How to use the insulation calculator
- Enter the area — the attic floor, or the wall area to insulate.
- For batts between studs or joists, subtract about 10% for framing; for blown-in over the top or rigid foam, leave it at 0.
- Set your target R-value (see the table below).
- Choose the insulation to add, and measure the depth and type of any existing insulation.
- For bag counts, enter the coverage per bag for your R-value from the bag’s chart.
The formula
Existing R = Existing depth × R per inch (existing type) R to add = Target R − Existing R Depth = R to add ÷ R per inch (new type) Volume = Area × Depth Bags = Area ÷ Coverage per bag (from the bag chart)
Worked example
A 1,000 ft² attic with 3½ in of old fiberglass batts, upgraded to R-49 with blown cellulose:
- Existing R = 3.5 × 3.2 = R-11.2
- R to add = 49 − 11.2 = R-37.8
- Depth = 37.8 ÷ 3.5 = 10.8 in of cellulose (blow a little deeper to allow for settling)
- At 40 ft² per bag for that R-value: 1,000 ÷ 40 = 25 bags
DOE recommended attic R-values
| Climate zone | Example areas | Uninsulated attic | Adding to 3–4 in existing |
|---|---|---|---|
| 1 | South Florida, Hawaii | R-30 to R-49 | R-25 to R-30 |
| 2–3 | Gulf Coast, Southern California, Georgia | R-30 to R-60 | R-25 to R-38 |
| 4–8 | Mid-Atlantic, Midwest, Northeast, Mountain West | R-49 to R-60 | R-38 to R-49 |
Walls: R-13 to R-15 in 2×4 cavities and R-19 to R-21 in 2×6 cavities, plus continuous exterior foam in colder zones. Check the Department of Energy’s map for your exact zone.
R-value per inch by insulation type (typical)
| Insulation | R per inch | Best for |
|---|---|---|
| Fiberglass batts | 3.1–3.4 | Open walls, floors, attics (DIY) |
| Mineral wool batts | 3.8–4.2 | Fire and sound resistance |
| Blown fiberglass | 2.2–2.7 | Attic floors |
| Blown cellulose | 3.2–3.8 | Attic floors, dense-pack walls |
| Open-cell spray foam | 3.5–3.7 | Air sealing, roof decks |
| Closed-cell spray foam | 6.0–7.0 | Rim joists, basements |
| Rigid XPS / EPS / polyiso | 5.0 / 3.8–4.2 / 5.6–6.5 | Continuous exterior and basement insulation |
Before you insulate
- Air-seal first: caulk and foam gaps around wires, pipes, top plates and the attic hatch.
- Keep vents open: install baffles at the eaves so soffit vents stay clear.
- Check recessed lights: only IC-rated fixtures can be covered; use covers or keep clearance around others.
- Mark the depth with rulers stapled to joists so blown insulation is level.
- Look for rebates: many utilities and states offer rebates for attic insulation and air sealing — check before you buy.
Blown insulation can usually be installed with a machine lent free by the home improvement store when you buy enough bags — one person feeds the hopper while the other works in the attic.