Quantity · Attic insulation
Attic Insulation Calculator
Start with a ruler on the attic floor — not a guess at R. Convert existing depth and material to approximate R, pick a US state tip for target R, then size inches and bags for a blown or batt top-up. Bag-only charts live on the blown-in calculator; full job dollars on the attic cost calculator.
Attic measurements
Ruler from deck to top of fill. Faced or unfaced batts between joists. ≈ R-3.2/in → about R-19 today.
Zone 5 planning attic ≈ R-49. Sets target when you change state — override if code differs.
Common attic top-up over existing fill.
Inches to add
8.2in
From ~R-19 → R-49 · Blown cellulose (top-up)
- Bags to buy
- 40 bags
- Final depth
- 14.2 in
- R to add
- R-30
- Rough installed
- $1,200–$2,300
~26 sq ft/bag at R-30 settled (open-attic chart)
- Measure depth × R/inch ≈ existing R (19)
- R gap = target − existing
- Inches and bags/packs from the top-up product chart
Price data updated . National US ranges from published supplier and contractor averages. Not a bid. Local quotes vary.
How attic insulation quantity is calculated
Existing R comes from measured depth × typical R per inch for the material you already have. The R gap to your target drives settled inches and bags for the top-up product. Final depth is existing inches plus inches added.
r_existing = depth_in × r_per_inch (existing material) r_needed = max(0, r_target − r_existing) inches_add = chart_or_r_per_inch(r_needed) bags/packs = ceil(area ÷ coverage(r_needed) × (1 + waste %)) final_depth = depth_in + inches_add
Worked example. A 1,000 sq ft Illinois attic with 6" of fiberglass batts (~R-19) targeting R-49 needs about R-30 more. Blown cellulose adds roughly 8.3 in settled (~38–39 bags before overblow, ~41 at 5% waste). Final depth lands near 14.3 in — check joist depth and baffles before you blow over the top plates.
Attic ruler chart — depth to approximate R
Measure from the attic deck to the top of the existing fill. Multiply by the R/inch for your material, or use this planning chart. Compressed batts underperform — use the lower end if fill looks flattened.
| Depth | Batt ≈ R | Blown FG ≈ R | Cellulose ≈ R | Not sure ≈ R |
|---|---|---|---|---|
| 3.5" | 11 | 9 | 12 | 10 |
| 5.5" | 18 | 14 | 19 | 15 |
| 7" | 22 | 18 | 25 | 20 |
| 9" | 29 | 23 | 32 | 25 |
| 10.5" | 34 | 26 | 37 | 29 |
| 13" | 42 | 33 | 46 | 36 |
| 15" | 48 | 38 | 53 | 42 |
| 17" | 54 | 43 | 60 | 48 |
| 20" | 64 | 50 | 70 | 56 |
- Assumes ~3.2 R/in batts, 2.5 blown fiberglass, 3.5 cellulose, 2.8 unknown/mixed.
- Manufacturer labels beat any website average — especially after settling.
Floor joist depth vs batt fit
Batts should match joist depth without compression. Deeper climate targets (R-49+) usually mean blown fill above the joists — keep soffit baffles clear.
| Joist | Cavity depth | Typical batt R | Notes |
|---|---|---|---|
| 2×6 | 5.5" | R-19 | R-38+ needs blown over joists |
| 2×8 | 7.25" | R-25 | Still short of many R-49 tips |
| 2×10 | 9.25" | R-30 | Top-up blown common for R-49 |
| 2×12 | 11.25" | R-38 | R-49–R-60 often still needs inches on top |
Attic Insulation calculator FAQ
- How much attic insulation do I need?
- Measure existing depth, convert to R with the ruler chart, then compare to your climate target (often R-38 to R-60). This calculator returns the R gap, inches to add, bags or batt packs, and a rough installed band for the top-up.
- How deep should attic insulation be?
- Depth follows the product and target R. Blown cellulose near R-49 is often around 13 in settled on a full fill; fiberglass needs more inches for the same R. Use the ruler chart for what you have today, then add the inches the calculator shows.
- Can I add blown insulation over old batts?
- Usually yes if the batts are dry and not blocking eave vents. Enter batt depth as existing, choose a blown top-up, and keep baffles open at the soffits.
- Why not just enter R-value like the general calculator?
- Most homeowners know inches from a tape measure, not labeled R. This page is ruler-first. If you already know existing R, the general insulation calculator is faster.
- Where do I get bag charts or a full cost breakdown?
- Open-attic fiberglass vs cellulose bag charts are on the blown-in calculator. For air sealing, baffles, removal, and access uplift dollars, use the attic insulation cost calculator.
- Is the dollar range a bid?
- No — it is a rough installed band for the top-up product only. Job add-ons live on the attic cost page.
Related tools
Attic insulation cost calculator · Blown-in bag calculator · Insulation calculator (R in / all products) · All insulation calculators