How the wall calculation works
1. Measure the net wall area
Choose Measure walls and enter the total length of the exterior walls (the house perimeter), the wall height, and how many doors and windows there are. The calculator works out:
Net wall area = perimeter × wall height − (doors × 21 sq ft) − (windows × 12 sq ft)
Those opening sizes are typical defaults. If you have big patio doors or picture windows, measure them and use Total area instead.
2. Find your target
For walls, the calculator uses the cavity-only option from the 2021 IECC, which ENERGY STAR's table is based on:
| Climate zone | Wall target |
|---|---|
| 1–2 | R-13 |
| 3 | R-20 |
| 4–8 | R-30 |
3. Check what fits
This is where walls differ from attics. Insulation between the studs can't be deeper than the studs. A 2×4 wall has a 3.5 in cavity and a 2×6 wall has 5.5 in. Pick your stud size under Cavity depth and the calculator caps the in-wall insulation at that depth. If the target doesn't fit, it shows the best you can get and warns you.
Wall batts come sized for the cavity: R-13 and R-15 are 3.5 in thick for 2×4s, and R-21 is 5.5 in for 2×6s. The calculator uses 15 in wide batts, made for studs 16 in on center.
Worked example: a one-story house in zone 4A
A 50 × 30 ft house has a 160 ft perimeter. With 9 ft walls, 2 doors and 10 windows:
- Gross wall area: 160 × 9 = 1,440 sq ft
- Minus 2 doors (42 sq ft) and 10 windows (120 sq ft): 1,278 sq ft net
- With 10% waste: 1,406 sq ft
- Zone 4A target: R-30
| Wall and product | Result | To buy | Cost |
|---|---|---|---|
| 2×4, fiberglass batts | R-15 (cavity full) | 21 bags of R-15 | $980–$1,438 |
| 2×6, fiberglass batts | R-21 (cavity full) | 21 bags of R-21 | $1,818–$2,044 |
| 2×6, closed-cell spray foam | R-30 at 4.62 in | Installed by a pro | $8,848–$14,746 |
Batts fall short of R-30 in both walls, and the calculator says so. Closed-cell spray foam gets there inside a 2×6, but its price is an installed cost for a professional job, and spray contractors often have a minimum charge.
The same 2×6 wall with R-21 batts in zone 3A meets that zone's R-20 target with the same 21 bags.
Why R-30 walls need continuous insulation
R-30 doesn't fit in a standard 2×4 or 2×6 cavity with batts. The usual answer is continuous insulation: rigid foam board over the outside of the studs, normally added when the siding comes off. Because it isn't interrupted by studs, the code accepts lower cavity-plus-board combinations as equal to R-30 cavity-only. The PNNL table of IECC requirements lists the combinations for each zone.
You can model this in two passes. First size the cavity insulation. Then set What's there now to "I know the R-value", enter the cavity R, and choose foam board. For the 2×6 wall above, entering R-21 and choosing polyiso gives one layer of 1.65 in R-10 board: 44 sheets, $1,649, for a total of R-31. The calculator simply adds nominal R-values, so treat that as a conservative answer, not the code minimum.
Dense-pack vs. batts
- Open walls (new framing or a gut renovation): batts are the DIY choice. Cut them to fit snugly with no gaps.
- Closed walls: an installer blows dense-pack cellulose or injects foam through holes drilled in the siding or drywall. At the R-3.65 per inch our cellulose data uses, a full 2×4 cavity works out to about R-13, so you get about the same R as a batt, just without opening the wall. The calculator doesn't model dense-pack yet.
Tips and common mistakes
- Don't compress batts. Split a batt around wires and pipes instead of pushing it behind them.
- Fill the whole cavity. Cut batts to fit around electrical boxes and fill the narrow bays beside windows and corners.
- Faced batts: staple the flanges to the sides or faces of the studs, facing the heated room. Whether you need a vapor retarder at all depends on your climate zone, so check local code.
- Match the width to the stud spacing. 15 in batts fit studs 16 in on center; attic batts are wider.
- Check for wiring problems before closing a wall, and never bury old knob-and-tube wiring without an electrician's sign-off.
For depth per R-value across materials, see the R-value chart. For basement walls, use the basement insulation calculator, and check targets on insulation R-values by climate zone.
Sources
- ENERGY STAR: Recommended home insulation R-values
- PNNL Building America Solution Center: 2009–2021 IECC minimum insulation requirements
- Owens Corning PINK Next Gen Fiberglas fact sheet
- ROCKWOOL Comfortbatt R-value fact sheet
- Johns Manville AP Foil-Faced polyiso
- HomeGuide: Spray foam insulation cost (2026)
- Benjamin Moore paint calculator (door and window deductions)