What R-value do I need in my attic?

Under the 2021 IECC it is R-30 in zone 1, R-49 in zones 2 and 3, and R-60 in zones 4 through 8. Your local jurisdiction may have adopted an earlier code edition with lower values, or amended the table.

Does doubling the R-value halve my heat loss?

Through that assembly, roughly yes — but heat loss through the whole house does not halve, because windows, doors, air leakage and other assemblies are unchanged. Returns diminish as you add more.

Insulation R-Value Calculator

Find the R-value your climate zone calls for and how many inches of a given insulation reach it.

Draft — figures checked, awaiting an editorial read. Not indexed.

Target R-value— R2021 IECC prescriptive minimum for this zone and assembly.

Thickness needed: — in
Material R per inch: — R/in
Loose-fill volumeEstimate only — follow the bag coverage chart.: — ft³

Formula: Target R comes from the IECC climate zone table. Thickness = target R ÷ R-value per inch of the chosen material.

These are the 2021 IECC prescriptive minimums, provided for planning only. Jurisdictions adopt different code editions on their own schedules and many amend the tables locally. Confirm the requirement with your building department before you buy or build — your local adopted code governs, not this page.

What this calculates

This gives the insulation R-value the energy code calls for in your climate zone, and converts it into a thickness for the material you are actually installing. R-value measures resistance to heat flow, and it is additive: two inches of something rated R-3.5 per inch gives you R-7. The reason thickness has to be calculated rather than looked up is that materials differ enormously per inch — reaching R-49 takes about fourteen inches of blown cellulose but only about eight inches of closed-cell spray foam, and that difference decides whether it fits in the cavity you have.

How it works

The target R-value comes from the 2021 IECC prescriptive table for wood-frame residential construction, indexed by climate zone and by where the insulation is going — attic, wall, or floor over unconditioned space. That target is then divided by the chosen material's R-value per inch to give the thickness. The optional area field multiplies through to a rough loose-fill volume in cubic feet, which is useful for sanity-checking a quote, though the bag's own coverage chart is the number to buy from.

Worked example

A house in climate zone 5 insulating the attic with blown cellulose. The 2021 IECC prescriptive ceiling value for zone 5 is R-60. Cellulose is about R-3.5 per inch, so 60 ÷ 3.5 = 17.1 inches of settled depth. Over a 1,000 square foot attic that is 1,000 × 17.1 ÷ 12 = 1,429 cubic feet of material. Using closed-cell foam at R-6.5 per inch instead, the same R-60 needs only 9.2 inches — which is the trade-off you are paying for.

Common mistakes

Treating the code minimum as the optimum. It is a floor, not a target, and in a cold zone the payback on going above it is often good. Second, ignoring compression: batts squashed into a too-shallow cavity lose R-value roughly in proportion to how much they are compressed. Third, insulating without air sealing — air leakage frequently moves more heat than conduction does, and insulation does very little to stop it.

Common questions

What R-value do I need in my attic?: Under the 2021 IECC it is R-30 in zone 1, R-49 in zones 2 and 3, and R-60 in zones 4 through 8. Your local jurisdiction may have adopted an earlier code edition with lower values, or amended the table.
Does doubling the R-value halve my heat loss?: Through that assembly, roughly yes — but heat loss through the whole house does not halve, because windows, doors, air leakage and other assemblies are unchanged. Returns diminish as you add more.