Wall R-Value Calculator: Total R-Value and U-Factor of a Wall
A 2×4 wall with studs 16 in on center, R-13 batts, 1/2 in gypsum, 7/16 in OSB and vinyl siding is R-15.53 through the insulation but R-11.83 effective (U-0.0845) as a whole wall, because the studs, about 25% of the wall, are wood at R-4.38 instead of R-13 insulation: the framing takes 24% off. Choose the framing, the cavity insulation and each layer to see the center-of-cavity R-value, the effective R-value and the U-factor. The figures are typical handbook and maker values, so treat the result as an estimate, not a tested wall rating.
Figures based on the sources listed belowUpdated How we calculate
Sets the cavity depth (3.5 in for a 2×4, 5.5 in for a 2×6) and the default framing fraction: 25% at 16 in on center, 22% at 24 in.
The share of the wall that is wood (studs, plates, headers, corners). Leave 0 for the ASHRAE default of 25% (16 in) or 22% (24 in). Advanced framing is often 15 to 20%.
Batts use their rated R-value; blown and sprayed fills use R per inch over the full cavity depth. A batt thicker than the cavity is scaled down. Typical figures: products vary.
The layer on the inside face of the studs.
Structural sheathing on the outside face of the studs.
Rigid foam board over the sheathing. It covers the studs too, so it adds its full R-value to both paths and is the usual way to beat the framing penalty.
The outer skin. Siding adds little; brick is better but is usually spaced off the wall by an air gap that is not counted here.
The thin layer of still air on each surface. ASHRAE and most code U-factor tables include them: 0.68 inside and 0.17 outside (15 mph wind), for a vertical wall.
The maximum U-factor in the 2021 IECC for a wood-frame wall (above grade). Your jurisdiction may have adopted another edition: check with the building department.
R-11.83 effective.
R-11.83 effective
- Effective (whole-wall) R-value: R-11.83, U-0.0845 (parallel path, framing 25%, 16 in on center)
- Center-of-cavity R-value: R-15.53, U-0.0644 (a path through the insulation only)
- Through-the-framing R-value: R-6.91 (a path through a stud: R-4.38 of wood in place of the insulation)
- Cost of the framing: framing lowers the wall from R-15.53 to R-11.83, 24% less
- 2021 IECC limit, climate zone 3: does not meet the wood-frame wall maximum of U-0.0600: this wall is U-0.0845, over it by 0.0245
- Layer 1, Inside air film: R-0.68 (still air, vertical wall)
- Layer 2, 1/2 in gypsum board: R-0.45
- Layer 3, Fiberglass batt R-13 / 2×4 stud: R-13.00 in the cavity, R-4.38 at the stud (cavity path / stud path (3.5 in × 1.25))
- Layer 4, 7/16 in OSB: R-0.62
- Layer 5, Vinyl siding: R-0.61
- Layer 6, Outside air film: R-0.17 (15 mph wind)
Shopping list
- Fiberglass batt insulation
- Mineral wool batts
- Rigid foam board insulation
- Insulation knife
- Respirator and gloves for insulation
Your project
- Wall framing: 2×4 studs, 16 in on center
- Framing fraction override (optional): 0 %
- Cavity insulation: Fiberglass batt R-13 (3.5 in)
- Interior finish: 1/2 in gypsum board (R-0.45)
- Sheathing: OSB 7/16 in (R-0.62)
- Continuous exterior foam: None
- Cladding: Vinyl siding, hollow-backed (R-0.61)
- Air films: Include inside (R-0.68) and outside (R-0.17) air films
- Compare with the 2021 IECC wood-frame wall limit: Climate zone 3 (U-0.060)
Also pick up
- Fiberglass batt insulation
- Mineral wool batts
- Rigid foam board insulation
- Insulation knife
- Respirator and gloves for insulation
