Insulation
Cellulose Insulation Calculator
Works out cellulose, which fills irregular spaces well and has to be installed at the right density.
Result
Required R-values are set by climate zone and by local energy code, and they differ for attics, walls and floors. The figures offered here are common code minimums rather than recommendations, and a minimum is a floor, not a target. Air sealing comes first: insulation slows heat conduction but does nothing about air leaking through gaps, and an uninsulated but sealed attic often outperforms an insulated leaky one.
Get a Free Business AuditHow the calculation works
Thickness from R target divided by R per inch, then bags from the manufacturer's coverage chart at the installed thickness. Cellulose is recycled paper treated with borate for fire and pest resistance, blown loose in attics or dense packed into closed cavities. Its strength is conformity: it flows into irregular spaces, around wiring and into gaps that batts bridge, so the installed result in a messy assembly is often better than a nominally higher rated batt poorly fitted.
What moves the number
- Installed density, which for dense pack is the whole specification.
- Whether it is loose blown or dense packed, since the applications and densities differ.
- Settling allowance on an open blow, handled through the coverage chart.
- Moisture exposure, because cellulose absorbs and holds water if the assembly leaks.
Where this one goes wrong
Density is the specification on dense pack and it is the thing that goes wrong. Under-dense cellulose in a closed wall settles, leaving a void at the top of the cavity that you cannot see and that undoes a meaningful share of the work. Installers working to a stated pounds per cubic foot and verifying by bag count against area are doing it correctly; blowing until material stops going in is not the same thing. On moisture, cellulose handles incidental humidity well and does not handle liquid water, so a roof or plumbing leak into cellulose is a bigger problem than into fibreglass, and it needs finding and drying rather than ignoring.
Before you quote
- Specify and verify density on any dense pack work, by bag count against area.
- Use the coverage chart's settled thickness for open blows.
- Fix any water leak before filling a cavity with cellulose.
Where the rules come from
Questions
About this calculator
Is cellulose better than fibreglass?
It conforms to irregular spaces and fills around obstructions better, so the installed result in a messy assembly is often superior even at similar rated values. Fibreglass handles moisture exposure better. Neither is universally better; the installation and the assembly decide it.
What does dense packed mean?
Blowing cellulose into a closed cavity at a specified density, measured in pounds per cubic foot, so that it cannot settle. The density is the specification, and it is verified by bag count against the area filled. Blowing until material stops going in is not a density specification.
Is cellulose a fire risk?
It is treated with borate for fire and pest resistance and is tested to the relevant standards. The practical caution is different: it is paper, and a water leak into it causes more trouble than into a mineral material, so leaks need resolving before and after installation.
The formula
thickness = (target R - existing R) / R-value per inch
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How to use this calculator
- Length (ft) (ft). Enter your own figure; the page starts at 40 ft.
- Width (ft) (ft). Enter your own figure; the page starts at 30 ft.
- Target R-value. Enter your own figure; the page starts at R-49, attic, cold climate.
- Material. Enter your own figure; the page starts at Blown cellulose (R-3.7 per inch).
- Existing R-value in place. Enter your own figure; the page starts at 0.
- Cost per square foot at that depth ($). Enter your own figure; the page starts at $1.25.
The result updates as you type, so you can try a figure, see what it does to thickness needed, and change it back. Nothing you enter is sent anywhere or saved.
A worked example
These are the figures the calculator opens with. Every number in the results column below is produced by the same calculation the page runs, so you can follow it through before putting your own figures in.
What goes in
| Input | Value |
|---|---|
| Length (ft) | 40 ft |
| Width (ft) | 30 ft |
| Target R-value | R-49, attic, cold climate |
| Material | Blown cellulose (R-3.7 per inch) |
| Existing R-value in place | 0 |
| Cost per square foot at that depth | $1.25 |
What comes out
| Result | Value |
|---|---|
| Thickness needed main result | 13.2 |
| Area | 1,200.0 |
| R-value still to add | 49.0 |
| Estimated material cost | $1,500 |
| Bags if blown | 52 |
You need 13.2 inches to add R-49.0 across 1,200.0 square feet, roughly $1,500 in material.
How sensitive the answer is
Holding everything else steady and changing only cost per square foot at that depth, this is what happens to thickness needed. It is worth knowing which of your inputs the answer actually hinges on, and which barely move it.
| Cost per square foot at that depth ($) | Thickness needed |
|---|---|
| $0.63 | 13.2 |
| $0.94 | 13.2 |
| $1.25 example above | 13.2 |
| $1.88 | 13.2 |
| $2.5 | 13.2 |
Every other input is held at the example value while this one changes, which is not how a real job behaves. Treat it as a guide to which number matters most, not as five separate quotes.
What each figure means
- Length (ft) (ft)
- Measured in ft, starting at 40 ft.
- Width (ft) (ft)
- Measured in ft, starting at 30 ft.
- Target R-value
- Choose from R-13, walls, mild, R-19, walls or floors, R-30, attic, mild climate, R-38, attic, R-49, attic, cold climate, R-60, attic, very cold.
- Material
- Choose from Blown fibreglass (R-2.5 per inch), Blown cellulose (R-3.7 per inch), Fibreglass batt (R-3.2 per inch), Rigid foam board (R-5 per inch), Open cell spray foam (R-3.7 per inch), Closed cell spray foam (R-6.5 per inch).
- Existing R-value in place
- A number, between 0 and 80, starting at 0.
- Cost per square foot at that depth ($)
- A money amount, starting at $1.25.
What the results tell you
| Result | What it is |
|---|---|
| Thickness needed | The headline result. A decimal figure. |
| Area | A decimal figure. |
| R-value still to add | A decimal figure. |
| Estimated material cost | A money figure. |
| Bags if blown | A number. |
Keep going
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