Insulation
Attic Insulation Calculator
Works out how much more attic insulation you need, where air sealing first matters more than the depth you add.
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
Required thickness is the target R-value less whatever is already there, divided by the R per inch of the material you are adding. Existing R comes from measuring the current depth and multiplying by that material's R per inch, which is why identifying what is up there matters before calculating. The target depends on your climate zone rather than being universal, and attics carry the highest recommended values of any assembly because heat rises and the attic is the largest unconditioned surface above the living space.
What moves the number
- Climate zone, which sets the target and varies substantially across the country.
- What is already there and how deep, since existing R counts toward the target.
- Whether the material has settled, as blown insulation loses depth and therefore R over time.
- Access and obstructions, which decide whether this is a straightforward blow or a crawl between trusses.
Where this one goes wrong
Air seal before insulating, because insulation slows conduction and does almost nothing about air moving through. Gaps around plumbing stacks, wiring penetrations, recessed lights, the attic hatch and the top plates leak conditioned air straight into the attic, and burying them under a foot of new insulation makes them permanently inaccessible while leaving the leak. Sealing those penetrations first is cheaper than the insulation and usually saves more. Two other things to respect: keep insulation out of the soffit vents by using baffles, since blocking them stops the attic ventilating and causes moisture and ice damming, and do not bury a recessed light that is not rated for contact, which is a fire risk.
Before you quote
- Air seal penetrations and the attic hatch before adding insulation.
- Fit baffles so insulation cannot block the soffit vents.
- Check recessed lights are contact rated before covering them.
- Measure existing depth and identify the material to get existing R right.
Where the rules come from
Questions
About this calculator
How much attic insulation do I need?
It depends on your climate zone, which is why the target is an input rather than a fixed number. Department of Energy recommendations and the adopted energy code both set values by zone, and attics carry the highest targets of any assembly. Subtract what is already there to get what you need to add.
Should I air seal before adding insulation?
Yes, and it is the single most common omission. Insulation slows conduction and does very little about air leaking through gaps around pipes, wiring, lights and the hatch. Sealing those costs less than the insulation, saves more energy, and becomes impossible once they are buried under a foot of new material.
Can I add insulation over what is already there?
Usually yes, and it is the normal approach. Blown material can go over batts, and unfaced batts can go over existing insulation. What you should not do is add a second vapour retarder facing down onto the first, because that traps moisture between them. Unfaced material over faced is the safe direction.
Why do I need baffles?
To keep insulation from blocking the soffit vents at the eaves. The attic needs to ventilate, and insulation pushed into the eaves stops the airflow, which causes moisture buildup and contributes to ice damming in cold climates. Baffles hold it back while letting air through.
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 12.
- 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 | 12 |
| Cost per square foot at that depth | $1.35 |
What comes out
| Result | Value |
|---|---|
| Thickness needed main result | 10.0 |
| Area | 1,120.0 |
| R-value still to add | 37.0 |
| Estimated material cost | $1,512 |
| Bags if blown | 37 |
You need 10.0 inches to add R-37.0 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 | 10.0 |
| $1.01 | 10.0 |
| $1.35 example above | 10.0 |
| $2.03 | 10.0 |
| $2.7 | 10.0 |
Every other input is held at the example value while this one changes, which is not how a real job behaves. Treat it as a guide to which number matters most, not as five separate quotes.
What each figure means
- Length (ft) (ft)
- Measured in ft, starting at 40 ft.
- Width (ft) (ft)
- Measured in ft, starting at 28 ft.
- Target R-value
- Choose from R-13, walls, mild, R-19, walls or floors, R-30, attic, mild climate, R-38, attic, R-49, attic, cold climate, R-60, attic, very cold.
- Material
- Choose from Blown fibreglass (R-2.5 per inch), Blown cellulose (R-3.7 per inch), Fibreglass batt (R-3.2 per inch), Rigid foam board (R-5 per inch), Open cell spray foam (R-3.7 per inch), Closed cell spray foam (R-6.5 per inch).
- Existing R-value in place
- A number, between 0 and 80, starting at 12.
- Cost per square foot at that depth ($)
- 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. |
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.