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R-value of each layer this calculator uses, and where it comes from
| Layer | R-value | Basis |
|---|---|---|
| Inside air film | R-0.68 | ASHRAE value as reproduced in the Seattle Energy Code and archtoolbox tables |
| Outside air film (15 mph wind) | R-0.17 | ASHRAE value as reproduced in the Seattle Energy Code and archtoolbox tables |
| 1/2 in gypsum board (R-0.45) | R-0.45 | ASHRAE value as reproduced in secondary tables |
| 5/8 in gypsum board (R-0.56) | R-0.56 | Typical; not confirmed in a table this site could read |
| OSB 7/16 in (R-0.62) | R-0.62 | Typical; published values run R-0.5 to R-0.6 |
| Plywood 1/2 in (R-0.63) | R-0.63 | ASHRAE value as reproduced in secondary tables (0.62 or 0.63) |
| Plywood 5/8 in (R-0.77) | R-0.77 | ASHRAE value as reproduced in secondary tables |
| Vinyl siding, hollow-backed (R-0.61) | R-0.61 | ASHRAE value as reproduced in secondary tables |
| Wood bevel lap siding (R-0.80) | R-0.80 | ASHRAE value as reproduced in secondary tables |
| Face brick, 4 in (R-0.44) | R-0.44 | ASHRAE value as reproduced in secondary tables; other tables give R-0.32 |
| Common brick, 4 in (R-0.80) | R-0.80 | ASHRAE value as reproduced in secondary tables |
| Fiberglass batt R-13 (3.5 in) | R-13 per batt | Manufacturer rating (Owens Corning) |
| Fiberglass batt R-15 (3.5 in) | R-15 per batt | Manufacturer rating; thickness not confirmed |
| Fiberglass batt R-19 (6.25 in) | R-19 per batt | Manufacturer rating (Owens Corning) |
| Fiberglass batt R-21 (5.5 in) | R-21 per batt | Manufacturer rating (Owens Corning) |
| Mineral wool batt R-15 (3.5 in) | R-15 per batt | Manufacturer rating (ROCKWOOL Comfortbatt) |
| Mineral wool batt R-23 (5.5 in) | R-23 per batt | Manufacturer rating (ROCKWOOL Comfortbatt) |
| Dense-pack cellulose (R-3.5 per inch) | R-3.5 per inch | Typical; sources give R-3.2 to R-3.8 per inch |
| Open-cell spray foam (R-3.6 per inch) | R-3.6 per inch | Typical (DOE Building America spray foam guide); varies by product |
| Closed-cell spray foam (R-6 per inch) | R-6 per inch | Typical; sources give R-6 to R-7 per inch |
| XPS foam board | R-5 per inch | Manufacturer (Owens Corning FOAMULAR data sheets) |
| EPS foam board | R-4 per inch | Typical nominal; one maker lists R-3.85 at 75°F and R-4.35 at 25°F |
| Polyiso foam board | R-6 per inch | Manufacturer (Johns Manville) at 75°F; lower in cold weather |
| Softwood framing | R-1.25 per inch | Typical ASHRAE-attributed figure; sources give about 1.1 to 1.5 |
How much framing costs: center of cavity against effective R-value
| Cavity insulation | Framing | Center of cavity | Effective | U-factor | Framing penalty |
|---|---|---|---|---|---|
| Fiberglass batt R-13 | 2×4 studs, 16 in on center | R-15.53 | R-11.83 | U-0.0845 | 24% |
| Fiberglass batt R-13 | 2×4 studs, 24 in on center | R-15.53 | R-12.18 | U-0.0821 | 22% |
| Fiberglass batt R-13 | 2×6 studs, 16 in on center | R-15.53 | R-13.36 | U-0.0749 | 14% |
| Fiberglass batt R-13 | 2×6 studs, 24 in on center | R-15.53 | R-13.58 | U-0.0736 | 13% |
| Fiberglass batt R-15 | 2×4 studs, 16 in on center | R-17.53 | R-12.66 | U-0.0790 | 28% |
| Fiberglass batt R-15 | 2×4 studs, 24 in on center | R-17.53 | R-13.10 | U-0.0764 | 25% |
| Fiberglass batt R-15 | 2×6 studs, 16 in on center | R-17.53 | R-14.42 | U-0.0694 | 18% |
| Fiberglass batt R-15 | 2×6 studs, 24 in on center | R-17.53 | R-14.73 | U-0.0679 | 16% |
| Fiberglass batt R-21 | 2×6 studs, 16 in on center | R-23.53 | R-17.11 | U-0.0585 | 27% |
