Insulation R-Value and Savings
Target R-value to the thickness you need, the rolls, bags, sheets, or board feet it takes, the whole-wall derate for framing, the annual fuel savings, and the payback.
A 2x4 wall at R-13 effective with fiberglass batts needs about 5.5 in of batt, not 4.2 in, because the studs, plate, and drywall screws short-circuit about a quarter of the cavity R-value. R-13 batt in a 2x4 wall is the IECC zone 4 baseline with R-10 continuous.
How the math works
- R per inch by product: standard fiberglass batt 3.14, high-density batt 3.7, dense-pack blown 3.7, mineral wool batt 3.3, loose-fill attic 2.2 to 2.3, open-cell spray 3.6, closed-cell spray 6.5, polyiso 6.0, XPS 5.0, EPS 4.0 per inch. Label R at the labeled thickness, and a batt compressed to fit a shallow cavity loses R.
- Whole-wall factor: a wood-frame wall is 15 to 25 percent framing by area, and the studs, plates, drywall fasteners, and outlet boxes carry heat straight through. The industry whole-wall derate is about 0.75 on cavity batts, 0.90 on dense-pack blown, 0.95 on spray foam, and about 1.00 on continuous rigid over the framing. The attic floor derate is about 0.95.
- Why R-13 in a 2x4 is not R-13: the cavity is 3.5 in, so R-13 batt fills it, and the whole wall measures about R-9 to R-11 with sheathing, films, and bridging. That is why the IECC moved to cavity plus continuous: R-13 cavity with R-10 continuous in zone 4, R-13 plus R-13 in zone 5, and R-21 with continuous in zones 6 to 8.
- Fuel savings: annual BTU saved is wall area times heating degree days times 24, times the difference in R, so (1/R-old minus 1/R-new). Convert at 1,000 BTU per therm, 91,500 per gallon of propane, 138,900 per gallon of oil, and 3,412 BTU per kWh, then divide by the equipment efficiency (0.92 gas, 0.88 oil, 1.00 electric resistance, about 2.0 for a cold-climate heat pump at the annual average).
- Air sealing first: leaks, not the R-value, run most heating bills. A dense-pack wall, spray foam, and the rim joist all address air movement, and 3 to 5 air changes per hour at 50 Pascals on a blower door is the practical target after the work.
Why is my wall R-value lower than the batt label?
Because the wall is not all cavity. Framing covers 15 to 25 percent of the wall area and conducts straight through, and a whole R-13 batt wall measures R-9 to R-11. The fix is continuous insulation over the sheathing or plate, which is exactly why code now asks for cavity plus continuous R.
How many rolls of R-13 insulation do I need?
At 40 sq ft nominal per roll (a 15 in by 32 ft roll for 16 in spacing), divide your wall area by 40 and add 10 percent for the cuts around headers, top plates, and outlets: 300 sq ft of wall is 9 rolls. Batt for 24 in spacing is a 20 in width, and it must be friction-fit, not compressed.
Is spray foam worth it over batt insulation?
On a per-inch basis, yes, and it is the air barrier in the same operation. Closed-cell at 2 in on a rim joist or a crawl ceiling is the classic payback job, and open-cell at 5 in in a 2x6 wall is the cheaper fill. Batt with the air sealed by hand, dense-pack, and the rim joist boxed with foam gets close for a fraction of the cost.
What R-value should I insulate to?
Cavity plus continuous at your climate zone prescriptive minimum is the code floor, and the retrofit sweet spot is 1.5 to 2x that: R-21 wall and R-49 to R-60 attic in a cold zone. The diminishing returns start where the payback crosses 12 to 15 years, which is where the fuel price and the comfort call converge.