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Insulation R-Values by Climate Zone for Every Part of the House

The 2021 IRC insulation figures for ceilings, walls, floors, basements and slabs by climate zone, what "13&5ci" means, and how ENERGY STAR's levels differ.

Updated
Reading time6 min
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Key takeaways

  • The 2021 IRC's ceiling figure is R-30 in zones 0 and 1, R-49 in zones 2 and 3, and R-60 in zones 4 to 8.
  • In "13&5ci", the first number is cavity insulation and the second is continuous insulation, so R-13 between the studs plus R-5 over them.
  • Section N1103.5.2 calls for R-3 insulation on hot-water pipes 3/4 inch and larger inside the house, and on several other runs.

Building codes set insulation by climate zone, with a figure for each part of the house. This page shows the 2021 International Residential Code (IRC) figures, explains the notation, and shows where ENERGY STAR's levels for existing homes differ. Your state or city may use another edition or amend the figures, so your building department has the final word.

Find your climate zone first

The IRC numbers its zones 0 to 8, and the higher numbers are colder. Zone 4 is split: marine zone 4 (4C) is grouped with zone 5, and the rest of zone 4 has its own row.

  1. Look up your county in the Guide to Determining Climate Zone by County from Pacific Northwest National Laboratory (PNNL). It gives the 2021 zone for every county and flags counties whose zone changed since the 2018 edition.
  2. Or read the zone map in the IRC, Figure N1101.7.
  3. Confirm with your building department, especially if your county sits near a zone line or your area hasn't adopted the 2021 edition.

An older chart may list your county in a different zone.

The 2021 IRC figures by zone

These are four of the columns in 2021 IRC Table N1102.1.3. Read the full table for mass walls, crawl spaces and the footnotes.

Zone Ceiling Wood-frame wall Floor Basement wall
0 and 1 R-30 13 or 0&10ci R-13 0
2 R-49 13 or 0&10ci R-13 0
3 R-49 20 or 13&5ci or 0&15ci R-19 5ci or 13
4, except marine R-60 30 or 20&5ci or 13&10ci or 0&20ci R-19 10ci or 13
Marine 4, 5 and 6 R-60 30 or 20&5ci or 13&10ci or 0&20ci R-30 15ci or 19 or 13&5ci
7 and 8 R-60 30 or 20&5ci or 13&10ci or 0&20ci R-38 15ci or 19 or 13&5ci

The colder zones' floor figures carry a footnote allowing another option, so read it before you buy. Other parts of the house:

  • Slab edges: no figure in zones 0 to 2, R-10 continuous to 2 feet deep in zone 3, and R-10 continuous to 4 feet in zones 4 to 8.
  • Crawl space walls: the same figures as basement walls.
  • Windows: a U-factor of no more than 0.40 in zone 2 and 0.30 in zones 3 to 8. The solar heat gain coefficient (SHGC) is no more than 0.25 in zones 0 to 3 and 0.40 in zones 4 and 5, with no requirement in zones 6 to 8.

Footnote a says the R-values are minimums and the U-factors and SHGC figures are maximums.

Reading "13&5ci" and the "or"s

Each "or" is a separate option, and you meet the figure by meeting any one of them. A zone 5 wall can be:

  • 30: R-30 in the stud cavity alone
  • 20&5ci: R-20 in the cavity plus R-5 continuous
  • 13&10ci: R-13 in the cavity plus R-10 continuous
  • 0&20ci: nothing in the cavity and R-20 continuous

In a pair, the first number is cavity insulation and the second is continuous insulation (ci), as PNNL's summary of the code tables explains. Continuous insulation runs unbroken over the framing, such as insulating sheathing under the siding, so it covers the studs as a batt between them can't.

ENERGY STAR's advice follows the same logic. For an insulated 2x4 wall in zones 4 to 8, it suggests adding R-10 insulating sheathing under new siding, the same pairing as the code's 13&10ci option.