| Fiberglass batt R-21 | 2×6 studs, 24 in on center | R-23.53 | R-17.69 | U-0.0565 | 25% |
| Mineral wool batt R-15 | 2×4 studs, 16 in on center | R-17.53 | R-12.66 | U-0.0790 | 28% |
| Mineral wool batt R-15 | 2×4 studs, 24 in on center | R-17.53 | R-13.10 | U-0.0764 | 25% |
| Mineral wool batt R-15 | 2×6 studs, 16 in on center | R-17.53 | R-14.42 | U-0.0694 | 18% |
| Mineral wool batt R-15 | 2×6 studs, 24 in on center | R-17.53 | R-14.73 | U-0.0679 | 16% |
| Mineral wool batt R-23 | 2×6 studs, 16 in on center | R-25.53 | R-17.87 | U-0.0560 | 30% |
| Mineral wool batt R-23 | 2×6 studs, 24 in on center | R-25.53 | R-18.54 | U-0.0539 | 27% |
| Dense-pack cellulose | 2×4 studs, 16 in on center | R-14.78 | R-11.50 | U-0.0869 | 22% |
| Dense-pack cellulose | 2×4 studs, 24 in on center | R-14.78 | R-11.82 | U-0.0846 | 20% |
| Dense-pack cellulose | 2×6 studs, 16 in on center | R-21.78 | R-16.39 | U-0.0610 | 25% |
| Dense-pack cellulose | 2×6 studs, 24 in on center | R-21.78 | R-16.89 | U-0.0592 | 22% |
| Open-cell spray foam | 2×4 studs, 16 in on center | R-15.13 | R-11.66 | U-0.0858 | 23% |
| Open-cell spray foam | 2×4 studs, 24 in on center | R-15.13 | R-11.99 | U-0.0834 | 21% |
| Open-cell spray foam | 2×6 studs, 16 in on center | R-22.33 | R-16.62 | U-0.0602 | 26% |
| Open-cell spray foam | 2×6 studs, 24 in on center | R-22.33 | R-17.15 | U-0.0583 | 23% |
| Closed-cell spray foam | 2×4 studs, 16 in on center | R-23.53 | R-14.69 | U-0.0681 | 38% |
| Closed-cell spray foam | 2×4 studs, 24 in on center | R-23.53 | R-15.38 | U-0.0650 | 35% |
| Closed-cell spray foam | 2×6 studs, 16 in on center | R-35.53 | R-20.97 | U-0.0477 | 41% |
| Closed-cell spray foam | 2×6 studs, 24 in on center | R-35.53 | R-22.05 | U-0.0453 | 38% |
2021 IECC maximum U-factor for a wood-frame wall
| Climate zone | Maximum U-factor | Equivalent effective R-value |
|---|---|---|
| 0 to 2 | 0.084 | R-11.90 |
| 3 | 0.060 | R-16.67 |
| 4 to 8 | 0.045 | R-22.22 |
Framing fraction by stud spacing
| Framing | Framing fraction | Cavity depth | Stud R-value (R-1.25 per inch) |
|---|---|---|---|
| 2×4 studs, 16 in on center | 25% | 3.5 in | R-4.38 |
| 2×4 studs, 24 in on center | 22% | 3.5 in | R-4.38 |
| 2×6 studs, 16 in on center | 25% | 5.5 in | R-6.88 |
| 2×6 studs, 24 in on center | 22% | 5.5 in | R-6.88 |
Methodology & assumptions
A wall is a stack of layers that heat passes through in series, so their R-values add. The tool builds the stack from inside to outside: the inside air film (R-0.68), the interior finish, the framing cavity, the sheathing, any continuous exterior foam, the cladding and the outside air film (R-0.17). It has a slot for each of these layers rather than a free list. Heat does not only go through the insulation, though: through a stud it meets wood instead, so two paths are worked. 1) Center of cavity: the sum of every layer with the cavity insulation in the framing cavity. Batts use their rated R-value; blown and sprayed fills use their R per inch times the cavity depth (3.5 in for a 2×4, 5.5 in for a 2×6). 2) Through the framing: the same sum with the stud in place of the insulation. Wood is taken as R-1.25 per inch of depth, so a 2×4 is R-4.38 and a 2×6 is R-6.88. 