Free Insulation R-Value Calculator – What R-Value Do I Need?

Code minimums are taken from 2021 IECC Table R402.1.3. Climate zone boundaries follow the DOE/IECC county map. Local jurisdictions adopt code editions on their own schedule and may amend them, so confirm with your building department before ordering material.

Related calculators: Insulating a new build? Start with the wall framing calculator to estimate your stud count, then use the concrete volume calculator for your foundation.

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What R-Value Do I Need for My Attic?

Under the 2021 IECC the ceiling requirement is R-30 in Zones 0 to 1, R-49 in Zones 2 to 3, and R-60 in Zones 4 through 8. Those are legal minimums, not targets. ENERGY STAR recommends going higher in cold climates, and the calculator above shows both figures side by side so you can tell which is which.

The attic is usually the best return on investment in the house, because heat rises and escapes through the roof in winter while summer attic heat radiates down into the rooms below. Blown cellulose and blown fiberglass are the cheapest way to reach R-49 or R-60, and both fill irregular framing easily. Air-seal the attic plane first: DOE and EPA put the combined saving from air sealing plus insulation at roughly 15 percent of heating and cooling costs, and leaks undermine depth far more than depth compensates for leaks.

R-Value by Climate Zone: 2021 IECC Minimums

The 2021 IECC divides the country into zones 0 through 8, with marine subzones. Zone 0 is new in the 2021 edition. Marine 4 (4C), which covers Seattle, Portland and the coastal Pacific Northwest, follows the Zone 5 row for walls and floors rather than the Zone 4 row. Wood-frame walls offer several compliance paths: a single cavity value, or a smaller cavity value plus continuous exterior insulation.

Zones 0 and 1: Ceiling R-30, wall R-13 or R-10 continuous, floor R-13. Basement, crawlspace wall and slab not required.
Zone 2: Ceiling R-49, wall R-13 or R-10 continuous, floor R-13. Basement, crawlspace wall and slab not required.
Zone 3: Ceiling R-49, wall R-20 or R-13 plus R-5 continuous, floor R-19, basement and crawlspace wall R-5 continuous or R-13 cavity, slab R-10 to 2 ft.
Zone 4: Ceiling R-60, wall R-30 or R-20 plus R-5 continuous, floor R-19, basement and crawlspace wall R-10 continuous or R-13 cavity, slab R-10 to 2 ft.
Marine 4C and Zone 5: Ceiling R-60, wall R-30 or R-20 plus R-5 continuous, floor R-30, basement and crawlspace wall R-15 continuous or R-19 cavity, slab R-10 to 2 ft.
Zone 6: Ceiling R-60, wall R-30 or R-20 plus R-5 continuous, floor R-30, basement and crawlspace wall R-15 continuous or R-19 cavity, slab R-10 to 4 ft.
Zones 7 and 8: Ceiling R-60, wall R-30 or R-20 plus R-5 continuous, floor R-38, basement and crawlspace wall R-15 continuous or R-19 cavity, slab R-10 to 4 ft.

R-Value Per Inch: Comparing Insulation Types

R-value per inch decides what can physically reach the target in the depth you have. This matters most in a 2×4 wall, where you only have 3.5 inches, and it is the reason the code offers continuous-insulation paths for Zones 4 and above.

Fiberglass batt: about R-3.2 per inch. Fits 2×4 and 2×6 cavities. Cheapest and most DIY-friendly.
Mineral wool batts: about R-3.1 per inch. Fire and moisture resistant, much better at sound.
Blown fiberglass: about R-2.5 per inch. Attic floors. Settles, so install deeper than the target implies.
Blown cellulose: about R-3.7 per inch. Recycled paper, treated with borate. Good for attics and retrofits.
Open-cell spray foam: about R-3.6 per inch. Air seals and fills gaps, but is vapour open.
Closed-cell spray foam: about R-6.5 per inch. The highest of any cavity fill, and its own vapour retarder.
Rigid polyiso: about R-5.6 per inch long-term. Note this is lower than the R-6 often printed on the label: polyiso derates in cold weather, exactly when you need it in Zones 5 to 8. It is exterior sheathing, not cavity fill.
Rigid XPS: about R-4.7 per inch aged. Fresh boards test near R-5 but drift down over time.
Rigid EPS: about R-3.9 per inch. The usual budget choice below grade.

Wall Insulation: Why R-30 Does Not Fit in a 2×4

A 2×4 cavity is 3.5 inches and a 2×6 cavity is 5.5 inches. In Zones 4 through 8 the 2021 IECC wants R-30 in a wood-frame wall, and no batt reaches that in either cavity. Only closed-cell spray foam gets close, at about R-35 in a 2×6. That is why the code lists alternatives: R-20 cavity plus R-5 continuous, R-13 plus R-10 continuous, or R-20 continuous with nothing in the cavity. Continuous exterior foam also covers the studs, which is where a quarter of the wall area is losing heat. The calculator flags any material that cannot fit and tells you how much continuous insulation would close the gap.

