How to calculate how much insulation you need
Insulation starts with the area you're covering. For batts and rolls, you divide that area by the coverage listed on each package. For blown-in attic insulation, the bag count depends on the R-value you're targeting, because higher R-values need more bags per hundred square feet.
area = length × width
batts: packages = round up((area × 1.05) ÷ coverage)
blown-in: bags = round up(area ÷ coverage-per-bag at R-value)
Worked example
A 30 ft × 25 ft attic is 750 sq ft. With batt packages covering 88 sq ft each and a 5% buffer, you need 792 ÷ 88 = 9 packages. For blown-in at R-49, coverage per bag is lower, so bag counts run higher.
What R-value do you need?
R-value measures resistance to heat flow — higher is warmer. Recommended levels depend on climate and location in the home:
| Location | Typical R-value |
|---|
| Attic (warm climate) | R-30 to R-38 |
| Attic (cold climate) | R-49 to R-60 |
| Exterior walls (2x4) | R-13 to R-15 |
| Exterior walls (2x6) | R-19 to R-21 |
| Floors | R-19 to R-30 |
Frequently asked questions
How much insulation do I need for a 750 sq ft attic?
For batts covering about 88 square feet per package, a 750 square foot attic needs roughly 9 packages including a small waste buffer. Blown-in needs vary by target R-value.
What R-value do I need for my attic?
Most attics call for R-30 to R-38 in warm climates and R-49 to R-60 in cold ones. Your local energy code sets the recommended minimum for your region.
Is blown-in or batt insulation better?
Batts are simple for open walls and floors between joists. Blown-in fills irregular attic spaces and around obstacles more completely, but needs a machine to install.
Can I put new insulation over old?
In attics you can usually add unfaced insulation over existing material to boost R-value. Never add a second vapor barrier, which can trap moisture between layers.