3) Parallel path: U = f ÷ R(framing) + (1 − f) ÷ R(cavity), where f is the framing fraction, and the effective R-value is 1 ÷ U. U is in BTU/hr·ft²·°F, the heat lost through a square foot for each degree of temperature difference, and a lower U is a better wall. The framing fraction is the share of the wall area that is wood, counting studs, plates, headers, corners and cripples: the ASHRAE Handbook of Fundamentals default, as quoted by the American Wood Council, is 25% for studs 16 in on center and 22% for 24 in on center. A wall with advanced framing can be 15 to 20%, so you can type your own figure. Anything outside the studs is continuous, so rigid foam, sheathing and cladding count in both paths; this is why exterior foam is the usual way to reduce the penalty for framing. A batt thicker than the cavity (an R-19 in a 2×4 wall, say) is compressed and loses R-value. This tool scales the rating down in proportion to the depth and shows a warning, which is only an estimate. A batt thinner than the cavity (an R-13 in a 2×6 wall) is counted at its rating, with nothing added for the gap behind it. Limits: the parallel-path method ignores heat that moves sideways between the two paths; it counts no air leakage, no extra thermal bridging at windows and doors, and it assumes the insulation is fitted with no gaps, voids or compression, which a real job seldom achieves. The air films are for a vertical wall; a ceiling or floor uses different values, so use this for walls. Brick cladding is counted as the brick only, without the air gap behind it, and an empty cavity is counted as R-1.01 of still air, a figure not checked against a table. The code comparison uses the maximum U-factors for a wood-frame wall in the 2021 International Energy Conservation Code: 0.084 for climate zones 0 to 2, 0.060 for zone 3 and 0.045 for zones 4 to 8. Your jurisdiction may use another edition or a different path (R-value tables rather than U-factor), and the code is the authority: ask the building department. The value for each material and where it was found is in the table below. Where a value is labeled typical, it is a typical figure that varies by product; use the maker’s figure when you have it.
- American Wood Council, DCA 7: Meeting Residential Energy Requirements with Wood Frame Construction (2012 IECC version): the ASHRAE Handbook of Fundamentals parallel-path method for wood-frame walls and its default framing factors of 25% for studs 16 in on center and 22% for 24 in on center
- American Wood Council, What are the thermal R-values for wood? (FAQ): the thermal resistance of softwood, about R-1.1 per inch in the 1993 ASHRAE figure it cites; other sources, and this tool, use R-1.25 per inch
- ICC, code change proposal REPI-33-21 to Table R402.1.2 of the 2021 IECC (IECC-RES-CAR-update): maximum assembly U-factors for a wood-frame wall of 0.084 (climate zones 0 to 2), 0.060 (zone 3) and 0.045 (zone 4 and above)
- Premier SIPS, Technical Bulletin TechBC-1, IECC U-Factor Requirements: the 2021 IECC wood-framed wall U-factors by climate zone, as a cross-check
- City of Seattle, Seattle Energy Code Appendix A (2021): default building-material R-values and air films as taken from the ASHRAE Handbook of Fundamentals, including the inside film of 0.68 and the outside film of 0.17. A secondary reproduction of ASHRAE, not the Handbook itself
- Architecture Toolbox, R-Values of common building materials: gypsum 1/2 in R-0.45, plywood 1/2 in R-0.62 and the air films. A secondary table that cites ASHRAE
- Residential Shipping Container Primer, Insulation Values for Selected Materials: vinyl siding R-0.61, wood bevel lap siding R-0.80, 4 in face brick R-0.44, common brick R-0.80, softwood R-1.25 per inch and 5/8 in plywood R-0.77. A secondary table, not the ASHRAE Handbook; these values could not be checked against it
