R-Value Calculator results show the whole-wall R of a wood stud wall once the framing is counted, or the R of an attic ceiling, with heat loss and the 2021 IECC path for your zone.
Enter the R printed on the batt or board label; framing is handled for you.
Layers and Heat Loss
2021 IECC Insulation by Zone
| Zone | Ceiling | Wood Wall | Floor |
|---|---|---|---|
| Zone 0–1 | R-30 | 13 or 0+10ci | R-13 |
| Zone 2 | R-49 | 13 or 0+10ci | R-13 |
| Zone 3 | R-49 | 20 or 13+5ci or 0+15ci | R-19 |
| Zone 4 | R-60 | 30 or 20+5ci or 13+10ci or 0+20ci | R-19 |
| Zone 5 | R-60 | 30 or 20+5ci or 13+10ci or 0+20ci | R-30 |
| Zone 6 | R-60 | 30 or 20+5ci or 13+10ci or 0+20ci | R-30 |
| Zone 7–8 | R-60 | 30 or 20+5ci or 13+10ci or 0+20ci | R-38 |
R-Value Calculator for Walls and Attics
The R on an insulation label is not the R of the wall. Wood studs carry heat around the batts, and the drywall, sheathing, siding and air films each add a little resistance of their own.
Builders and remodelers use the result to compare a thicker batt against foam board, and to check a wall or attic against the energy code. Homeowners use it to see what their existing insulation is worth and how much heat it lets through.
The R-Value Calculator works out the assembly the way code calculations do. It gives the whole-wall or ceiling R-value, the R through the cavity and through the studs, the U-factor and heat loss, and the 2021 IECC path closest to what you have.
Wall Mode, Attic Mode and the Layers Behind Them
Wall mode in the R-Value Calculator takes the framing as 2×4 or 2×6 at 16 or 24 inches on center, the cavity insulation R and any continuous insulation over the studs. Enter the R printed on the batt or board label, since the tool handles the framing itself.
Attic mode takes the installed R of the ceiling insulation at its settled depth. Both modes use the IECC climate zone for the code check, and US mode works in R and °F while metric mode works in RSI and °C.
The accordion holds the interior finish, sheathing and siding, plus an Other Layers field for anything else in the assembly. It also takes the area and the inside-to-outside temperature difference used for heat loss.
Why Framing Pulls the Whole-Wall R-Value Down
Layers stacked one behind another simply add. The cavity path runs through the air films, finish, sheathing, siding, continuous insulation and the batt, while the stud path swaps the batt for solid wood.
$$R_{cavity} = R_{layers} + R_{batt}$$
$$R_{stud} = R_{layers} + 1.25 \times d$$
Softwood resists about R-1.25 per inch, so a 3.5 inch 2×4 stud is worth about R-4.4 against R-13 in the cavity beside it. Heat then flows through both paths side by side, which is why the two are combined as U-factors weighted by the framing fraction f.
$$U = \frac{f}{R_{stud}} + \frac{1 – f}{R_{cavity}}$$
$$R_{wall} = \frac{1}{U}$$
The California Energy Commission’s wall U-factor tables use this parallel heat flow method from the ASHRAE Handbook. They take 25% framing for studs at 16 inches and 22% at 24 inches, the same fractions the R-Value Calculator uses, with R-0.68 for the inside air film and R-0.17 outside.
The default wall is a 2×4 at 16 inches with R-13 batts, R-5 foam, 1/2 inch drywall, 7/16 inch OSB and hardboard siding. Its cavity path is R-20.6 and its stud path R-12.0, so the whole wall lands at R-17.4, about 15% below the cavity alone.
The common mistake is averaging the two R-values by area, which overstates the wall. A GreenBuildingAdvisor thread on framing factors makes the same point, and it notes that ASHRAE raised its framing fractions from 15% and 12% to 25% and 22% because real houses carry far more wood than stud spacing suggests.
Continuous Insulation or a Deeper Cavity
A plain 2×4 wall with R-13 batts works out to about R-11.9 whole wall. Upgrading to R-15 batts only reaches R-12.7, because the studs still bridge every 16 inches.
Adding R-5 of foam board over the same R-13 wall lifts it to R-17.4, since continuous insulation covers the studs as well as the bays. That matches a 2×6 wall with R-21 batts and no foam, which comes out at R-17.2 at 16 inches.
In the R-Value Calculator, the fourth result card shows this for your own wall. It adds R-5 and R-10 of continuous insulation to the current assembly, so you can see the new whole-wall R and the cut in heat loss before buying anything.
Siding and sheathing values come from the ASHRAE Handbook table reproduced in ASHRAE’s published errata, such as R-0.62 for 7/16 inch OSB, R-0.68 for 1/2 inch OSB and R-0.67 for 7/16 inch hardboard siding. An empty stud bay counts as about R-0.91 of still air, so a 2×4 wall with nothing in it drops to about R-4.0.
