Insulation
Insulation Quantity Calculator
Totals the material for a job, where the waste and the framing deduction are what estimates miss.
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
Quantity is the area to be insulated converted into bags or batts through the coverage figure for the material and installed thickness. Two adjustments make it a usable number rather than a theoretical one. In a cavity application the framing occupies part of the gross area, so the net insulated area is lower, which matters for ordering and matters again for claiming whole-assembly performance. And cutting around obstructions generates waste, which is modest on an open attic blow and significant on batt work in a framed wall with many penetrations.
What moves the number
- Gross area against net insulated area once framing is deducted.
- Waste from cutting, which varies greatly between an open blow and detailed batt work.
- Whether partial bags are usable on the next job or are effectively waste.
- Access, which affects whether material can be delivered where it is needed.
Where this one goes wrong
Order a little over rather than exactly, because running short mid-job on insulation is disproportionately disruptive: a blowing machine set up and a crew on site idle while somebody drives to a supplier costs more than the spare bags. Count the framing honestly in the other direction though, since a wall is not all cavity. The thing most often omitted entirely is the small stuff that makes the job work: baffles, air sealing materials, fasteners, netting for dense pack, and support wire for floors. Those are cheap individually and all of them are needed, and a material list without them means a second trip.
Before you quote
- Deduct framing to get net insulated area for cavity work.
- Add a waste allowance matched to how much cutting the job needs.
- Include baffles, air sealing materials, netting and fasteners in the list.
Where the rules come from
Questions
About this calculator
How much extra insulation should I order?
Enough that you do not stop. Running short with a crew and a blowing machine on site costs far more in idle time than a few spare bags, and loose fill keeps. The place to be precise is the framing deduction on cavity work, where a wall is not all cavity.
Do I subtract the framing from the area?
For cavity work, yes, since studs and plates occupy area that takes no insulation. It affects both how much you order and what the assembly actually achieves, because the framing conducts heat around the insulation. For an open attic blow over a floor, there is no equivalent deduction.
What gets left off insulation material lists?
The accessories: soffit baffles, air sealing caulk and foam, netting for dense pack, support wire for floor bays, and fasteners. Each is inexpensive and each is necessary, and a list with only insulation on it means a second trip to the supplier partway through the job.
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 28 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 $1.35.
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) | 28 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 | $1.35 |
What comes out
| Result | Value |
|---|---|
| Thickness needed main result | 13.2 |
| Area | 1,120.0 |
| R-value still to add | 49.0 |
| Estimated material cost | $1,512 |
| Bags if blown | 49 |
You need 13.2 inches to add R-49.0 across 1,120.0 square feet, roughly $1,512 in material.
How sensitive the answer is
Holding everything else steady and changing only cost per square foot at that depth, this is what happens to thickness needed. It is worth knowing which of your inputs the answer actually hinges on, and which barely move it.
| Cost per square foot at that depth ($) | Thickness needed |
|---|---|
| $0.68 | 13.2 |
| $1.01 | 13.2 |
| $1.35 example above | 13.2 |
| $2.03 | 13.2 |
| $2.7 | 13.2 |
Every other input is held at the example value while this one changes, which is not how a real job behaves. Treat it as a guide to which number matters most, not as five separate quotes.
What each figure means
- Length (ft) (ft)
- Measured in ft, starting at 40 ft.
- Width (ft) (ft)
- Measured in ft, starting at 28 ft.
- Target R-value
- Choose from R-13, walls, mild, R-19, walls or floors, R-30, attic, mild climate, R-38, attic, R-49, attic, cold climate, R-60, attic, very cold.
- Material
- Choose from Blown fibreglass (R-2.5 per inch), Blown cellulose (R-3.7 per inch), Fibreglass batt (R-3.2 per inch), Rigid foam board (R-5 per inch), Open cell spray foam (R-3.7 per inch), Closed cell spray foam (R-6.5 per inch).
- Existing R-value in place
- A number, between 0 and 80, starting at 0.
- Cost per square foot at that depth ($)
- A money amount, starting at $1.35.
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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