- Owens Corning, FOAMULAR XPS insulation product data sheets (400, 600, 1000; copy hosted by a distributor): R-5 per inch for 1 in boards at 75°F
- Johns Manville, XSPECT ISOfoam polyisocyanurate board product data: R-6.0 for a 1 in board at 75°F. Polyiso tests lower at cold temperatures
- Insulfoam, R-Tech EPS overview data sheet: R-3.85 per inch at 75°F, 4.17 at 40°F and 4.35 at 25°F, so R-4 per inch is a nominal figure
- ROCKWOOL, Comfortbatt product page and R-value expansion fact sheet: mineral wool batts of R-15 at 3.5 in and R-23 at 5.5 in. The product page also lists R-14 and R-22, so the rating depends on the edition of the sheet
- Owens Corning, EcoTouch PINK Fiberglas Insulation product data: fiberglass batts from R-11 to R-49 in 3.5 to 14 in thicknesses; R-13 at 3.5 in, R-19 at 6.25 in and R-21 at 5.5 in on distributor listings
- US Department of Energy, Building America Spray Foam Guide: open-cell spray foam about R-3.6 per inch. Closed-cell figures run about R-6 to R-7 per inch in other sources, so R-6 is used; cellulose at R-3.5 is the middle of the R-3.2 to R-3.8 range given in trade sources
- Building America Solution Center (PNNL), Typical R-values and vapor retarder classifications of common insulation materials: R per inch ranges for EPS, XPS, polyiso and mineral wool
- About these figures: the air film, gypsum, plywood, OSB, siding and brick values are ASHRAE Handbook of Fundamentals figures as reproduced in published secondary tables. The R-15 batt thickness, 5/8 in gypsum (0.56), OSB (0.62), the R-1.01 empty cavity and R-6 per inch for closed-cell foam are typical values; product data sheets vary, so use your product’s label when you have it
How to find the R-value and U-factor of a wall
- Pick the framing. Choose 2×4 or 2×6 studs at 16 or 24 in on center. This sets the cavity depth and the framing fraction (25% or 22%).
- Choose the cavity insulation. Pick a batt by its R-value, or a blown or sprayed fill by its R per inch, which is multiplied by the cavity depth.
- Choose the other layers. Pick the interior finish, the sheathing, any continuous exterior foam and its thickness, and the cladding. Leave the air films in unless you want the layers alone.
- Add the layers. Add every layer from inside to outside to get the center-of-cavity R-value, then add the layers again with the stud in place of the insulation to get the through-the-framing R-value.
- Combine the two paths. U = framing fraction ÷ framing R + (1 − framing fraction) ÷ cavity R. The effective R-value is 1 ÷ U.
- Compare with the code. Choose your climate zone to see whether the U-factor is at or under the 2021 IECC limit for a wood-frame wall, and check with your building department.
Worked examples
| Project | Result |
|---|---|
| 2×4 at 16 in, R-13 batt, gypsum, OSB, vinyl siding | R-11.83 effective, U-0.0845 (center of cavity R-15.53) |
| The same wall with R-15 mineral wool | R-12.66 effective, U-0.0790 |
| The same wall with 1 in of XPS under the siding | R-17.38 effective, U-0.0575 |
| 2×6 at 16 in with R-21 batts | R-17.11 effective, U-0.0585 (center of cavity R-23.53) |
| 2×6 at 24 in with R-21 batts (22% framing) | R-17.69 effective, U-0.0565 |
| 2×6 at 16 in, R-21 batts and 2 in of XPS | R-28.37 effective, U-0.0353 |
| 2×6 at 16 in, closed-cell spray foam filling the cavity | R-20.97 effective, U-0.0477 |
| Uninsulated 2×4 wall (empty cavity) | R-4.03 effective, U-0.2481 |
Frequently asked questions
- How do you calculate the R-value of a wall?
