San Juan County, WA Insulation Payback
San Juan County attic insulation, by the numbers
DOE classifies San Juan County as climate zone 4, marine — temperate and damp most of the year, with neither heating nor cooling asking for much. Its recommended attic R-value for bare joists is R-60. Topping up a typical 1,000-square-foot attic from bare joists (R-2) to R-60 runs an estimated $3,480, and saves about $1,388 a year in heating and cooling — a payback around 2.5 years. Topping up the attic pays for itself fast here.
Where the savings come from
Split it out and it's roughly $1,383 a year on the heating side, $5 on cooling. By estimated insulation savings, San Juan County lands in the 45th percentile nationally. Zone 4 alone covers 825 counties in this dataset, and San Juan County comes in 409th by savings. Washington averages $1,583/year across the counties tracked here; San Juan County comes in 12% lower.
The climate data behind this
The heating and cooling degree days behind this estimate — 5,171 HDD and 32 CDD (base 65°F, 1991-2020 normals) — come from FRIDAY HARBOR AP, WA US, the nearest NOAA station with published normals, 3.8 miles from San Juan County's center. By total degree days, San Juan County sits at the 31th percentile nationally. One local detail the R-value alone won't tell you: San Juan County sits in moisture regime C (marine), which can affect the vapor-barrier choice.
Already have some insulation?
A shallower starting point changes the math: with R-11 already down, only the gap to R-49 needs filling, at roughly $2,280 for an estimated $202/year saved (~11.3-year payback). Call this borderline rather than a clear win.
Next steps for a San Juan 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 San Juan County's estimated $1,388/year on the line.
Where these figures come from
San Juan 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.