San Mateo County, CA Insulation Payback
San Mateo County attic insulation, by the numbers
San Mateo County, California is zone 3 under DOE's climate map (Marine), which means temperate and damp most of the year, with neither heating nor cooling asking for much. 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 $984 a year — payback in roughly 2.9 years. Topping up the attic pays for itself fast here.
Where the savings come from
Split it out and it's roughly $982 a year on the heating side, $2 on cooling. By estimated insulation savings, San Mateo County lands in the 16th percentile nationally. Zone 3 alone covers 690 counties in this dataset, and San Mateo County comes in 503rd by savings. California averages $1,056/year across the counties tracked here; San Mateo County comes in 7% lower.
Where these numbers come from
The heating and cooling degree days behind this estimate — 3,701 HDD and 12 CDD (base 65°F, 1991-2020 normals) — come from HALF MOON BAY, CA US, the nearest NOAA station with published normals, 5.6 miles from San Mateo County's center. By total degree days, San Mateo County sits at the 1th percentile nationally. One local detail the R-value alone won't tell you: San Mateo County sits in moisture regime C (marine), 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 $133/year saved — roughly 12.2 years to break even. Call this borderline rather than a clear win.
Getting it done in San Mateo County
Actual quotes will vary by attic access, existing debris, and whether the job needs air sealing first. Treat the $2,820 figure above as a planning number for San Mateo County, not a bid - contractors typically charge a $600 minimum regardless of job size.
Where these figures come from
San Mateo 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.