Insulation is sized two ways at once: the R-value (how well it resists heat flow) and the quantity (how many batts or bags cover the area). Get the R-value from your climate zone, get the quantity from the square footage, and you have a complete shopping list. Here are both.
R-value targets by location
R-value is additive, so a thicker or denser product in the same cavity gets you a higher number. The Department of Energy's recommended ranges:
| Where | Cold North (zones 5-8) | Middle (zones 3-4) | Warm South (zones 1-2) |
|---|---|---|---|
| Attic | R-49 to R-60 | R-38 to R-49 | R-30 to R-49 |
| 2x4 wall | R-13 to R-15 | R-13 to R-15 | R-13 to R-15 |
| 2x6 wall | R-19 to R-21 | R-19 to R-21 | R-19 to R-21 |
| Floor over crawlspace | R-25 to R-30 | R-19 to R-25 | R-13 to R-19 |
Walls barely change with climate because the stud depth caps how much fits: a 2x4 cavity is 3.5 inches and holds about R-13 to R-15, a 2x6 holds R-19 to R-21. Attics are where cold-climate homes pile on depth, because there is nothing stopping you from adding more.
How many batts or bags
Insulation is sold by coverage per package, printed on the label, so the count is just area divided by that number plus a little waste. A typical bag of R-13 wall batts or R-38 attic rolls covers around 40 square feet; blown-in bags list coverage at a target depth.
| Area to insulate | Packages (at 40 sq ft) |
|---|---|
| 12 x 12 ft ceiling (144 sq ft) | 4 |
| 200 sq ft wall section | 6 |
| 20 x 20 ft attic (400 sq ft) | 11 |
| 30 x 20 ft attic (600 sq ft) | 16 |
The insulation calculator takes your exact area and your product's own label coverage, so you can plug in the real number instead of the 40 sq ft estimate above and get a package count you can buy to.
Batts, rolls, or blown-in?
- Batts and rolls (fiberglass or mineral wool) are the DIY default for open stud walls and accessible joist bays. Cut them to friction-fit, never compress them, and split the batt around wiring rather than stuffing it behind.
- Blown-in (cellulose or fiberglass) wins in an existing attic: it flows into every gap and hits a uniform depth. Rent the blower, mark your target depth on the trusses first, and it is a two-person afternoon.
- Faced vs unfaced: the kraft facing is a vapor retarder and faces the warm-in-winter side (the living space). Never add a second faced layer on top; use unfaced when you are building up an attic.
The detail that undoes good R-value
Air leaks beat insulation. A gap around a recessed light or an un-sealed top plate lets conditioned air escape straight past whatever R-value you installed. Seal penetrations with caulk or foam before you insulate, and treat attic hatches and knee-wall doors as full-depth insulation too, not afterthoughts.
Insulating during a remodel? The wall framing calculator sizes the studs the batts go between, and the drywall calculator picks up once the cavities are full and the vapor barrier is on.