Meeting the 2021 IECC Wall and Ceiling Minimums
The R-Value Calculator’s third card checks your insulation against 2021 IECC Table R402.1.3. That table lists the R of the installed insulation, not the whole assembly, so the check compares your batt and board labels with each listed path.
| Climate Zone | Ceiling | Wood-Frame Wall | Floor |
|---|---|---|---|
| 0 and 1 | R-30 | 13 or 0+10ci | R-13 |
| 2 | R-49 | 13 or 0+10ci | R-13 |
| 3 | R-49 | 20 or 13+5ci or 0+15ci | R-19 |
| 4 except Marine | R-60 | 30 or 20+5ci or 13+10ci or 0+20ci | R-19 |
| 5 and Marine 4 | R-60 | 30 or 20+5ci or 13+10ci or 0+20ci | R-30 |
| 6 | R-60 | 30 or 20+5ci or 13+10ci or 0+20ci | R-30 |
| 7 and 8 | R-60 | 30 or 20+5ci or 13+10ci or 0+20ci | R-38 |
In the wall column, 13+10ci means R-13 in the cavity plus R-10 of continuous insulation, and meeting any one path complies. The default R-13 plus R-5 wall falls short in Zone 4, and the closest fix is another R-5 of foam to reach 13+10ci.
The paths line up with the code’s U-factor alternative. Delaware’s summary of the 2021 changes shows the Zone 4 wall U-factor tightening to 0.045, and an R-13 plus R-10 wall on a 2×4 works out to U-0.044, or R-22.7 whole wall.
For ceilings, the tool notes that R-49 can count as R-60 where the full depth extends over the top plate at the eaves. Many states adopt an older or amended edition, so confirm the version your building department enforces.
Heat Loss Through the Wall or Ceiling
Once the U-factor is known, conducted heat loss is the U-factor times the area times the temperature difference. The result is in Btu per hour in US mode and watts in metric mode.
$$Q = U \times A \times \Delta T$$
The default 1,000 sq ft of wall at a 50 °F difference loses about 2,866 Btu/h through U-0.0573. Windows, doors and air leaks are not part of this figure, so the Heat Loss Calculator is the place for a whole-room total.
Used as an attic R-value calculator, the tool treats the ceiling insulation as unbroken, so its R adds straight to the drywall and air films. Ceiling joists buried under deep loose fill have little effect, which is why topping up an attic pays back more directly than adding to a framed wall.
R-Value Questions About Walls, Attics and Air
What is the R-value of a 6 inch wall?
A 2×6 wall with R-21 batts at 16 inches on center works out to about R-17.2 whole wall with drywall, OSB and hardboard siding. At 24 inches on center, less framing lifts it to about R-17.8.
The label says R-21, but the 5.5 inch studs bridge a quarter of the wall at only about R-6.9 each. Adding R-5 of foam board outside the same wall covers those studs and pushes it to about R-23.
What is a good R-value for a wall?
Under the 2021 IECC, a good wall depends on your zone. Zones 0 to 2 accept R-13 cavity insulation, and Zone 3 calls for R-20 or R-13 plus R-5 continuous. Zones 4 to 8 call for R-30, R-20 plus R-5, R-13 plus R-10 or R-20 continuous.
In whole-wall terms, those Zone 4 and colder paths land at about R-22 or more once framing is counted. A typical older 2×4 wall with R-11 or R-13 batts sits near R-11 to R-12.
What is the R-value of 1 inch of air?
An enclosed, still air space between 1/2 inch and 4 inches thick is worth about R-1, according to the material table in the Alaska Housing Finance Corporation building manual. Making the gap thicker adds almost nothing, because air starts to circulate and carry heat across.
A reflective foil face on one side of the space raises it noticeably. Air that moves freely, such as a leaky empty wall bay, is worth less than the still-air figure.
Is R-30 or R-38 better?
R-38 is better. Over 1,000 sq ft of ceiling at a 50 °F difference, R-30 lets through roughly 1,600 Btu/h and R-38 roughly 1,270 Btu/h, a 20% cut once drywall and air films are added.
Both fall below the 2021 IECC in most of the country. R-30 meets the ceiling minimum only in Zones 0 and 1, Zones 2 and 3 call for R-49, and Zone 4 and colder call for R-60.
How do I check the R-value of my walls?
Look for a label on exposed batts in the attic, basement or an outlet box opening, or measure the cavity depth and identify the insulation type. The Alaska Housing Finance Corporation table puts fiberglass batts at about R-3.14 per inch, so a full 3.5 inch bay holds about R-11.
Enter that R as cavity insulation with your stud size and spacing. The R-Value Calculator then adds the framing, sheathing and siding to give the whole-wall figure.
How do I convert m²K/W to an R-value?
Multiply RSI in m²K/W by 5.678 to get the US R-value in ft²·°F·h/Btu, or multiply R by 0.1761 to go the other way. An RSI 3.52 wall is about R-20, and an R-13 batt is about RSI 2.29.
Switching the R-Value Calculator to metric shows every input and result in RSI. The line under the main result also gives the other unit, so both figures appear together.