Insulation R-Value Calculator
Find the insulation thickness needed to reach a target R-value or the ENERGY STAR / IECC level for your climate zone, top up existing attic insulation, compare materials and check what a cavity depth delivers, in R and RSI.
How it works
R to add = target R − existing depth × existing R per inch · thickness = R to add ÷ material R per inch · R in a cavity = depth × R per inch · RSI = R ÷ 5.678 · λ (W/m·K) = 0.1442 ÷ R per inch · U = 1 ÷ R
R-value measures resistance to heat flow, and layers in series simply add up. A material’s R per inch times its thickness gives its R, so the thickness to reach a target is the target divided by the R per inch. Choose a custom target, or pick your IECC climate zone and the part of the house to use the ENERGY STAR retrofit levels, which follow the 2021 IECC: R-30 to R-60 for attics, R-13 to R-38 for floors over unheated space, and for walls R-13 to R-20 in the cavity plus foam sheathing in zones 4–8. For a top-up, enter the depth and type of the insulation already there; the calculator subtracts its R and gives only what you need to add. The material table shows how much thinner a high-R foam is than loose-fill, the zone table gives the level and thickness for every zone, and the cavity check tells you whether a stud or joist bay is deep enough. R is in US units (ft²·°F·h/BTU); RSI and λ (lambda) are shown for metric product data.
Worked example
An attic in climate zone 5 should reach R-60. It already has 4 in of old blown fiberglass at R-2.5 per inch, which is R-10, so R-50 must be added. With blown cellulose at R-3.5 per inch that is 50 ÷ 3.5 = 14.3 in; with fiberglass batts at R-3.2 it is 15.6 in. The top-up cuts heat flow through the ceiling insulation by 1 − 10 ÷ 60 = 83%. For a custom target, R-38 with fiberglass batts needs 38 ÷ 3.2 = 11.9 in, while a 9.25 in 2×10 joist bay holds only R-29.6.
Sources: ENERGY STAR — Recommended Home Insulation R-Values · DOE Energy Saver — Insulation · 2021 International Energy Conservation Code, Table R402.1.3
Frequently asked questions
What R-value do I need?
It depends on your climate zone and the part of the house. ENERGY STAR, following the 2021 IECC, recommends R-30 in zone 1, R-49 in zones 2–3 and R-60 in zones 4–8 for an uninsulated attic; R-13 to R-38 for floors over unheated space; and for wood-frame walls R-13 in zones 1–2, R-20 in zone 3 and R-20 plus R-5 foam sheathing in zones 4–8. Choose “Climate zone” to use these levels.
How much should I add to an attic that already has insulation?
Measure the depth and note the type — fluffy fiberglass is about R-2.5 per inch, cellulose about R-3.5. Enter both and the calculator subtracts the existing R from the target. ENERGY STAR’s rule of thumb is to add R-49 in zones 4–8 (R-38 in zones 2–3) over 3–4 in of old insulation.
Why does compressing insulation lower its R-value?
A batt is rated at its full lofted thickness. Squeezing an R-19 batt into a 3.5 in wall cavity drops it to about R-13, because R depends on thickness. Match the batt to the cavity depth, or use a higher R per inch material when the bay is shallow.
Which material gives the most R in the least space?
Closed-cell spray foam (about R-6.5 per inch) and foil-faced polyiso board (about R-6) lead, then XPS (R-5) and EPS (R-4). Batts and cellulose are R-3.2 to R-3.7, and loose-fill fiberglass about R-2.5, so they need two to two and a half times the depth for the same R.
How do I convert R-value to RSI or λ?
RSI = R ÷ 5.678, so R-38 is RSI 6.7 m²·K/W. A material’s conductivity λ in W/m·K is 0.1442 ÷ its R per inch, so R-3.2 per inch equals λ 0.045. You can enter a target in RSI or a material by λ.
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Estimates only — add for waste and confirm against local building codes and supplier specs.