Floors, Crawl Spaces and Basement Walls

These are three different requirements and they are commonly confused. A floor over an unheated space follows the floor row: R-19 in Zones 3 to 4, R-30 in Zones 4C to 6, R-38 in Zones 7 to 8. A sealed, conditioned crawlspace is insulated at the walls instead, and follows the crawlspace wall row: R-5 continuous in Zone 3, R-10 in Zone 4, R-15 in Zones 5 to 8. A conditioned basement wall follows the same values as the crawlspace wall. They are not interchangeable: in Zone 5 the floor above wants R-30 while the crawlspace wall wants R-15 continuous, a two-fold difference behind what many articles treat as one choice. Pick the matching assembly in the calculator rather than assuming one number covers all three.

Rigid foam is the usual basement choice because it tolerates moisture. Glue it to the concrete and fasten mechanically, then cover with fire-rated drywall as code requires. Two inches of XPS is about R-9.4, so Zones 5 to 8 need roughly 3.2 inches to reach R-15 continuous. The rim joist above a conditioned basement follows the basement wall value, not a separate number, and closed-cell spray foam reaches R-15 there in about 2.3 inches.

Vapor Barriers and Insulation

In cold climates a vapour retarder belongs on the warm-in-winter side, to stop moisture migrating into the cavity and condensing. Kraft-faced batts have one built in. In hot humid climates the drive reverses and the retarder belongs toward the exterior. Closed-cell spray foam is its own vapour retarder; open-cell is not. Continuous exterior foam changes the dew point inside the wall, which is part of why the code pairs specific cavity and continuous values rather than letting you mix freely. Local codes vary, so confirm before you build.

Insulation R-Value FAQs

What is a good R-value for exterior walls?

Under the 2021 IECC it depends on your zone, not on your stud size. Zones 0 to 2 require R-13 cavity. Zone 3 requires R-20, or R-13 cavity plus R-5 continuous. Zones 4 through 8 require R-30, or R-20 cavity plus R-5 continuous, or R-13 cavity plus R-10 continuous. That last group is the important one: R-30 does not fit in a 2×4 cavity with any material, and only closed-cell spray foam reaches it in a 2×6, which is precisely why the code offers the continuous-insulation paths.

Is R-30 enough for an attic?

Only in Zones 0 and 1. The 2021 IECC ceiling requirement is R-30 in Zones 0 to 1, R-49 in Zones 2 to 3, and R-60 in Zones 4 through 8. R-30 was the older value and is no longer sufficient across most of the country.

Can you have too much attic insulation?

There is a point of diminishing returns, generally around R-60 for most climates. Beyond that the energy savings rarely justify the cost. Air sealing matters more than the last few R-values: leaks reduce effective performance far more than adding depth does, so seal first and insulate second.

What is the highest R-value insulation for walls?

Closed-cell spray foam, at roughly R-6.5 per inch. A 5.5-inch 2×6 cavity filled with closed-cell reaches about R-35. Polyiso is higher per inch on paper but is a continuous sheathing product, not cavity fill, and it derates in cold weather, so this calculator no longer offers it as a stud-cavity option.

Does higher R-value always mean better insulation?

No. R-value measures resistance to conductive heat flow only. It does not account for air infiltration or radiant heat. A well-installed R-13 batt in an air-sealed wall outperforms R-21 batts with gaps around them. Framing also matters: R-13 in a 2×4 wall delivers roughly R-11 once thermal bridging through the studs is counted.

What is the R-value of a 2×4 wall?

An uninsulated 2×4 stud wall is about R-4 to R-5 from framing and sheathing alone. Filling it with R-13 batts gives R-13 in the cavity, but the whole-wall value, accounting for studs at about 25 percent of the wall area, is closer to R-11. Continuous exterior rigid foam is what raises that number, because it covers the studs as well as the cavities.

How thick is R-38 insulation?

It depends on the material, and it should always be rounded up to a depth an installer can actually place. Fiberglass batts need 12 inches, mineral wool 12.5 inches, blown cellulose 11 inches including settling allowance, blown fiberglass 16.5 inches including settling, open-cell spray foam 11 inches, closed-cell spray foam 6 inches, XPS 8.5 inches, polyiso 7 inches and EPS 10 inches. Enter 38 as a custom target in the calculator to see this for your own assembly.

What climate zone am I in?

The 2021 IECC uses zones 0 through 8, plus marine subzones. Zone 0 was added in 2021 for tropical territories. Broadly: Hawaii and south Florida are Zone 1, the Gulf Coast and Phoenix are Zone 2, the Southeast is Zone 3, Virginia and Tennessee are Zone 4, Seattle and Portland are Marine 4C, most of Kansas and Ohio and New England are Zone 5, Minnesota and Montana are Zone 6, northern Minnesota is Zone 7, and Arctic Alaska is Zone 8. Many states span two or more zones, so confirm by county on the DOE zone map rather than by state.