Insulation
Floor Insulation Calculator
Works out floor insulation, where keeping it in contact with the subfloor is the whole job.
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 is the target R less existing divided by R per inch, applied to the floor assembly over an unconditioned space such as a crawl space, an unheated basement or a garage below. The calculation is straightforward; the installation is where floors differ. Insulation in a floor is fighting gravity, so it has to be held up against the subfloor, and material that sags away leaves an air gap that defeats much of its value even though the insulation is still physically present.
What moves the number
- Joist depth, which caps cavity thickness as it does in a wall.
- How the insulation is supported, since anything that sags loses much of its effect.
- Whether pipes and ducts run in the joist bays, which complicates the fill and raises freezing risk.
- Whether the space below is vented or sealed, which changes the right approach entirely.
Where this one goes wrong
Insulation has to touch the subfloor to work. Batts held up by friction alone sag over time, and a gap between the insulation and the floor above lets air circulate across the underside of the floor, which is most of what you were trying to stop. Support it properly with wire, strapping or netting rather than relying on the fit. Watch pipes too: insulating below a water pipe puts it on the cold side of the insulation, which is how pipes freeze in a crawl space that was previously marginal. Pipes should be kept on the warm side, which sometimes means insulating the crawl space walls instead of the floor.
Before you quote
- Support the insulation in contact with the subfloor, not by friction.
- Keep water pipes on the warm side of the insulation.
- Decide whether insulating the floor or the perimeter walls is the right approach.
Where the rules come from
Questions
About this calculator
Why does floor insulation need supporting?
Because gravity pulls it away from the subfloor. Batts held by friction sag over time, and the air gap that opens up lets air move across the underside of the floor, which defeats much of the insulation even though it is still in the bay. Wire, strapping or netting keeps it in contact.
Will insulating my floor freeze the pipes?
It can, and this is a real risk. Insulating below pipes in a crawl space puts them on the cold side, in a space that is now colder because the heat from above is no longer reaching it. Pipes need to stay on the warm side, which sometimes means insulating the crawl space walls instead.
Should I insulate the floor or the crawl space walls?
It depends on whether the crawl space is vented or sealed. A sealed, conditioned crawl space is better insulated at its perimeter walls, which keeps pipes and ducts inside the thermal envelope. A vented crawl space is normally insulated at the floor above. Mixing the two approaches causes problems.
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-30, attic, mild climate.
- Material. Enter your own figure; the page starts at Fibreglass batt (R-3.2 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) | 28 ft |
| Target R-value | R-30, attic, mild climate |
| Material | Fibreglass batt (R-3.2 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 | 9.4 |
| Area | 1,120.0 |
| R-value still to add | 30.0 |
| Estimated material cost | $1,400 |
| Bags if blown | 30 |
You need 9.4 inches to add R-30.0 across 1,120.0 square feet, roughly $1,400 in material.
How sensitive the answer is
Holding everything else steady and changing only cost per square foot at that depth, this is what happens to thickness needed. It is worth knowing which of your inputs the answer actually hinges on, and which barely move it.
| Cost per square foot at that depth ($) | Thickness needed |
|---|---|
| $0.63 | 9.4 |
| $0.94 | 9.4 |
| $1.25 example above | 9.4 |
| $1.88 | 9.4 |
| $2.5 | 9.4 |
Every other input is held at the example value while this one changes, which is not how a real job behaves. Treat it as a guide to which number matters most, not as five separate quotes.
What each figure means
- Length (ft) (ft)
- Measured in ft, starting at 40 ft.
- Width (ft) (ft)
- Measured in ft, starting at 28 ft.
- Target R-value
- Choose from R-13, walls, mild, R-19, walls or floors, R-30, attic, mild climate, R-38, attic, R-49, attic, cold climate, R-60, attic, very cold.
- Material
- Choose from Blown fibreglass (R-2.5 per inch), Blown cellulose (R-3.7 per inch), Fibreglass batt (R-3.2 per inch), Rigid foam board (R-5 per inch), Open cell spray foam (R-3.7 per inch), Closed cell spray foam (R-6.5 per inch).
- Existing R-value in place
- A number, between 0 and 80, starting at 0.
- Cost per square foot at that depth ($)
- A money amount, starting at $1.25.
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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