How to calculate insulation
Insulation is estimated by area and the coverage of each bag or roll:
Bags or rolls = area ÷ coverage per bag/roll (rounded up)
For batts, coverage is the square footage one roll or bundle covers. For blown-in insulation, the coverage per bag drops as the R-value rises — a deeper layer covers fewer square feet, so always read the bag's coverage chart for your target R-value.
Common R-value targets
| Location | Typical R-value |
|---|---|
| Attic | R-38 to R-49 |
| 2x4 walls | R-13 to R-15 |
| 2x6 walls | R-19 to R-21 |
| Floors | R-25 to R-30 |
Batts vs. blown-in
- Batts/rolls suit open walls, floors, and standard joist spacing — cut to fit between framing.
- Blown-in covers attic floors fast and fills awkward gaps, but needs a blower (often free to rent with purchase).
- Wear protection. Gloves, a mask, and goggles are a must with any insulation.