Johnston County, NC Insulation Payback
Attic insulation payback for Johnston County, NC
Johnston County, North Carolina is zone 3 under DOE's climate map (Mixed-Humid), which means heating load in winter and cooling load in summer, both worth taking seriously here. An uninsulated attic there should be built up to R-49. For an attic around 1,000 square feet, closing the gap from R-2 to R-49 costs around $2,820 and saves an estimated $1,115 a year — payback in roughly 2.5 years. Hard to find a home improvement with a faster payback than this.
Where the savings come from
Of that, about $817/year comes from reduced heating and $298/year from reduced cooling load through the ceiling. Johnston County beats 25% of the counties in this 3,162-county dataset on estimated insulation savings. Among the 690 counties DOE places in zone 3, Johnston County ranks 222nd. Against North Carolina's own average of $1,169/year, Johnston County runs 5% below that.
Where these numbers come from
The heating and cooling degree days behind this estimate — 3,078 HDD and 1,871 CDD (base 65°F, 1991-2020 normals) — come from SMITHFIELD, NC US, the nearest NOAA station with published normals, 1.3 miles from Johnston County's center. That combined heating-and-cooling load ranks Johnston County 2390th nationally by total degree days — the 24th percentile. DOE also classifies Johnston County as moisture regime A (humid) - ask a local contractor whether that changes the vapor-barrier approach for this job.
The smaller top-up scenario
Already have a few inches up there? Topping R-11 up to R-38 runs about $1,620 for about $150/year saved — roughly 10.8 years to break even. It is close - the payback only just fits a reasonable timeframe.
Next steps for a Johnston County attic
Get more than one quote before committing. Attic access and any needed air sealing move the price around, and most contractors won't take a job below roughly $600 regardless of size - worth asking upfront given Johnston County's estimated $1,115/year on the line.
More on how this was built
Nothing here is a guess: the methodology guide names the DOE climate map and NOAA station data behind every number on this page. See the DIY vs. contractor guide for whether a job this size is realistic to do yourself.