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United States

U.S. Climate Zone 3C home maintenance

San Diego is DOE’s example for Zone 3C. Use the official PNNL county file to identify the zone for your home.

Official profile

3CWarm–MarineRepresentative: San Diego, California

A confirmed Zone 3C attic uses the R49/R38 comparison, depending on whether it is empty or already has 3–4 inches. San Diego is an example location, not an address lookup.

What the official table recommends

Official insulation comparison for U.S. Climate Zone 3C home maintenance
ConditionGuidanceScope
Uninsulated atticR49ENERGY STAR retrofit recommendation for Zone 3.
Existing 3–4 inchesR38Official retrofit table condition

This table compares attic R-values. It does not calculate bags or measure the gap in performance. Assess the existing insulation first, then match the required added R-value to a current product label.

Estimate insulation bags

What to check through the year

  1. Spring

    Inspect exterior joints and wood finish condition.

    A measured condition survey should precede recoating.

  2. Summer

    Map attic air leaks and uneven insulation.

    Air sealing comes before top-up.

  3. Fall

    Check roof drainage and interior staining.

    Moisture sources must be repaired rather than concealed.

Before you plan the work

  • Confirm the actual project zone using the cited official lookup or the authority that applies locally.
  • Inspect existing material, moisture, air leakage, safe access, wiring, fixtures, and combustion equipment before top-up.
  • Use the coverage chart on the current product label; do not transfer bag data between U.S. and Canadian products.
  • Treat representative locations as classification examples, not local forecasts or code boundaries.

Sources and notes

Source links reviewed July 16, 2026. A review date is not the document's publication date.

  1. U.S. Department of Energy: Prototype Building Models — Climate Zone RepresentativesUnited States · government standard

    Representative locations classify a zone; they are not a substitute for local weather or code data.

  2. Pacific Northwest National Laboratory: Guide to Determining Climate Zones by CountyUnited States · government standard

    Use the county files for a project address; representative cities are examples rather than boundaries.

  3. ENERGY STAR: Recommended Home Insulation R-ValuesUnited States · government guide

    Recommendations are presented by ENERGY STAR using 2021 IECC climate zones.

  4. ENERGY STAR: DIY Checks and InspectionsUnited States · government guide

    Do not disturb suspected vermiculite insulation; obtain qualified guidance before work.