Fairfax City County is in IECC Climate Zone 4A (Mixed - Humid). Get accurate Manual J load calculations with official ASHRAE design temperatures for Fairfax City County.
Fairfax City County, Virginia is classified under IECC Climate Zone 4A (Mixed - Humid), a mixed-climate region with humid moisture conditions. With 78 ZIP codes and 1 communities, every HVAC installation in Fairfax City County must use the ASHRAE design temperatures of 12°F (99% winter heating) and 94°F (1% summer cooling) for accurate Manual J load calculations.
With balanced heating and cooling loads, Fairfax City County homes require well-rounded insulation — R-20 walls, R-49 ceilings, R-19 floors — and windows with both U-factor 0.32 and SHGC 0.4 to handle both winter lows and summer highs.
Minimum insulation R-values and window performance for Climate Zone 4A in Fairfax City County, VA.
| Building Component | Minimum Requirement | Unit |
|---|---|---|
| Ceiling / Attic Insulation | R-49 | R-value |
| Wall Insulation | R-20 | R-value |
| Floor Insulation | R-19 | R-value |
| Window U-Factor | 0.32 | BTU/hr-ft²-°F |
| Window SHGC | 0.4 | Coefficient |
| Air Infiltration | 0.15 | ACH50 |
Climate Zone 4A requires precise load calculations. Upload your Fairfax City County blueprint and get a professional Manual J report in 60 seconds— first report is FREE. Pre-loaded with Fairfax City County's exact design temperatures (12°F winter / 94°F summer).
Enter your ZIP code to confirm it's in Fairfax City County and get climate zone details.
Official ASHRAE design temperatures for Fairfax City County, VA, used for accurate Manual J load calculations.
Minimum insulation and window requirements for Fairfax City County per IECC 2021 energy code.
Climate Zone 4A: Mixed - Humid - True mixed climate with hot, humid summers and cold winters. Equal emphasis on heating and cooling.
1 cities and communities in Fairfax City County share Climate Zone 4A requirements.
Compare climate zones with other counties in Virginia. Different zones require different HVAC equipment sizing.
Upload your Fairfax City County blueprint and get a professional Manual J load calculation using official Zone 4A design temperatures. First report is FREE.
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