IECC Figure R301.1. Your building department can confirm.
Sets the maximum R-value that will physically fit.
Area 1
Total length of wall to insulate.
Total opening area to deduct.
Advanced options
0 = use the code minimum for your zone.
Drives the thermal bridging loss.
Rigid foam outside the studs. Adds its full value.
0 = use the typical figure for the R-value.
This insulation calculator gives the R-value your climate zone requires, then checks whether it will physically fit the cavity you have — and reports what the wall actually performs at once the studs are counted.
The short answer
Under the 2021 IECC, zones 4–8 need R-60 ceilings and R-30 walls. Zones 2–3 need R-49 ceilings, zone 1 R-30.
But an R-30 wall cavity is not buildable with batts. A 2×6 filled with fiberglass reaches R-21. That is why the code also offers R-20 + R-5 continuous, or R-13 + R-10 continuous — and why the continuous layer matters more than its number suggests.
On this page
How to use this insulation calculator
Pick your climate zone, material and cavity depth, then enter the area. Four modes:
- Walls — total wall run, height, and openings to deduct
- Attic floor — length and width of the space below the roof
- Floor or cathedral — over a crawl space, or a sloped ceiling
- Known area — you’ve already measured
The header toggle switches between 2021 and 2018 IECC. Check which your jurisdiction has adopted, because they differ substantially and adoption is patchy.
The results tell you three things: the code minimum, whether your material and cavity can reach it, and what the assembly actually performs at after thermal bridging.
What the code requires
2021 IECC Table R402.1.3, the prescriptive R-value path.
| Zone | Where | Ceiling | Wood-frame wall | Floor |
|---|---|---|---|---|
| 1 | S. Florida, Hawaii | R-30 | R-13 | R-13 |
| 2 | Gulf Coast, S. Texas | R-49 | R-13 | R-13 |
| 3 | Carolinas to N. Texas | R-49 | R-20 or 13+5ci | R-19 |
| 4 | Mid-Atlantic, Tennessee | R-60 | R-30 or 20+5ci | R-19 |
| 5 | Midwest, New England | R-60 | R-30 or 20+5ci | R-30 |
| 6 | N. Plains, Vermont | R-60 | R-30 or 20+5ci | R-30 |
| 7 | N. Minnesota, N. Maine | R-60 | R-30 or 20+5ci | R-38 |
| 8 | Northern Alaska | R-60 | R-30 or 20+5ci | R-38 |
“ci” means continuous insulation — a layer outside the studs, uninterrupted by framing. R-13 + R-10ci is a third wall path in zones 4 and above.
2018 versus 2021 — and why published tables disagree
Insulation requirements are one of the worst areas on the web for confidently wrong numbers. In researching this page I found multiple sources citing “2021 IECC” while publishing 2018 values, and one that contradicted itself two paragraphs apart — giving R-49 ceilings for zones 4–8 in one place and R-60 in another.
The confusion is understandable, because 2021 changed a lot:
| Zone | Ceiling 2018 | Ceiling 2021 | Wall 2018 | Wall 2021 |
|---|---|---|---|---|
| 1 | R-30 | R-30 | R-13 | R-13 |
| 2–3 | R-38 | R-49 | R-13 / R-20 | R-13 / R-20 |
| 4–8 | R-49 | R-60 | R-20 | R-30 |
Code adoption is state and sometimes county level, and it lags. A significant number of jurisdictions were still on the 2018 IECC well after 2021 was published, and several adopted 2021 with amendments that roll the attic values back to R-49 — the NAHB published model amendments doing exactly that.
So “what does the code require” has no single national answer. The calculator gives both editions; your building department gives the one that counts.
Will it actually fit?
This is the question most insulation calculators never ask. They take your target R-value and multiply out quantities, without checking whether that R-value can exist in the cavity you have.
