Insulation is the one material where the number on the package matters more than any default. A bag of batts might cover 83 sq ft or 176 sq ft depending on the batt, so the estimate has to start from the label.
Area to insulate = length × width (or length × height for a wall)
Packages = area × (1 + allowance) ÷ coverage per package
Round up to a whole package
A 30 × 20 ft attic
The floor to cover is 600 sq ft. If your product’s bag says it covers 96 sq ft, the calculation is 6.25 packages and the list says 7 packages.
This example assumes
- 96 sq ft per bag, a figure you would take from your own label
- No extra allowance, the planner’s starting value for insulation
Worked example 30 × 20 ft area, 96 sq ft per package
| Material | Calculated | Buy |
|---|---|---|
| Insulation96 sq ft per package (the value you entered) | 6.25 packages | 7 packages |
Why coverage per package varies
Johns Manville’s fiberglass fact sheet shows the pattern. A bag of R-19 batts in 16 × 48 inch pieces covers 96 sq ft; a bag of R-30 batts covers 83.42 sq ft; and a bag of R-13 batts in 24 × 96 inch pieces covers 176 sq ft. Coverage per package depends on batt thickness, width, length, pieces per bag and how the product is packaged, so no single factor explains the spread. The same attic takes a very different number of bags:
| Coverage on the bag | Calculated | Packages |
|---|---|---|
| 83.42 sq ft | 7.19 packages | 8 |
| 96 sq ft | 6.25 packages | 7 |
| 176 sq ft | 3.41 packages | 4 |
Those three figures belong to different products with different R-values, so the table shows how much the count depends on the product and not that any of them is the right choice for your project. Which R-value you need is a code and climate question this guide does not answer.
What changes the estimate?
- Coverage per package for the exact product, width and thickness
- Whether you add an allowance for trimming
- Areas that need a different product than the rest
- Obstructions such as framing or ducts, which you may subtract
Where the area comes from
The planner measures a wall (length × height, minus openings), an attic or floor (length × width), or takes an area you already know. The measuring guide covers each.