Charleston County, SC Insulation Payback
Charleston County attic insulation, by the numbers
Charleston County, South Carolina is zone 3 under DOE's climate map (Hot-Humid), which means summers that stay hot and humid while winters barely register on the energy bill. An uninsulated attic there should be built up to R-49. Topping up a typical 1,000-square-foot attic from bare joists (R-2) to R-49 runs an estimated $2,820, and saves about $821 a year in heating and cooling — a payback around 3.4 years. Topping up the attic pays for itself here.
The breakdown behind the number
Split it out and it's roughly $431 a year on the heating side, $390 on cooling. By estimated insulation savings, Charleston County lands in the 5th percentile nationally. Zone 3 alone covers 690 counties in this dataset, and Charleston County comes in 680th by savings. South Carolina averages $1,016/year across the counties tracked here; Charleston County comes in 19% lower.
Where these numbers come from
Charleston County's climate figures — 1,623 heating and 2,448 cooling degree days a year — are read from CHARLESTON CITY, SC US, 1.5 miles away, NOAA's nearest 1991-2020 normals station. That combined heating-and-cooling load ranks Charleston County 3068th nationally by total degree days — the 3th percentile. One local detail the R-value alone won't tell you: Charleston County sits in moisture regime A (humid), which can affect the vapor-barrier choice.
Already have some insulation?
Already have a few inches up there? Topping R-11 up to R-38 runs about $1,620 for about $111/year saved — roughly 14.7 years to break even. Call this borderline rather than a clear win.
Getting it done in Charleston County
Real bids depend on how easy the attic is to reach and whether old insulation needs removing first, so treat $2,820 as a starting estimate for a Charleston County job, not a quote. Expect at least a $600 minimum charge from most contractors.
Where these figures come from
Charleston County's R-49 target and its degree-day figures both come from named federal sources - see the methodology guide for exactly which ones. The guide to what R-value actually means explains why R-49 is the target here specifically.