Insulation
Batt Insulation Calculator
Works out batt insulation, where the installation quality decides whether you get what you paid 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 from the R target divided by R per inch, then batts are bought to the cavity depth and stud spacing rather than cut to a thickness. Batts come sized for nominal framing, so the practical approach is choosing the batt rated for your cavity and checking what R that gives you against the target. The quantity is area divided by the coverage per bag, with an allowance for cutting around obstructions.
What moves the number
- Cavity depth and stud spacing, since batts are made to fit standard framing.
- How much cutting the framing requires, which is where gaps and compression creep in.
- Faced or unfaced, which depends on climate and whether a retarder already exists.
- Obstructions such as wiring and pipes, which have to be split around rather than compressed over.
Where this one goes wrong
Batts are the easiest insulation to install badly and the hardest to tell afterwards. Compressed behind a wire instead of split around it, stuffed into a cavity that is too narrow, left short at the top of a bay, or cut sloppily around an outlet box, each leaves gaps and compression that cut real performance well below the rating on the packet. The material is cheap and the installation is everything. On facing, do not add a second vapour retarder facing a different direction from an existing one, and in a climate where the retarder goes on the warm side, putting it on the wrong side is the classic way to condense moisture inside a wall.
Before you quote
- Split batts around wires and pipes rather than compressing over them.
- Cut neatly to fill the bay completely, including at the top.
- Use unfaced batts where a vapour retarder already exists.
- Confirm which side the vapour retarder belongs on for your climate.
Where the rules come from
Questions
About this calculator
Why do batts underperform their rating?
Because installation quality dominates. Compressed material, gaps at the ends of bays, batts stuffed around wiring instead of split around it, and sloppy cuts at boxes all reduce real performance well below the rated figure, and none of it is visible once the drywall goes on.
Should I use faced or unfaced batts?
Faced where a vapour retarder is needed and none exists, unfaced where one is already present. The thing to avoid is two retarders facing different directions, which traps moisture between them. Which side the retarder belongs on depends on climate, and getting it backwards causes condensation in the wall.
How do I insulate around wiring and pipes?
Split the batt and pass it behind the obstruction so material fills the cavity on both sides. Compressing the batt over a wire leaves a thin spot and an uninsulated area behind it. It takes a little longer and it is the difference between the rated performance and noticeably less.
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 100 ft.
- Width (ft) (ft). Enter your own figure; the page starts at 9 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 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 $0.95.
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) | 100 ft |
| Width (ft) | 9 ft |
| Target R-value | R-13, walls, mild |
| Material | Fibreglass batt (R-3.2 per inch) |
| Existing R-value in place | 0 |
| Cost per square foot at that depth | $0.95 |
What comes out
| Result | Value |
|---|---|
| Thickness needed main result | 4.1 |
| Area | 900.0 |
| R-value still to add | 13.0 |
| Estimated material cost | $855 |
| Bags if blown | 11 |
You need 4.1 inches to add R-13.0 across 900.0 square feet, roughly $855 in material.
How sensitive the answer is
Holding everything else steady and changing only cost per square foot at that depth, this is what happens to thickness needed. It is worth knowing which of your inputs the answer actually hinges on, and which barely move it.
| Cost per square foot at that depth ($) | Thickness needed |
|---|---|
| $0.48 | 4.1 |
| $0.71 | 4.1 |
| $0.95 example above | 4.1 |
| $1.42 | 4.1 |
| $1.9 | 4.1 |
Every other input is held at the example value while this one changes, which is not how a real job behaves. Treat it as a guide to which number matters most, not as five separate quotes.
What each figure means
- Length (ft) (ft)
- Measured in ft, starting at 100 ft.
- Width (ft) (ft)
- Measured in ft, starting at 9 ft.
- Target R-value
- Choose from R-13, walls, mild, R-19, walls or floors, R-30, attic, mild climate, R-38, attic, R-49, attic, cold climate, R-60, attic, very cold.
- Material
- Choose from Blown fibreglass (R-2.5 per inch), Blown cellulose (R-3.7 per inch), Fibreglass batt (R-3.2 per inch), Rigid foam board (R-5 per inch), Open cell spray foam (R-3.7 per inch), Closed cell spray foam (R-6.5 per inch).
- Existing R-value in place
- A number, between 0 and 80, starting at 0.
- Cost per square foot at that depth ($)
- A money amount, starting at $0.95.
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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