Insulation
Basement Insulation Calculator
Works out basement insulation, where getting the moisture direction wrong is worse than no insulation.
Result
Basement walls are below grade and damp. Rigid or closed cell foam against the concrete keeps warm indoor air away from the cold surface where it would otherwise condense. Fibreglass batts directly against a foundation wall are a recipe for mould. 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 is the target R less existing divided by R per inch, applied to the basement walls where the basement is or will be conditioned. The arithmetic is the easy part. A basement wall is concrete or block in contact with soil, so it is cool and it transmits moisture, and the insulation strategy has to let that assembly dry. That is why the material choice here is driven by moisture tolerance rather than by R per inch.
What moves the number
- Whether the basement has a water problem, which has to be fixed before insulating.
- Material moisture tolerance, since fibrous insulation against damp concrete is a mould risk.
- Whether the insulation is continuous on the wall or in a framed stud wall inside it.
- Code requirements for ignition or thermal barriers over foam in a basement.
Where this one goes wrong
Fix water before insulating. A basement that takes water, even occasionally, will put that water behind whatever you install, and fibreglass batts against damp concrete with a polyethylene sheet over them is a reliable way to grow mould in a wall nobody can see. Rigid or spray foam directly against the concrete is the common approach because it is not damaged by moisture and it keeps the concrete surface above the dew point, which is what stops condensation forming. Avoid creating a cavity that cannot dry in either direction, and check the ignition barrier requirement for exposed foam.
Before you quote
- Resolve any water entry before insulating.
- Use a moisture tolerant material against the concrete.
- Avoid trapping a cavity between two vapour-impermeable layers.
- Confirm the ignition or thermal barrier requirement over foam.
Where the rules come from
Questions
About this calculator
Can I use fibreglass batts in a basement?
It is the approach most likely to cause problems. Basement walls are cool and transmit moisture, and fibrous insulation held against damp concrete, especially behind a vapour barrier, is a known mould risk. Foam against the concrete is the usual recommendation because it tolerates moisture and keeps the surface above the dew point.
Do I need to fix dampness before insulating?
Yes, and it is not optional. Insulating over a wall that takes water puts the water behind a surface you can no longer see or dry. Any insulation work on a basement should follow drainage, grading and waterproofing, not precede them.
Why is a vapour barrier risky in a basement?
Because it can trap moisture rather than keep it out. A basement wall needs to be able to dry, and a sheet of polyethylene on the inside face stops drying inward while the soil side cannot dry outward. The result is a cavity that stays wet. Assemblies that let the wall dry in one direction are safer.
The formula
thickness = (target R - existing R) / R-value per inch
Put this calculator on your site
Free to embed. Paste this where you want it to appear. It stays up to date automatically because it loads from BDEVY.
Please keep the attribution line. It is the only thing we ask in return.
How to use this calculator
- Length (ft) (ft). Enter your own figure; the page starts at 120 ft.
- Width (ft) (ft). Enter your own figure; the page starts at 8 ft.
- Target R-value. Enter your own figure; the page starts at R-13, walls, mild.
- 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 $2.1.
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) | 120 ft |
| Width (ft) | 8 ft |
| Target R-value | R-13, walls, mild |
| Material | Rigid foam board (R-5 per inch) |
| Existing R-value in place | 0 |
| Cost per square foot at that depth | $2.1 |
What comes out
| Result | Value |
|---|---|
| Thickness needed main result | 2.6 |
| Area | 960.0 |
| R-value still to add | 13.0 |
| Estimated material cost | $2,016 |
| Bags if blown | 11 |
You need 2.6 inches to add R-13.0 across 960.0 square feet, roughly $2,016 in material.
How sensitive the answer is
Holding everything else steady and changing only cost per square foot at that depth, this is what happens to thickness needed. It is worth knowing which of your inputs the answer actually hinges on, and which barely move it.
| Cost per square foot at that depth ($) | Thickness needed |
|---|---|
| $1.05 | 2.6 |
| $1.58 | 2.6 |
| $2.1 example above | 2.6 |
| $3.15 | 2.6 |
| $4.2 | 2.6 |
Every other input is held at the example value while this one changes, which is not how a real job behaves. Treat it as a guide to which number matters most, not as five separate quotes.
What each figure means
- Length (ft) (ft)
- Measured in ft, starting at 120 ft.
- Width (ft) (ft)
- Measured in ft, starting at 8 ft.
- Target R-value
- Choose from R-13, walls, mild, R-19, walls or floors, R-30, attic, mild climate, R-38, attic, R-49, attic, cold climate, R-60, attic, very cold.
- Material
- Choose from Blown fibreglass (R-2.5 per inch), Blown cellulose (R-3.7 per inch), Fibreglass batt (R-3.2 per inch), Rigid foam board (R-5 per inch), Open cell spray foam (R-3.7 per inch), Closed cell spray foam (R-6.5 per inch).
- Existing R-value in place
- A number, between 0 and 80, starting at 0.
- Cost per square foot at that depth ($)
- A money amount, starting at $2.1.
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
Related calculators
Estimating is the easy part
The jobs you never hear about cost more than the ones you mis-measure. BDEVY builds the marketing, systems and automation that bring them in.