Measure and MendStart measuring

United States

U.S. Climate Zone 4C home maintenance

Seattle is DOE’s example for Zone 4C. Confirm your project zone in the official PNNL county data.

Official profile

4CMixed–MarineRepresentative: Seattle, Washington

For Zone 4C, the retrofit comparison is R60 for an empty attic and R49 when 3–4 inches remain. Confirm the county rather than assuming every Seattle-like climate belongs to Zone 4C.

What the official table recommends

Official insulation comparison for U.S. Climate Zone 4C home maintenance
ConditionGuidanceScope
Uninsulated atticR60ENERGY STAR groups Zone 4C with Zones 5 and 6 for this retrofit table.
Existing 3–4 inchesR49Official 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

    Trace roof and wall water paths.

    Wet assemblies must be corrected before insulation or finish work.

  2. Summer

    Inspect attic air sealing and ventilation details.

    Intentional ventilation and leakage controls are different.

  3. Fall

    Review exterior wood and drainage condition.

    Preparation follows observed moisture and coating condition.

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.