Batts are sold at fixed label R-values determined by the framing they’re made for:
| Cavity | Depth | Fiberglass batt | Mineral wool batt | Closed-cell foam |
|---|---|---|---|---|
| 2×4 | 3½ in | R-15 | R-15 | R-23 |
| 2×6 | 5½ in | R-21 | R-23 | R-36 |
| 2×8 | 7¼ in | R-25 | R-30 | R-47 |
| 2×10 | 9¼ in | R-30 | R-38 | R-60 |
Now look back at the 2021 requirement for zones 4–8: R-30 in the wall cavity. A 2×6 filled with fiberglass gives R-21. Mineral wool gives R-23. Neither reaches it. The only cavity-only routes to R-30 are closed-cell foam in a 2×6, or 2×8 framing — both expensive, and 2×8 walls are not standard residential construction.
That is not an oversight in the code. It’s the point. The R-30 cavity path is effectively theoretical for normal framing, which pushes you toward the alternatives — and those alternatives are where the real performance is.
An R-21 batt squashed into a 2×4 cavity does not give R-21. Compression collapses the air pockets that do the insulating, and you end up somewhere near R-13 — while having paid for R-21.
The reverse matters too: a batt that is too small leaves gaps, and gaps hurt far more than their area suggests because air moves through them. Buy the batt made for the cavity.
Thermal bridging — what the label doesn’t tell you
An R-21 batt gives R-21 through the batt. But a wall is not all batt. Roughly a quarter of it is wood, and wood insulates at about R-1.25 per inch.
Uavg = (1 − framing %) × Ucavity + framing % × Ustud
Whole-wall R = 1 ÷ Uavg
Heat takes both routes at once, and averages by conductance, not R-value
A 2×6 stud is only R-6.9 against the batt’s R-21
| Assembly | Nominal | Whole-wall | Lost |
|---|---|---|---|
| 2×4, R-13, 16 in oc | R-15.5 | R-11.8 | 24% |
| 2×4, R-13, 24 in oc | R-15.5 | R-12.1 | 22% |
| 2×6, R-21, 16 in oc | R-23.5 | R-17.1 | 27% |
| 2×6, R-21, 24 in oc | R-23.5 | R-18.5 | 21% |
| 2×6, R-21 + R-5ci | R-28.5 | R-22.1 | 23% |
Note the deeper the cavity, the worse the penalty — a 2×6 loses more proportionally than a 2×4, because the stud is deeper too but doesn’t get any better at insulating. Advanced framing at 24 inch centres claws back a few points by simply having less wood.
Why continuous insulation is worth more than its number
Continuous insulation goes outside the studs, so nothing interrupts it. Every square foot of it works at its full rating.
R-5 more cavity
Adds ~R-1.8
Only the cavity portion improves. The studs are unchanged, so the whole-wall gain is diluted by the framing fraction — on a 2×6 you get about a third of what you paid for.
R-5 continuous
Adds R-5.0
Uninterrupted by framing, so the full value lands. It also warms the sheathing, which reduces condensation risk inside the wall.
The trade-off
Detailing
Windows, doors and trim all have to move outward by the foam thickness. It’s the reason builders resist it, not the material cost.
Material comparison
| Material | R per inch | Strengths | Watch out for |
|---|---|---|---|
| Fiberglass batt | 3.2 | Cheapest, DIY friendly | Performance collapses if fitted badly |
| Mineral wool batt | 4.2 | Fire and water resistant, holds shape, good acoustically | Roughly twice the cost of fiberglass |
| Blown fiberglass | 2.6 | Fills irregular spaces, no gaps | Lowest R per inch; can be wind-washed in attics |
| Blown cellulose | 3.6 | Recycled, dense-packs well, good in attics | Settles ~10% unless dense-packed |
| Open-cell foam | 3.7 | Air seals as it insulates, expands into gaps | Vapour permeable, so needs a retarder in cold zones |
| Closed-cell foam | 6.5 | Highest R per inch, air and vapour barrier, adds racking strength | Most expensive; hard to reach code R in thin cavities without it |
Attics — where the depth actually goes
Attics are the cheapest place to add R-value, because there’s no cavity limit. But R-60 is genuinely deep:
| Target | Blown cellulose | Blown fiberglass |
|---|---|---|
| R-30 | 8.3 in | 11.5 in |
| R-38 | 10.6 in | 14.6 in |
| R-49 | 13.6 in | 18.8 in |
| R-60 | 16.7 in | 23.1 in |
A standard truss sits directly on the top plate, leaving only a few inches of height where the roof meets the wall. You cannot get 17 inches of insulation into a 4 inch space, so the R-value thins out to almost nothing exactly at the perimeter — which is a large fraction of the ceiling on a typical house.
