Insulation Calculator
Inches of insulation to get from your existing R-value to a target, with bags, batts or sheets and cost — using DOE R-per-inch figures for ten materials.
Insulation Calculator: with the default inputs, thickness to add is 8.6 in.
Attic floor, wall area or crawlspace ceiling — the flat area, not the roof slope.
0 for bare framing. Roughly R-2.5 per inch of old settled fiberglass, R-3.2 per inch of cellulose.
DOE recommends R-49 to R-60 for attics in most of the US, R-30 to R-49 in the far south.
Depth of new insulation needed on top of what's already there.
- R-value added
- 30
- RSI added (metric)
- 5.28R-value ÷ 5.678, the metric m²·K/W figure used outside the US.
- Packages to buy
- 103
- Sold as
- bags of loose-fill
- R per inch used
- 3.5
- Material volume
- 857.1 ft³
- Material cost
- $0.00
Assumptions
- R-per-inch values are DOE published figures for the material alone, at standard test conditions; framing, gaps and air leakage reduce whole-assembly performance.
- R-values are US units (ft²·°F·h/Btu). RSI = R ÷ 5.678.
- Package counts use the coverage figures under More options — check them against the bag or bundle you are actually buying.
- Loose-fill is counted at installed depth; cellulose settles a few percent after blowing.
| Climate zone | Roughly where | Attic target | Inches of cellulose, blown / loose-fill from bare |
|---|---|---|---|
| 1 | Hawaii, Guam, south Florida and south Texas | R-30 to R-49 | 8.6 |
| 2 | Gulf coast, southern California, Arizona | R-30 to R-60 | 8.6 |
| 3 | Carolinas, Tennessee, coastal Pacific | R-30 to R-60 | 8.6 |
| 4 | Mid-Atlantic, Midwest, Pacific Northwest | R-38 to R-60 | 10.9 |
| 5 | New England, upper Midwest, Rockies | R-49 to R-60 | 14 |
| 6 | Northern Minnesota, Maine, mountain west | R-49 to R-60 | 14 |
| 7–8 | Interior and northern Alaska | R-49 to R-60 | 14 |
Zones follow the IECC climate map; your county's zone is on the DOE map linked below. The last column is the depth to reach the bottom of each range starting from bare framing — you only need the difference from what is already up there.
| Material | R per inch (used here) | Published range | Inches for R-30 |
|---|---|---|---|
| Fiberglass batt | 3.2 | R-3.1 to R-3.4 per inch | 9.4 |
| Mineral wool (rock wool) batt | 3.3 | R-3.0 to R-3.3 per inch | 9.1 |
| Fiberglass, blown / loose-fill | 2.5 | R-2.2 to R-2.7 per inch | 12 |
| Cellulose, blown / loose-fill | 3.5 | R-3.2 to R-3.8 per inch | 8.6 |
| Mineral wool, blown | 3.3 | R-3.0 to R-3.3 per inch | 9.1 |
| Spray foam, open cell | 3.6 | R-3.5 to R-3.6 per inch | 8.3 |
| Spray foam, closed cell | 6.3 | R-6.0 to R-6.5 per inch | 4.8 |
| Rigid board, EPS | 4 | R-3.6 to R-4.2 per inch | 7.5 |
| Rigid board, XPS | 5 | R-4.5 to R-5.0 per inch | 6 |
| Rigid board, polyisocyanurate | 6 | R-5.6 to R-8.0 per inch | 5 |
R-per-inch is a laboratory figure at a standard mean temperature. Real assemblies lose ground to thermal bridging through framing, gaps and air movement.
How this is worked out
The formula
R to add = target R − existing R Thickness = R to add ÷ R per inch Loose-fill bags = (area × inches) ÷ bag coverage in ft²·in Batts = area ÷ area per batt Rigid sheets = ⌈area ÷ 32 ft²⌉ × ⌈inches ÷ board thickness⌉ Spray foam board-feet = area × inches RSI = R ÷ 5.678
Open How it’s calculated above to see this worked through with your own numbers.
