Every insulation sticker rates the material in the cavity. But houses are not made of cavities — they are made of cavities interrupted every 16 inches by a wooden stud, and wood conducts heat roughly three times faster than fiberglass. The wall's real number is lower. Here is the math the sticker skips.
The framing factor
A typical wall is ~23–25% framing by area: studs, plates, headers, and corners. The whole-wall R-value is the area-weighted combination:
Rwall = 1 / (0.25/Rstud + 0.75/Rcavity)
For a 2×6 wall with R-19 batts (studs ≈ R-6.8 effective): R ≈ 1 / (0.25/6.8 + 0.75/19) ≈ 15. The R-19 wall is an R-15 wall before exterior finishes — and headers, corners, and penetrations drag it further.
Where this matters most
| Assembly | Cavity R | Realistic whole-assembly R |
|---|---|---|
| 2×4 wall, R13 batts | 13 | ~10 |
| 2×6 wall, R19 batts | 19 | ~15 |
| Attic joists, R49 loose fill | 49 (flat) | ~42 (joists at 12%) |
| Continuous exterior rigid R5 | +5 | +~5.5 (no framing to bridge — the point) |
The last row is the punchline: continuous insulation outside the framing is the only R that delivers its full sticker value, because it covers the studs instead of dodging them.
What to do with the insight
- Attic first, always: the flat attic plane has the least framing fraction and the cheapest R. The framing-factor penalty is smallest exactly where the payback is best.
- When re-siding, add exterior rigid: the only time continuous wall insulation is cheap — it rides on the siding project.
- Detail the weak points: headers, corners, and rim joists are where framing concentrates; foam board at the rim joist is a weekend project with outsized value.
- Don't chase cavity R into foam: swapping R19 batts for R23 high-density fiberglass buys ~R1.5 whole-wall — real, but small next to an inch of continuous rigid.
The honest expectations
A "R-19" house with R-15 walls, R-42 attic, and leaky rim joists benefits most from the checklist nobody markets: seal the rim, top the attic, and spend the wall budget when the siding is coming off anyway. The sticker number is material; the house number is geometry — and the geometry is where the money hides.