King County, WA Insulation Payback
King County attic insulation, by the numbers
DOE classifies King County as climate zone 4, marine — mild, damp weather year-round with modest heating load and little cooling. Its recommended attic R-value for bare joists is R-60. Going from R-2 to the recommended R-60 costs roughly $3,480 installed for a typical attic (about 1,000 sq ft), and pays for itself in about 2.6 years at an estimated $1,359/year saved. Topping up the attic pays for itself fast here.
Where the savings come from
Split it out and it's roughly $1,335 a year on the heating side, $24 on cooling. By estimated insulation savings, King County lands in the 43th percentile nationally. Zone 4 alone covers 825 counties in this dataset, and King County comes in 467th by savings. Washington averages $1,583/year across the counties tracked here; King County comes in 14% lower.
King County's actual weather, not a national average
The heating and cooling degree days behind this estimate — 4,991 HDD and 151 CDD (base 65°F, 1991-2020 normals) — come from SNOQUALMIE FALLS, WA US, the nearest NOAA station with published normals, 3.5 miles from King County's center. By total degree days, King County sits at the 30th percentile nationally. One local detail the R-value alone won't tell you: King County sits in moisture regime C (marine), which can affect the vapor-barrier choice.
If the attic isn't bare
If King County's attic already has 3-4 inches of old insulation (about R-11) rather than bare joists, the recommended target drops to R-49. That smaller top-up costs around $2,280 and saves an estimated $198/year — payback about 11.5 years. Call this borderline rather than a clear win.
Quotes and materials for this job
Real bids depend on how easy the attic is to reach and whether old insulation needs removing first, so treat $3,480 as a starting estimate for a King County job, not a quote. Expect at least a $600 minimum charge from most contractors.
Where these figures come from
King County's R-60 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-60 is the target here specifically.