Measure the attic and the existing insulation
Start with the insulated ceiling footprint, measured in square feet. Divide an irregular attic into rectangles, calculate each area, and add them. Do not subtract the joists from an open attic floor because insulation must cover across them to reduce thermal bridging.
Measure existing depth in several undisturbed spots and identify the material. If it is wet, compressed, pest-damaged, or possibly vermiculite, pause the calculation. Those conditions need a scope decision before new material goes over the top.
Calculate the R-value you need to add
Subtract the estimated existing R-value from the target. A 1,200-square-foot attic at roughly R-19 with an R-49 target needs about R-30 added. Then select the R-30 row on the exact product's coverage chart.
If that row lists 40 square feet of maximum coverage per bag, divide 1,200 by 40 and round up, giving 30 bags. This is only an example. Bag weight and coverage differ enough that substituting another brand's chart can spoil the estimate.
Check the result against the bag label
A calculator is a planning and quote-checking tool. The product label remains the installation instruction, and local code can add requirements for fire clearances, ventilation, and access.
- Minimum number of bags for the measured area.
- Minimum settled or installed thickness, using the label's exact wording.
- Minimum weight per square foot when the chart provides it.
- Extra material for uneven framing and a small amount of unavoidable hose loss.
- Depth markers installed before blowing so coverage can be checked across the attic.
Primary sources
Product details, health guidance, and code references were checked against these sources.
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