- Add the R-value of every layer from the inside air film to the outside air film. A 2×4 wall with R-13 batts, 1/2 in gypsum, 7/16 in OSB and vinyl siding adds up to R-15.53 through the insulation. That is the center-of-cavity value. Studs are not insulated, so the wall as a whole is lower: with 25% framing the effective R-value is R-11.83 effective (U-0.0845).
- Why is the effective R-value lower than the R-value of the insulation?
- Heat takes the easy path. A stud is wood, about R-1.25 per inch, so a 3.5 in stud is R-4.38 against R-13 for the batt beside it, and about a quarter of the wall is stud. The parallel-path method averages the two paths by their U-factors, not their R-values: 0.25 ÷ 6.91 + 0.75 ÷ 15.53 = 0.0845, and 1 ÷ 0.0845 = 11.83.
- What is a U-factor, and how does it relate to R-value?
- U is the rate of heat flow through a square foot of the wall for each degree Fahrenheit of temperature difference, in BTU/hr·ft²·°F, and it is the inverse of R: U = 1 ÷ R. The wall above is U-0.0845. Energy codes such as the IECC often state a maximum U-factor for a whole assembly, because it already counts the framing. A lower U is better.
- What framing fraction should I use?
- The ASHRAE Handbook of Fundamentals default, as quoted by the American Wood Council, is 25% for studs 16 in on center and 22% for 24 in on center, counting the studs, plates, headers, corners and cripples. A house with advanced framing (two-stud corners, insulated headers and fewer jack studs) can come in at 15 to 20%. Enter your own figure in the override field if you have counted it.
- How much does exterior foam help?
- Foam over the sheathing covers the studs as well, so all of its R-value counts in both paths. One inch of XPS takes the default wall from R-11.83 to R-17.38 effective, and two inches of EPS gives R-20.56 effective. A 2×6 wall with R-21 batts at 16 in is R-17.11 effective; with 2 in of XPS it is R-28.37 effective.
- Is an R-21 batt in a 2×6 wall an R-21 wall?
- No. At 16 in on center it is R-23.53 through the cavity and R-17.11 effective for the whole wall (27% less). With studs 24 in on center the framing is 22% and the wall is R-17.69 effective.
- Do the air films count?
- Yes in the usual code and handbook calculations: R-0.68 inside and R-0.17 outside, which together add R-0.85. Leaving them out takes the default wall from R-11.83 to R-10.83. Use the “leave them out” setting to compare with a figure quoted for the layers alone.
- Does my wall meet the energy code?
- Choose your climate zone to compare the U-factor with the 2021 IECC maximum for a wood-frame wall: 0.084 for zones 0 to 2, 0.060 for zone 3 and 0.045 for zones 4 to 8. A 2×6 wall with R-21 batts at 24 in and 1 in of XPS is U-0.0426, at or under the 0.045 limit. The code is the authority: your jurisdiction may use another edition or accept an R-value table instead, and this tool does not count windows, doors, air leakage or installation quality.
- How accurate is this?
- It is a handbook calculation from typical material values, not a tested rating. Real walls differ: batts are rarely perfect, the framing fraction varies with the openings, and insulation values change with temperature and age. For scale, an empty 2×4 cavity comes to R-4.03 effective and a closed-cell foam 2×6 wall to R-20.97 effective. Brick on a 2×6 R-21 wall is R-16.90 effective, advanced framing at 17% on the default wall gives R-12.81 effective, and R-15 mineral wool in place of the R-13 fiberglass batt gives R-12.66 effective.