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R-49 is the common target for the roof or ceiling in cold and mixed climates. That is about 20 inches of blown fibreglass, 15 inches of blown cellulose, 16 inches of fibreglass batt or about 8 inches of closed-cell spray foam. Air seal the ceiling plane before adding any of it.
An R-13 batt does not make an R-13 wall. Studs are wood, and wood insulates about R-1.25 per inch, so heat takes a shortcut through the framing. Counting studs, drywall, sheathing and the air films, a 2×4 wall with R-13 batts works out to about R-11 overall. Enter your wall and see its effective R-value and U-factor, and what exterior foam adds.
With these inputs:
| Result | Value |
|---|---|
| Effective whole-wall R-value | R-11.1 |
| U-factor | 0.09 |
| Through the insulated cavity | R-14.92 |
| Through a stud | R-6.3 |
| Framing fraction used | 25% |
Why exterior foam helps more than a thicker batt. Continuous foam outside the sheathing covers the studs as well as the cavities, so every square foot of wall gets its full R-value. Raising the batt from R-13 to R-15 only improves the 75% of the wall that is cavity. The table shows the difference: R-5 of foam on a 2×4 wall adds more to the effective R-value than the batt upgrade. Foam on the outside also keeps the sheathing warmer in winter, which lowers the risk of condensation inside the wall. The building code's vapor retarder rules take exterior foam thickness into account, so check them for your climate zone before choosing a thickness.
Each path = inside air film 0.68 + drywall + cavity (or stud) + sheathing + foam + siding + outside air film 0.17 · Stud R = depth × 1.25 per inch · U = framing fraction ÷ R(stud path) + (1 − framing fraction) ÷ R(cavity path) · Effective R = 1 ÷ U
| Wall | Insulation on the label | Effective R | U-factor |
|---|---|---|---|
| 2×4 16 in o.c., R-13 batts | R-13 | R-11.1 | 0.09 |
| 2×4 16 in o.c., R-15 batts | R-15 | R-11.9 | 0.084 |
| 2×4 16 in o.c., R-13 + R-5 foam | R-18 | R-16.7 | 0.06 |
| 2×6 16 in o.c., R-21 batts | R-21 | R-16.4 | 0.061 |
| 2×6 24 in o.c., R-21 batts | R-21 | R-16.9 | 0.059 |
| 2×6 24 in o.c., R-21 + R-10 foam | R-31 | R-28.2 | 0.035 |
Computed with this calculator by the parallel-path method. Headers, sills, corners and plates add more framing than the standard fraction, so real walls can be a little lower.
About R-11 with 1/2 in drywall, 7/16 in OSB and air films, using the parallel-path method with 25% framing at 16 in on center. The studs, at about R-4.4 for 3.5 in of wood, pull the whole-wall figure below the batt's rating.
Add up the R-values of each layer along two paths, one through the insulation and one through a stud, including the inside (R-0.68) and outside (R-0.17) air films. Then weight them by area: U = framing fraction ÷ R(stud path) + cavity fraction ÷ R(cavity path), and effective R = 1 ÷ U.
ASHRAE Standard 90.1 uses 25% for wood studs at 16 in on center and 22% at 24 in. Walls with many windows, corners and headers have more framing than that.
No. Prescriptive code tables list the R-value of the insulation you install, such as R-20 or R-13 + R-5; the code's alternative U-factor tables compare whole assemblies, which is what this calculator's U-factor gives you.
Need the whole roof at once? The free roofing calculator works out roof area, squares, shingle bundles, underlayment, drip edge, ridge cap and installed cost from one set of measurements.