Two fixes. Baffles at every rafter bay hold the required 1 inch ventilation gap open and stop insulation being pushed into the soffit. Raised-heel trusses on new work lift the roof at the wall so full depth carries all the way out — and the 2021 IECC allows R-49 instead of R-60 where one is used, which is a meaningful concession.
Batts, bags and board feet
| Form | Sold as | How it’s counted |
|---|---|---|
| Batts | Bags covering 30–90 ft² | Area ÷ coverage per bag. Higher R means less per bag |
| Blown | Bags of loose fill | Coverage falls as depth rises — check the bag’s own chart |
| Spray foam | Board feet | One board foot = 1 ft² at 1 inch. Area × thickness |
Every bag of blown insulation carries a coverage chart printed on it, giving square feet per bag at each target R-value along with the minimum settled depth and the bag count per 1,000 ft². Use it — it is manufacturer-specific and more accurate than any general figure, including the ones here.
Add 10% for waste on batts, which lose material to cuts around outlets, pipes and blocking. Blown insulation wastes almost nothing, but attics need 10–20% extra to account for the joists and ducts that displace it.
Air sealing comes first
R-value measures resistance to conduction only. It says nothing about air moving through the assembly, and air movement carries far more heat than conduction through a well-built wall ever will.
Seal top and bottom plates, rim joists, every penetration for wiring and plumbing, and the attic hatch — before insulating, while you can still reach them. It is cheap, it is fast, and it does more per dollar than any amount of extra R-value.
This is also why blown and foam products often outperform their numbers relative to batts: they fill irregular gaps that a batt bridges over.
Insulation mistakes to avoid
- Compressing batts to fit. An R-21 batt in a 2×4 cavity performs like R-13.
- Trusting the label as the wall’s R-value. Studs cost you 21–27%.
- Assuming a 2×6 can hit R-30. Batts reach R-21; you need continuous insulation.
- Using a table without checking the edition. 2018 and 2021 differ by R-11 on ceilings.
- Blocking the eave vents. Fit baffles, or you trade a moisture problem for an R-value.
- Cutting around wiring instead of splitting the batt. Split it and slide half behind.
- Skipping air sealing. It’s the highest-return step and it has to happen first.
- Interior vapour barriers in hot-humid zones. In zones 1–2 they trap condensation against the sheathing.
Frequently asked questions
What R-value do I need?
Under the 2021 IECC, zones 4–8 need R-60 ceilings and R-30 walls, zones 2–3 need R-49 ceilings, and zone 1 needs R-30. Wall requirements are R-13 in zones 1–2 and R-20 in zone 3. Check which code edition your jurisdiction has adopted, because the 2018 values are noticeably lower.
Why do insulation R-value charts disagree?
Mostly because they mix code editions. The 2021 IECC raised ceilings from R-49 to R-60 in zones 4–8 and walls from R-20 to R-30, so a chart built on 2018 values looks wrong against 2021 and vice versa. Several published charts cite 2021 while listing 2018 numbers. Some jurisdictions also adopt 2021 with amendments that roll the attic values back.
Can a 2×6 wall meet R-30?
Not with batts. A 2×6 cavity takes an R-21 fiberglass batt or R-23 mineral wool. Closed-cell spray foam reaches about R-36 in the same cavity but costs several times as much. The practical routes are the code’s alternatives: R-20 cavity plus R-5 continuous, or R-13 cavity plus R-10 continuous.
What is thermal bridging?
Heat bypassing the insulation by travelling through the studs, which insulate at only about R-1.25 per inch. At 16 inch centres roughly a quarter of a wall is framing, so a 2×6 wall with R-21 batts performs at about R-17 whole-wall rather than R-23.5 — a 27% loss.
What does “ci” mean in R-13+5ci?
