Insulation
Blown-In Insulation Calculator
Works out blown insulation, where coverage is per bag at a stated depth and settling is the thing to plan for.
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 comes from the R target divided by R per inch, and the bag count then comes from the manufacturer's coverage chart, which states the area one bag covers at a given installed thickness. That chart is the figure to use rather than a volume calculation, because it accounts for the material's installed density. Blown material suits open attics particularly well, since it flows around obstructions and fills irregular spaces that batts leave gapped.
What moves the number
- Installed depth required, which drives bags through the coverage chart.
- Attic area, measured rather than taken from the house footprint, which includes the garage.
- Whether the product is fibreglass or cellulose, since coverage and settling differ.
- Obstructions and access, which affect labour more than material.
Where this one goes wrong
Install to the manufacturer's stated depth and bag count rather than to whichever runs out first, because the two are how coverage is actually verified. Blowing fewer bags over the right depth gives lower density and less R than the chart says, and it is invisible once the attic is full. Depth markers through the attic are the standard way to show it was done. Settling is the related issue: loose fill settles somewhat after installation, which the coverage chart accounts for by distinguishing initial and settled thickness, so installing to settled depth means blowing deeper initially. And baffles still have to keep it out of the soffit vents.
Before you quote
- Use the manufacturer's coverage chart rather than a volume estimate.
- Install depth markers so the installed depth can be verified.
- Blow to the initial depth that gives the settled depth you need.
- Fit baffles before blowing to keep material out of the vents.
Where the rules come from
Questions
About this calculator
How many bags of blown insulation do I need?
From the manufacturer's coverage chart, which states area covered per bag at a given installed thickness. That accounts for installed density in a way a volume calculation does not. Measure the actual attic area rather than using the house footprint, which usually includes spaces you are not insulating.
Does blown insulation settle?
Loose fill settles somewhat after installation, which is why coverage charts distinguish initial from settled thickness. Installing to the settled depth you want means blowing deeper to begin with. A job blown to the final depth on day one ends up below target.
How do I know enough material was installed?
Depth markers through the attic and the bag count against the coverage chart. Those two together are how it is verified, because once the attic is covered you cannot tell density by looking. An installer using fewer bags over the right depth has given you less R than specified.
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 $1.3.
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 | $1.3 |
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 | $1,560 |
| Bags if blown | 52 |
You need 13.2 inches to add R-49.0 across 1,200.0 square feet, roughly $1,560 in material.
How sensitive the answer is
Holding everything else steady and changing only cost per square foot at that depth, this is what happens to thickness needed. It is worth knowing which of your inputs the answer actually hinges on, and which barely move it.
| Cost per square foot at that depth ($) | Thickness needed |
|---|---|
| $0.65 | 13.2 |
| $0.98 | 13.2 |
| $1.3 example above | 13.2 |
| $1.95 | 13.2 |
| $2.6 | 13.2 |
Every other input is held at the example value while this one changes, which is not how a real job behaves. Treat it as a guide to which number matters most, not as five separate quotes.
What each figure means
- Length (ft) (ft)
- Measured in ft, starting at 40 ft.
- Width (ft) (ft)
- Measured in ft, starting at 30 ft.
- Target R-value
- Choose from R-13, walls, mild, R-19, walls or floors, R-30, attic, mild climate, R-38, attic, R-49, attic, cold climate, R-60, attic, very cold.
- Material
- Choose from Blown fibreglass (R-2.5 per inch), Blown cellulose (R-3.7 per inch), Fibreglass batt (R-3.2 per inch), Rigid foam board (R-5 per inch), Open cell spray foam (R-3.7 per inch), Closed cell spray foam (R-6.5 per inch).
- Existing R-value in place
- A number, between 0 and 80, starting at 0.
- Cost per square foot at that depth ($)
- A money amount, starting at $1.3.
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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