What you enter
- Area to insulate
- Attic floor, wall area or crawlspace ceiling — the flat area, not the roof slope.in ft², m², yd² · 0 or more · defaults to 1200
- Existing R-value
- 0 for bare framing. Roughly R-2.5 per inch of old settled fiberglass, R-3.2 per inch of cellulose.from 0 to 120 · defaults to 19
- Target R-value
- DOE recommends R-49 to R-60 for attics in most of the US, R-30 to R-49 in the far south.from 1 to 150 · defaults to 49
- Insulation type
- Choose one of 10 options.Fiberglass batt — R-3.1 to R-3.4 per inch · Mineral wool (rock wool) batt — R-3.0 to R-3.3 per inch · Fiberglass, blown / loose-fill — R-2.2 to R-2.7 per inch · Cellulose, blown / loose-fill — R-3.2 to R-3.8 per inch · Mineral wool, blown — R-3.0 to R-3.3 per inch · Spray foam, open cell — R-3.5 to R-3.6 per inch · Spray foam, closed cell — R-6.0 to R-6.5 per inch · Rigid board, EPS — R-3.6 to R-4.2 per inch · Rigid board, XPS — R-4.5 to R-5.0 per inch · Rigid board, polyisocyanurate — R-5.6 to R-8.0 per inch
- Override R per inch(under More options)
- Leave 0 to use the published figure for the type above. Use the number on the bag if you have it.from 0 to 12 · defaults to 0
- Bag coverage(under More options)
- From the bag's coverage chart: a bag that covers 28 ft² at 3.5 in is 98 ft² at 1 in.1 or more · defaults to 100
- Area per batt(under More options)
- A 15 in × 93 in batt for 16-in-on-centre framing is 9.7 ft²; a 23 in × 93 in batt for 24-in centres is 14.9 ft².0.5 or more · defaults to 9.7
- Rigid board thickness(under More options)
- Sheets are 4 × 8 ft = 32 ft²; thicker boards mean fewer layers.in in, mm, cm · 0.25 or more · defaults to 2
- Price per package(under More options)
- Per bag, per batt, per sheet or per board-foot of spray foam. Leave 0 to skip.in dollars · 0 or more · defaults to 0
What you get back
- Thickness to addmain answer
- Depth of new insulation needed on top of what's already there.
- R-value added
- RSI added (metric)
- R-value ÷ 5.678, the metric m²·K/W figure used outside the US.
- Packages to buy
- Sold as
- R per inch used
- Material volume
- Material cost
What this assumes
- R-per-inch values are DOE published figures for the material alone, at standard test conditions; framing, gaps and air leakage reduce whole-assembly performance.
- R-values are US units (ft²·°F·h/Btu). RSI = R ÷ 5.678.
- Package counts use the coverage figures under More options — check them against the bag or bundle you are actually buying.
- Loose-fill is counted at installed depth; cellulose settles a few percent after blowing.
About this calculator
Insulation is sold by thickness and rated by R-value, and the only number that matters is R added on top of whatever is already up there. This calculator takes your existing R, your target R and the material, and gives you the depth and the package count.
Getting the existing R-value right
Go into the attic with a ruler. Old settled fiberglass runs about R-2.5 per inch, cellulose about R-3.2, and anything visibly compressed or wet is worth less than that. Twelve inches of tired fiberglass is roughly R-30, not the R-38 it was installed at. If you can see the tops of the joists, you have less than R-25.
The US Department of Energy recommends R-49 to R-60 in attics across most of the country, dropping to R-30–R-49 in the far south. Walls are limited by cavity depth: a 2×4 wall tops out near R-15 with high-density batts, and a 2×6 wall near R-21.
The materials, honestly
- Blown cellulose (R-3.5/in) is the best value in an open attic, packs into odd shapes, and is what most retrofits use. It settles a little; blow to the depth marked on the ruler cards, not the depth you want after settling.
- Blown fiberglass (R-2.5/in) is lighter and doesn't settle much, but needs 40% more depth for the same R.
- Batts (R-3.2/in) are for open, regular framing you can reach. Over existing loose-fill they bridge and lose most of their value.
- Closed-cell spray foam (R-6.3/in) is the highest R per inch and doubles as an air and vapour barrier, but it costs several times as much per R and belongs where depth is the constraint — rim joists, cathedral ceilings, van conversions.
- Rigid board goes on the outside of sheathing or under a slab, where continuous insulation defeats thermal bridging.
Reading the results
Thickness to add is the depth of new material. Packages uses the coverage figures under More options — check them against the actual bag or bundle, because they vary by product more than by brand. The table shows what every other material would need for the same R, which is the fastest way to see the depth-versus-cost trade.
One caveat that outweighs all of this: air sealing first. A leaky attic plane with R-60 over it performs worse than a sealed one with R-38. Seal top plates, chases and can lights before you add a single inch.
Frequently asked questions
▸How many inches of insulation do I need for R-49?
From bare joists: 14 in of blown cellulose (R-3.5/in), 20 in of blown fiberglass (R-2.5/in), 15 in of fiberglass batt, or 8 in of closed-cell spray foam. From an existing R-19 you only need the difference — about 8½ in of cellulose.
▸What R-value should my attic be?
The DOE recommends R-49 to R-60 for attics in most US climate zones, and R-30 to R-49 in the hottest southern zones. Check your zone on the energy.gov insulation page before spending.
▸How many bags of cellulose do I need?
Multiply the area by the inches you're adding to get ft²·inches, then divide by the bag's coverage. A typical bag covers about 100 ft² at 1 in, so 1,200 ft² at 8.6 in is around 103 bags.
▸Can I put batts over existing blown insulation?
You can, but you usually shouldn't. Batts need to sit flat on a clean surface; laid over lumpy loose-fill they bridge and trap air gaps, and any facing on the underside becomes a second vapour retarder. Blow more loose-fill instead.
▸Does R-value per inch tell the whole story?
No. It's a lab figure for the material alone. Real walls lose 15–25% to thermal bridging through studs, and any assembly loses far more to air leakage than to conduction. Seal first, insulate second.
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