Continuous insulation — a layer outside the framing, usually rigid foam or mineral wool board, uninterrupted by studs. R-13+5ci means R-13 in the cavity plus R-5 continuous. Because nothing interrupts it, continuous insulation delivers its full rated value, which makes it worth more than the same R-value added to the cavity.
How deep is R-60 insulation?
About 16.7 inches of blown cellulose or 23 inches of blown fiberglass. That depth rarely fits at the eaves on a standard truss, so use baffles to protect ventilation and consider raised-heel trusses on new work — the 2021 IECC allows R-49 instead of R-60 where one is fitted.
Can I compress insulation to fit a cavity?
No. Compression collapses the air pockets that do the insulating, so an R-21 batt squeezed into a 2×4 cavity performs at roughly R-13 while costing R-21 money. Buy the batt sized for the framing you have.
How many batts do I need?
Divide the area by the coverage printed on the bag, then add about 10% for cuts around outlets, pipes and blocking. Coverage falls as R-value rises — an R-13 bag covers around 88 ft² while an R-30 bag covers about 42 ft².
Which insulation is best?
It depends on the cavity. Fiberglass batts are cheapest and fine when fitted carefully. Mineral wool costs more but holds its shape and resists fire and water. Blown cellulose suits attics. Closed-cell foam gives the highest R per inch and is often the only way to hit code in a thin cavity, at the highest cost.
Do I need a vapour barrier?
It depends on the climate zone, and getting it backwards causes real damage. In cold zones a vapour retarder belongs on the warm interior side. In hot-humid zones 1–2 an interior Class I barrier traps condensation against the sheathing and causes mould. Check your zone’s requirement rather than applying a general rule.
Is air sealing more important than insulation?
It comes first, and it’s cheaper. R-value only resists conduction; air moving through an assembly carries far more heat. A well-sealed R-13 wall outperforms a leaky R-30 one. Seal plates, rim joists, penetrations and the attic hatch before insulating, while you can still reach them.
Can I use this insulation calculator in metric?
Dimensions are in feet and inches and R-values are imperial (ft²·°F·h/BTU). For metric RSI, divide the R-value by 5.678 — so R-30 is about RSI 5.3. Multiply square feet by 0.0929 for square metres.
Standards and sources
- 2021 IECC Table R402.1.3 — prescriptive R-value alternative for residential envelopes
- 2018 IECC Table R402.1.2 — the equivalent table in the previous edition, still enforced in many jurisdictions
- 2021 IECC R402.2.1 — the raised-heel truss allowance permitting R-49 in place of R-60
- Whole-wall figures use the parallel-path (isothermal planes) method with framing fractions of 25% at 16 in and 22% at 24 in centres, and softwood framing at R-1.25 per inch
- Coverage figures are typical; every bag of insulation carries a manufacturer coverage chart that takes precedence
Related calculators
How this insulation calculator works out its numbers
The code minimum comes from IECC Table R402.1.3 for 2021 or R402.1.2 for 2018, indexed by climate zone and by which assembly you selected. Wall, ceiling and floor requirements differ within the same zone, so the mode you choose changes the target.
The cavity check compares your target against what the material can deliver in the depth available. Batts use their actual label R-values, because that is how they are manufactured and sold — a 2×6 fiberglass batt is R-21 regardless of what R-per-inch arithmetic suggests. Loose fill and foam use R per inch, since their depth is continuous.
Whole-wall R uses the parallel-path method: conductances are averaged by area fraction, then inverted back to an R-value. Framing fraction is 25% at 16 inch centres and 22% at 24 inch, with softwood at R-1.25 per inch and R-2.5 for drywall, sheathing and air films. Continuous insulation is added after the averaging, because nothing interrupts it. Attics are treated as unbridged, since blown insulation covers the joists rather than sitting between them.
This produces quantities and a code check, not an energy model. Actual performance depends on installation quality — RESNET Grade I versus Grade III installation of the same batt differs substantially — and on air sealing, which R-value does not measure at all. Local amendments to the IECC are common and override these tables. Confirm your zone, your adopted code edition and any amendments with your building department before ordering.