R-Value for Attic Insulation: What Do You Need by Climate Zone?

ENERGY STAR suggests R30 for an uninsulated attic in climate zone 1, R49 in zones 2 and 3, and R60 in zones 4 to 8. With 3 to 4 inches already in place, the targets drop to R25, R38 and R49.

An uninsulated attic in zone 4A has an ENERGY STAR target of R60. If a zone 4A attic already has 3 to 4 inches and that layer is about R-11, the target is R49, so you add R38 (49 − 11).

What R-value do you need for attic insulation?

R-value measures resistance to heat flow, and a higher number resists more. For an existing wood-framed house, ENERGY STAR publishes a table of cost-effective insulation levels by climate zone. Its footnote says the guidance is based on Table R402.1.3 of the 2021 International Energy Conservation Code (IECC). Here are the two attic columns.

The numbers are the total R-value you want in the attic, not the amount to add. The same ENERGY STAR page also lists floor targets. This guide stays with attics. To turn a target into packages or bags, see how much insulation you need.

Why do the DOE and ENERGY STAR numbers differ?

The Department of Energy’s Consumer Guide to Home Insulation (DOE/EE-2476, 2021) gives ranges instead of single numbers. It suggests R-values by zone when you add insulation to uninsulated or under-insulated attics.

The two sources group the zones slightly differently. ENERGY STAR puts 4C with zones 5 and 6, and the DOE puts marine zone 4 with zone 5. They are different publications with different formats, so neither is a mistake. ENERGY STAR’s single number falls inside the DOE range in every row. A practical reading: the ENERGY STAR figure is a clear target, and the DOE range shows how much room there is above or below it. Neither is a building code.

How do you find your climate zone?

The DOE guide includes a map of the United States with the zones marked, and the table above is keyed to it. Find your location on that map, then read across your row. A few notes printed with the map: zone 1 also includes Hawaii, Guam, Puerto Rico and the Virgin Islands, and Alaska is zone 7 except for select interior boroughs near and north of the Arctic Circle.

If you need a zone for a specific county, the Pacific Northwest National Laboratory publishes a county-by-county guide to the 2021 IECC zones. See the sources below. Your local building department can also tell you which zone applies.

How do you account for insulation you already have?

R-values of layers add up. The DOE fact sheet says that for a multi-layered installation, the R-values of the individual layers are added. So the amount to add is the target minus what is there:

R to add = target R − R you already have
(if the result is 0 or less, the attic already meets the target)

• Quick check. ENERGY STAR says that if your insulation is just level with or below the floor joists, so you can easily see them, you should add more.
• The ENERGY STAR column for 3 to 4 inches already assumes a layer that thin. If your existing layer is thicker or thinner, use the uninsulated or the 3-to-4-inch column that is closer, and treat the table as guidance.
• Example: a zone 4A attic with about R-11 and a target of R49 needs R38 more (49 − 11 = 38).
• Mixing types is fine. ENERGY STAR says you do not have to use the same type of insulation as the existing layer. You can add loose-fill on top of batts and the other way round. If you put fiberglass over loose-fill, use unfaced batts with no paper or foil backing.
• No vapor barrier between layers. The DOE fact sheet warns against one when adding insulation over existing insulation.

The R-value of an old layer is whatever the product label or installer says. This guide has no source for a per-inch figure for unlabeled old insulation, so it does not give one.

How many inches of insulation is that?

It depends on the product, because R per inch differs between materials and brands. Two blown-in manufacturers show it. The minimum installed thickness for the same R-value differs between them.

These are minimum installed thicknesses from each maker’s chart, and they apply to those products only. Both makers say you also have to install the required number of bags. Meeting the thickness alone does not deliver the stated R-value. Use the chart for the product you buy.

What about new construction?

The tables above are for adding insulation to an existing wood-framed house. For new construction, the required R-value comes from the energy code adopted where you build, so check with your local building department. This guide gives no new-construction numbers, because the code tables are not reproduced here.

What should you check before adding attic insulation?

• Air-seal first. ENERGY STAR’s attic guidance starts from an air-sealed attic.
• Keep vents clear. If the attic is ventilated, never cover or block the vents. Use baffles or rafter vents to keep loose-fill from clogging them (DOE).
• Recessed lights, flues and chimneys. Keep insulation away from heat sources, as the manufacturer and local code require. Owens Corning says not to insulate on top of or within 3 inches of a recessed fixture unless it is labeled IC (rated for insulation contact).
• Knob-and-tube wiring. The National Electric Code forbids loose, rolled or foam-in-place insulation around it. Have an electrician look first.
• Vapor retarders. Follow local code. For walls, the DOE notes that where moisture moves both ways for significant parts of the year, you may be better off with no vapor retarder at all.
• Do not compress batts. Compressed insulation does not give its full rated R-value.

Assumptions, common errors and limits

• These are ENERGY STAR and DOE recommendations for adding insulation to an existing house. They are not code, and your local code may require more.
• The two tables differ in format and grouping. This guide shows both and does not average them.
• Mixing up total and added R. The table gives the total you want. Subtract what you have.
• Picking the wrong zone. Check the map or ask the building department.
• Counting thickness only. Blown-in R-value needs both the thickness and the bag count from the chart.
• This is not an energy audit. It makes no savings claims, and it does not size an HVAC system or evaluate moisture or structural conditions.

Common questions