One climate model, county by county
Every county page starts from DOE's IECC climate zone classification (1 through 8) and the matching recommended attic R-value from ENERGY STAR's own table, for both a currently uninsulated attic and one with 3-4 inches already present. Los Angeles County, CA and Cook County, IL sit in different zones with very different targets — the R-value recommendation is a climate fact, not a guess.
Where the degree days come from
Heating and cooling degree days (base 65°F) are NOAA's own 1991-2020 climate normals, matched to each county by its nearest reporting station with published annual normals — the same nearest-station approach used across this factory's other climate-driven models. Some stations lack degree-day normals even when they report other data, so each county checks up to six nearby candidates before being included.
How the cost and savings are calculated
Install cost uses a blended national rate per square foot per R-value point added, from 2026 Angi and HomeGuide contractor-cost data, with a $600 minimum job floor. Savings are calculated from the actual reduction in heat lost and gained through the ceiling at the new R-value, converted to dollars using national average energy rates (18¢/kWh electricity, $19.83/Mcf gas — EIA, 2026), not a per-state rate. Every constant used is named in the model source.
What isn't modeled, and why
The federal §25C tax credit used to cover 30% of insulation cost, but it expired December 31, 2025 — this site does not add it back into any estimate, even though older guides elsewhere still quote it. Connecticut's county-equivalent planning regions (adopted 2022) aren't covered, the same DOE crosswalk gap several other county-level climate datasets in this factory run into. A handful of counties nationally are dropped entirely where no nearby NOAA station publishes degree-day normals at all.
Where to go next
See the range this plays out across: North Slope Borough, AK, Richmond County, NY, Río Grande Municipio, PR.
What Attic R-Value Actually Means (and Why the Target Changes by Climate) · Attic Insulation: DIY or Hire a Contractor?