New homes versus existing homes

The IRC table sets minimums for the work your local code covers. ENERGY STAR publishes separate recommended levels for retrofitting existing wood-framed buildings, based on the 2021 International Energy Conservation Code (IECC). It gives a lower attic figure when some insulation is already there.

Zone 2021 IRC ceiling ENERGY STAR, uninsulated attic ENERGY STAR, 3 to 4 in already ENERGY STAR floor
1 R-30 R-30 R-25 R-13
2 R-49 R-49 R-38 R-13
3 R-49 R-49 R-38 R-19
4A and 4B R-60 R-60 R-49 R-19
4C, 5 and 6 R-60 R-60 R-49 R-30
7 and 8 R-60 R-60 R-49 R-38

For walls, ENERGY STAR's advice applies when the siding comes off. Blow insulation into empty cavities, then add insulating sheathing: R-5 in zone 3 and R-5 to R-10 in zones 4 to 8. For basement and crawl space walls, it gives R-5 sheathing or an R-13 batt in zone 3, R-10 or R-13 in zones 4A and 4B, and R-15 or R-19 in zones 4C and 5 to 8.

Ask your building department whether the code figure applies before an addition or major renovation.

Worked example

You live in zone 4A and the attic has about 3.5 inches of old blown fiberglass. The attic insulation calculator uses R-2.2 per inch for blown fiberglass and R-3.7 for blown cellulose, both from a secondary table, so check your product.

  • What's there: 3.5 × 2.2 = R-7.7
  • ENERGY STAR level for zone 4A with 3 to 4 inches already: R-49. So 49 − 7.7 = R-41.3 to add, and 41.3 ÷ 3.7 = about 11.2 inches of cellulose.
  • IRC ceiling figure: R-60. So 60 − 7.7 = R-52.3 to add, and 52.3 ÷ 3.7 = about 14.1 inches of cellulose.

The two targets differ by about 2.9 inches. The bag's coverage chart gives the real depth and bag count.

Hot-water pipes: R-3

Section N1103.5.2 of the 2021 IRC calls for R-3 insulation on hot-water piping:

  • 3/4 inch and larger inside conditioned space
  • Outside conditioned space
  • Under a floor slab
  • Buried in the ground
  • From the water heater to a distribution manifold
  • In circulation loops

Common mistakes

  • Using an older zone map. Some counties changed zones in the 2021 edition.
  • Adding the pair and comparing totals. "13&10ci" isn't the same as any wall with R-23 in it, because the continuous layer has to cover the framing.
  • Mixing up the two sources. The IRC figure is a code minimum, and ENERGY STAR's is a retrofit recommendation.
  • Missing the zone 4 split. Marine 4 uses the zone 5 row for floors and basement walls.
  • Squeezing a thick batt into a thin cavity. PNNL's summary notes the installed R-value still has to meet the table, so a compressed batt can fall short of its label.

Frequently asked questions

What R-value do I need for exterior walls?

Under the 2021 IRC, a wood-frame wall needs R-13 (or R-10 continuous) in zones 0 to 2, R-20 or a combination in zone 3, and R-30 or a combination in zones 4 to 8.

What climate zone am I in?

Look up your county in PNNL's county guide or the IRC's Figure N1101.7, then confirm locally.

What does "ci" mean in insulation?

Continuous insulation: a layer that runs unbroken across the framing, such as insulating sheathing. In "13&5ci", R-13 goes in the cavity and R-5 is continuous.

Do I have to reach R-60 in an existing attic?

Not necessarily. R-60 is the 2021 IRC figure for zones 4 to 8, while ENERGY STAR's retrofit level there is R-49 if you already have 3 to 4 inches. Your building department can say what applies.

Do hot-water pipes need insulating?

Under Section N1103.5.2, yes in many places: R-3 on pipes 3/4 inch and larger inside, and on pipes outside conditioned space, under slabs, buried, to a manifold or in circulation loops.

Sources

We use primary sources where we can: building codes, federal rules and manufacturer documents. Read our editorial standards.