Width or entered directly
Bags = ⌈ Area (sq ft) × (1 + Extra ÷ 100) × Bags per 1,000 sq ft ÷ 1,000 ⌉
Own product: Bags = ⌈ Area × (1 + Extra ÷ 100) ÷ Coverage per bag ⌉
⌈ ⌉ means rounding up to the next whole bag. Metric areas are converted to square feet before the chart is applied.
Worked example in words
A 40 × 30 ft attic floor is 1,200 sq ft. For R-38, Owens Corning’s AttiCat chart calls for 19 bags per 1,000 sq ft, so 1,200 × 19 ÷ 1,000 = 22.8 – buy 23 bags. Each bag covers at most 1,000 ÷ 19 ≈ 52.6 sq ft, and the insulation must be at least 13.5 in deep once installed.
Reading a coverage chart
Every bag of blown-in insulation carries a chart with one row per R-value. The columns that matter are the minimum number of bags per 1,000 sq ft (or the maximum area one bag may cover) and the minimum installed thickness. Owens Corning’s fact sheet states that the R-value is provided by installing the required number of bags at no less than the minimum thickness, and that falling short on either lowers the R-value. Both charts quoted here also list extra bags and depth for very cold Minnesota winter design temperatures.
Products and charts
The two presets are quoted from Owens Corning’s AttiCat fact sheet (27.5 lb bag) and Johns Manville’s Climate Pro B7600 attic card. Bag weights and charts change between products and over time, so compare the chart on the bag you buy, and use “my own product” for cellulose or any other blowing wool.
Worked example
Inputs
- Measure by
- Attic length × width
- Attic length
- 40 ft
- Attic width
- 30 ft
- Coverage chart
- Owens Corning AttiCat (27.5 lb bag)
- R-value to reach
- R-38
- Extra allowance
- 0 %
Result
- Area to insulate
- 1,200 sq ft
- Coverage per bag
- 52.63 sq ft
- Minimum installed thickness
- 13.5 in
- Bags
- 23
Common mistakes
- Buying by depth alone. The R-value on the chart needs both the minimum number of bags and the minimum thickness – blowing loosely can reach the depth with too little material.
- Using one brand’s chart for another. Bag weights and coverage differ between products; at R-38, for example, the AttiCat chart calls for 19 bags per 1,000 sq ft and the Climate Pro chart for 17.7.
- Measuring the roof instead of the attic floor. Blown-in insulation is laid on the attic floor, so measure the ceiling below it.
- Ignoring existing insulation. These charts are for the R-value of the new layer; how much old insulation adds depends on its type and condition.
- Covering soffit vents or recessed lights. Keep vents clear with baffles and follow the fixture maker’s rules for clearance.
Assumptions and limits
- In dimension mode the attic floor is one rectangle of length × width.
- Coverage comes from the maker’s attic chart for open-blown applications at the chosen R-value; the bag count is rounded up after any extra allowance.
- Joists, trusses and other framing are not subtracted – the charts are stated per net area of attic floor.
- The calculator does not account for existing insulation, the Minnesota cold-climate additions printed on both charts, walls or enclosed cavities.
- This is a material quantity only. It does not choose an R-value, air sealing, baffles or a vapor retarder – those follow your local code.
Frequently asked questions
How many bags of blown-in insulation do I need for 1,000 sq ft?
It depends on the product and the R-value. Owens Corning’s AttiCat chart calls for 14.6 bags per 1,000 sq ft for R-30, 19 for R-38, 25 for R-49 and 31.5 for R-60. Johns Manville’s Climate Pro chart calls for 13.7, 17.7, 23.4 and 29.2 bags for the same R-values.
How deep should blown-in insulation be for R-38?
For AttiCat the chart’s minimum installed thickness for R-38 is 13.5 in; for Climate Pro it is 13.4 in. Other products differ, so use the chart on your bag.
How much does one bag of blown-in insulation cover?
Less as the R-value rises. An AttiCat bag covers at most 52.6 sq ft at R-38 and 31.8 sq ft at R-60, according to Owens Corning’s chart. The chart on the bag lists the maximum coverage for each R-value.
What R-value do I need in my attic?
That is set by the energy code where you live and depends on your climate zone. This calculator doesn’t recommend a value – check with your local building department.
Can I blow new insulation over old insulation?
Often, but the old layer’s R-value depends on its type, depth and condition, and the charts here are for the new layer alone. Enter the R-value you want the new layer to add, and ask the maker or an energy auditor if you are unsure.
Sources
- AttiCat Expanding Blown-In PINK Fiberglas Insulation – Fact Sheet (Pub. No. 10011287-E) — Owens Corning. Attic chart, 27.5 lb bag: bags per 1,000 sq ft and minimum thickness from R-13 (5.9 bags, 5 in) to R-60 (31.5 bags, 20.5 in), e.g. R-38 19 bags, 13.5 in; R-value needs both the bag count and the minimum thickness.
- Climate Pro B7600 Fiberglass Blowing Wool Insulation – Attic Coverage — Johns Manville. Attic coverage chart: minimum installed thickness and bags per 1,000 sq ft from R-11 (4.2 in, 4.8 bags) to R-60 (20.2 in, 29.2 bags), e.g. R-38 13.4 in, 17.7 bags; extra bags for Minnesota winter design temperatures.
- NIST Handbook 44, Appendix C – General Tables of Units of Measurement — National Institute of Standards and Technology. Exact unit definitions: 1 in = 2.54 cm, 1 yd³ = 27 ft³, 1 US gal = 231 in³, 1 lb = 0.45359237 kg.
Method reviewed November 5, 2026