Insulation
Crawl Space Insulation Calculator
Works out crawl space insulation, where the decision to seal or vent comes before any R-value.
Result
Where the insulation goes depends on whether the crawl space is vented or sealed. A vented crawl space is insulated at the floor above; a sealed and conditioned one is insulated at the perimeter walls with a ground vapour barrier. Getting this the wrong way round traps moisture and causes rot. 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.
Get a Free Business AuditHow the calculation works
Thickness follows the same arithmetic, but a crawl space has two quite different strategies and the calculation applies to whichever you choose. In a vented crawl space the insulation goes in the floor above and the space stays outside the thermal envelope. In a sealed, conditioned crawl space the insulation goes on the perimeter walls, a ground vapour barrier is installed, the vents are closed and the space is brought inside the envelope. The second is now widely preferred in most climates and it is a different scope rather than a variation.
What moves the number
- Whether the space is to be sealed or stay vented, which decides where insulation goes.
- Ground moisture, since a sealed crawl space needs a continuous vapour barrier over the soil.
- Whether ducts and pipes run in the space, which argues strongly for sealing it.
- Local code requirements for sealed crawl spaces, which commonly specify conditioning or dehumidification.
Where this one goes wrong
Do not half seal a crawl space. Closing the vents without installing a ground vapour barrier and providing some conditioning or dehumidification creates a damp, unventilated space, and that is how crawl spaces grow mould and rot joists. Either vent it properly and insulate the floor, or seal it properly with a ground barrier, insulated walls and a moisture strategy. Also be careful with foam in a crawl space: exposed foam plastic generally needs an ignition barrier or thermal barrier under the adopted code, and that requirement is routinely missed on retrofit work.
Before you quote
- Decide vented or sealed before specifying anything.
- Install a continuous ground vapour barrier in a sealed crawl space.
- Provide conditioning or dehumidification if the vents are closed.
- Check the ignition barrier requirement for any exposed foam.
Where the rules come from
Questions
About this calculator
Should I seal or vent my crawl space?
Sealing and conditioning is now widely preferred in most climates, especially where ducts or pipes run through the space, because it brings them inside the thermal envelope. It has to be done properly though: a ground vapour barrier, insulated walls, closed vents and a moisture strategy together, not just closing the vents.
What happens if I close the vents without doing anything else?
You get a damp, unventilated space, which is how crawl spaces grow mould and rot framing. The vents were doing something, even if imperfectly. Closing them only makes sense as part of a sealed assembly with a ground barrier and a way of managing humidity.
Does foam insulation in a crawl space need covering?
Generally yes. Exposed foam plastic usually requires an ignition or thermal barrier under the adopted code, and it is a requirement that gets missed on retrofits. The specifics depend on the foam and the space, so it is worth confirming rather than leaving it exposed.
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 36 ft.
- Width (ft) (ft). Enter your own figure; the page starts at 26 ft.
- Target R-value. Enter your own figure; the page starts at R-19, walls or floors.
- Material. Enter your own figure; the page starts at Rigid foam board (R-5 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 $1.9.
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) | 36 ft |
| Width (ft) | 26 ft |
| Target R-value | R-19, walls or floors |
| Material | Rigid foam board (R-5 per inch) |
| Existing R-value in place | 0 |
| Cost per square foot at that depth | $1.9 |
What comes out
| Result | Value |
|---|---|
| Thickness needed main result | 3.8 |
| Area | 936.0 |
| R-value still to add | 19.0 |
| Estimated material cost | $1,778 |
| Bags if blown | 16 |
You need 3.8 inches to add R-19.0 across 936.0 square feet, roughly $1,778 in material.
How sensitive the answer is
Holding everything else steady and changing only cost per square foot at that depth, this is what happens to thickness needed. It is worth knowing which of your inputs the answer actually hinges on, and which barely move it.
| Cost per square foot at that depth ($) | Thickness needed |
|---|---|
| $0.95 | 3.8 |
| $1.42 | 3.8 |
| $1.9 example above | 3.8 |
| $2.85 | 3.8 |
| $3.8 | 3.8 |
Every other input is held at the example value while this one changes, which is not how a real job behaves. Treat it as a guide to which number matters most, not as five separate quotes.
What each figure means
- Length (ft) (ft)
- Measured in ft, starting at 36 ft.
- Width (ft) (ft)
- Measured in ft, starting at 26 ft.
- Target R-value
- Choose from R-13, walls, mild, R-19, walls or floors, R-30, attic, mild climate, R-38, attic, R-49, attic, cold climate, R-60, attic, very cold.
- Material
- Choose from Blown fibreglass (R-2.5 per inch), Blown cellulose (R-3.7 per inch), Fibreglass batt (R-3.2 per inch), Rigid foam board (R-5 per inch), Open cell spray foam (R-3.7 per inch), Closed cell spray foam (R-6.5 per inch).
- Existing R-value in place
- A number, between 0 and 80, starting at 0.
- Cost per square foot at that depth ($)
- A money amount, starting at $1.9.
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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