Mixed Climate Windows
Window selection guide for IECC Climate Zone 4: Mixed climates including Seattle, Portland, Mid-Atlantic, and transitional regions.
Energy Code Requirement
U-0.32
U-Factor Maximum
Solar Heat Gain
No specific SHGC requirement (optional 0.30-0.40 for balance)
SHGC Requirement
Heating Degree Days
4,000-5,500 annually
Annual Average
Climate Zone Zone 4 Characteristics
Temperature & Climate
- Climate Type
- Mixed (Marine and Mixed-Humid)
- Average Summer
- 75-85°F (24-29°C)
- Average Winter
- 30-50°F (-1 to 10°C)
- Cooling Degree Days
- 800-2,000 annually
Energy Priorities
- Primary Concern
- Balanced heating and cooling needs
- Secondary Concern
- Moisture management (marine climates)
Specific Challenges for Mixed Climates
- ▸Four distinct seasons require balanced window performance
- ▸Marine climates (4C): constant moisture and condensation issues
- ▸Mixed-humid (4A): high summer humidity increases condensation
- ▸Moderate heating and cooling costs (roughly equal)
- ▸Spring/fall temperature swings stress window seals
- ▸Condensation risk in winter from high humidity
Energy Code Requirements
- Energy Code
- IECC 2018/2021 Climate Zone 4
- Air Leakage Maximum
- ≤0.3 cfm/ft²
- Additional Notes
- Balance is key. U-0.32 prevents winter heat loss, moderate SHGC allows passive solar gain. Marine climates need moisture-resistant frames.
Recommended Window Configurations
Balanced Performance
Budget- Pane Count
- Double-pane
- Low-E Coating
- Low-E2 or Low-E4
- Gas Fill
- Argon fill
- Frame Material
- Vinyl
- U-Factor
- U-0.28 to U-0.32
- SHGC
- SHGC 0.30 to 0.40
- Cost Range
- $450-$750 per window
- Payback Period
- 6-9 years
High-Performance Balanced
Premium- Pane Count
- Triple-pane (recommended for marine climates)
- Low-E Coating
- Low-E4 (balanced coating)
- Gas Fill
- Argon or krypton fill
- Frame Material
- Fiberglass or vinyl
- U-Factor
- U-0.20 to U-0.26
- SHGC
- SHGC 0.28 to 0.35
- Cost Range
- $700-$1,100 per window
- Payback Period
- 8-12 years
Common Window Problems in Mixed Climates
- Condensation in winter from marine moisture (Zone 4C)
- Mold growth around frames in high-humidity areas
- Water infiltration through aging seals
- Balancing heating efficiency with passive solar gain
- Rot in wood frames from constant moisture (marine)
- Fog/haze between panes from humidity and temperature cycling
Energy Cost Analysis
Annual Heating
$1,000-$1,600
Annual Cooling
$400-$800
Potential Savings
$300-$500/year with U-0.26 windows
Utility Rate
$0.10-$0.13/kWh
Recommended Window Products
Andersen
A-Series with Low-E4
SHGC 0.32, U-0.27, Marine climate rated
Pella
Architect Series with LoĒ4 Plus
SHGC 0.35, U-0.25, Moisture resistant
Marvin
Ultimate with LoĒ³-366
SHGC 0.30, U-0.28, Coastal warranty
Note: Product availability and specifications may vary. Always verify current specs with manufacturers and local dealers.
Cities in Climate Zone Zone 4
Seattle, WA
4800 HDD • U-0.30 or lower for residential windows
Spokane, WA
6800 HDD • U-0.30 or lower for residential windows
New York, NY
4900 HDD • U-0.32 or lower for residential windows
Philadelphia, PA
4800 HDD • U-0.32 or lower for residential windows
Washington, DC
4200 HDD • U-0.32 or lower for residential windows