Window Energy (U-Value) Calculator

Estimate annual heat loss and energy cost through windows by U-value, climate zone, and fuel type. Compare glazing options and check ENERGY STAR compliance.

Window Configuration

Number of Windows

windows

Width (inches)

Height (inches)

Window U-Value

BTU/hr·ft²·°F

SHGC (Solar Heat Gain Coefficient)

Climate & Energy

IECC Climate Zone

HDD

Heating Fuel

Fuel Cost (per unit)

$ /therm

Heating System Efficiency (%)

Annual Heating Cost Through Windows

—

per year

—

Total Window Area (ft²)

—

Annual Heat Loss (kBTU)

—

Therms/yr

—

U-Value Used

Annual Heat Loss Cost by Glazing Type

Comparison uses your current window area and climate settings across all four standard U-values.

ENERGY STAR U-Value & SHGC Requirements

Requirements for windows, doors, and skylights certified under ENERGY STAR Version 6 (2015–present). Northern and Southern zones have different SHGC needs.

ℹ U-factor measures heat loss — lower is better for energy conservation.

☀ SHGC measures solar heat gain — higher helps heating in cold climates; lower reduces cooling in hot climates.

✅ Your current configuration status is highlighted in the table below.

Condensation Risk Assessment

Window condensation forms when the glass surface temperature drops below the dew point of indoor air. Higher U-values (less insulating windows) dramatically increase risk.

Interior RH Limits by Window Type

Maximum recommended indoor relative humidity (at 70°F inside, 0°F outside) to avoid condensation on window glass surfaces.

Tips to Reduce Condensation

How to Use This Calculator

Enter window count and dimensions — Input the number of windows and their typical width/height in inches to calculate total glazing area.

Select U-value — Choose from Single, Double, Double LowE, or Triple glazing presets, or enter a custom U-value from your window specification sheet.

Enter SHGC — The Solar Heat Gain Coefficient (0–1) measures how much solar energy passes through. Find this on the NFRC label or window spec sheet.

Choose climate zone — Select your IECC zone (1–8) or enter custom Heating Degree Days (HDD) for your city.

Select fuel type and cost — Choose your heating fuel and enter the current cost per unit (therms for gas, kWh for electric, gallons for oil/propane).

Review tabs — Use Heat Loss Comparison to compare glazing options; check ENERGY STAR Zones for code compliance; review Condensation Risk for humidity guidance.

Window Energy Terminology

U-Value (U-Factor)Rate of heat transfer through the window assembly. Lower = more insulating. Measured in BTU/hr·ft²·°F. Inverse of R-value.

SHGCSolar Heat Gain Coefficient. Fraction of solar radiation admitted through the window. Range 0–1. Lower reduces cooling loads; higher helps heating in cold climates.

HDD (Heating Degree Days)Measure of cold weather severity. Sum of degrees per day below 65°F base. High HDD = cold climate = higher heating demand.

Low-E CoatingMicroscopically thin metallic coating applied to glass. Reflects long-wave infrared radiation, significantly reducing U-value without visible effect on light transmission.

NFRC LabelNational Fenestration Rating Council label on windows. Shows certified U-factor, SHGC, VT (visible transmittance), and air leakage ratings.

Dew PointTemperature at which water vapor in air condenses to liquid. When window surface temperature falls below dew point, condensation forms on the glass.

ENERGY STAREPA program certifying energy-efficient products. Window certification requires meeting U-factor and SHGC thresholds for the product's climate zone.

ThermUnit of heat energy equal to 100,000 BTU. Used for natural gas billing. One therm ≈ 100 cubic feet of natural gas at standard pressure.

Frequently Asked Questions

What U-value should I choose for new windows?

For most U.S. climates (Zones 4–6), Double LowE windows (U ≈ 0.27–0.32) offer the best balance of cost and performance and meet ENERGY STAR requirements. Triple pane (U ≈ 0.15–0.22) provides the best insulation but costs significantly more — it makes economic sense in very cold climates (Zones 6–8) where the energy savings justify the premium.

Why does the heat loss calculation use Heating Degree Days?

Heating Degree Days (HDD) quantify cold weather severity by summing the degrees per day that outdoor temperature falls below 65°F. Multiplying U-value × area × HDD × 24 gives annual BTU lost through the window assembly in a simplified steady-state model. Actual losses vary with solar gain, wind, and thermal mass, but the HDD method closely matches real-world monitored data.

How does Low-E coating affect SHGC?

Low-E coatings come in two main types: hard-coat (pyrolitic) and soft-coat (sputtered). Hard-coat has higher SHGC (~0.40–0.55) and is better for cold climates to capture solar heat. Soft-coat has lower SHGC (~0.20–0.35) and is better for hot climates to reject solar heat. Both types significantly reduce U-value. Check your window's NFRC label for the specific ratings.

Why does condensation form on the inside of my windows?

Interior condensation forms when the cold glass surface temperature drops below the dew point of your indoor air. High U-value (poorly insulating) windows have colder glass surfaces. The fix is either to upgrade to lower U-value windows or reduce indoor humidity by using exhaust fans, dehumidifiers, or improving ventilation. A rule of thumb: keep indoor RH below 30–35% with single-pane windows in cold weather.

Does this calculator account for window frames?

The U-value you enter should be the whole-window rated U-factor (including frame and spacers), not just the center-of-glass value. NFRC-certified windows always display the whole-window U-factor. Center-of-glass U-values are typically 10–20% lower than the whole-window value and will underestimate actual heat loss.