Energy efficient windows with Low-E coatings and argon gas achieve U-factors of 0.25-0.30, meeting ENERGY STAR requirements for the DC, Virginia, and Maryland area. They block 85-95% of infrared radiation, reduce heating and cooling costs by 10-25%, and qualify for federal tax credits up to $600 per year. This guide explains U-factor, SHGC, Low-E coatings, gas fills, frame materials, and how to read the NFRC label.
Last Updated: March 2026
The National Fenestration Rating Council (NFRC) label on every ENERGY STAR window displays five key ratings. Understanding these numbers helps you choose the right window for your climate and orientation.
Range: 0.15 - 1.20 | Lower is better
Measures heat transfer rate through the entire window assembly. A U-factor of 0.25 means the window transfers 0.25 BTU per hour per square foot per degree F of temperature difference. For DMV homes, target 0.30 or lower to meet ENERGY STAR. U-factor is the inverse of R-value.
Range: 0 - 1.0 | Lower blocks more heat
Fraction of solar radiation that passes through as heat. An SHGC of 0.30 means 30% of solar energy becomes indoor heat. For DMV homes, 0.25-0.35 balances summer cooling and winter solar heating. South-facing windows benefit from slightly higher SHGC.
Range: 0 - 1.0 | Higher means more daylight
Fraction of visible light that passes through the window. A VT of 0.50 means 50% of daylight enters. Higher VT reduces the need for artificial lighting. The VT-to-SHGC ratio (LSG) should be 1.0+ for optimal light with minimal heat gain.
Range: 0.1 - 0.3 | Lower is tighter
Measures cubic feet of air passing through one square foot of window per minute. Lower values indicate better air sealing. ENERGY STAR requires AL of 0.30 or less. High-quality windows achieve 0.10-0.15. Air leakage depends primarily on the window frame and operable hardware, not the glass.
Range: 1 - 100 | Higher resists more
Rates the window's ability to resist interior condensation. Higher numbers mean the interior glass surface stays warmer. Windows with CR above 50 perform well in the DMV climate. Triple pane windows typically achieve CR of 60-80.
DC / Virginia / Maryland Requirements
U-factor: 0.30 or lower. SHGC: 0.40 or lower. These are the minimum requirements for ENERGY STAR certification in the DMV area. Many high-performance windows exceed these significantly. Exceeding the minimums qualifies for additional utility rebates in some jurisdictions.
Low-emissivity coatings are the single most impactful technology in modern energy efficient windows. Understanding the types and placement options helps you optimize window performance for your specific needs.
Applied in a vacuum chamber after glass manufacturing. Multiple ultra-thin layers of silver and metal oxides are deposited on the glass surface. Soft-coat Low-E reflects 92-95% of infrared radiation and is the highest-performing option. It must be placed on a protected interior surface (surface 2 or 3 in an IGU) because the coating is delicate. This is the standard for residential ENERGY STAR windows.
Applied during glass manufacturing while the glass is still hot. A tin oxide layer fuses to the glass surface, creating a durable coating that can be exposed to weather. Hard-coat Low-E reflects 85-88% of infrared radiation. While less thermally efficient than soft-coat, it is more scratch-resistant and can be used on surface 1 (exterior face) for special applications like storm windows.
The sealed gas cavity and spacer system work together with Low-E coatings to achieve the complete energy performance of an insulated glass unit.
The standard gas fill for most residential windows. Reduces cavity heat conduction by 30% compared to air. Costs only marginally more than air-filled units. Optimal in 1/2 to 5/8 inch cavity widths. Combined with Low-E coating, argon fill is sufficient to meet ENERGY STAR requirements in the DMV.
Premium gas fill that reduces cavity heat conduction by 64% compared to air. Performs best in narrower 3/8 inch cavities, making it ideal for triple-pane windows with slim profiles. Costs significantly more than argon and is justified mainly for passive house construction or projects requiring maximum thermal performance in a thin assembly.
Traditional aluminum spacers conduct heat rapidly at the glass edge, creating cold spots and condensation. Warm-edge spacers (stainless steel, structural foam, or silicone) reduce edge heat transfer by 50-70%, improving whole-window U-factor by 0.02-0.04 and significantly reducing edge condensation. Look for warm-edge spacers in any quality IGU.
U-factor measures the rate of heat transfer through a window, expressed in BTU/hr/sq ft/degree F. Lower U-factor means better insulation. For the DC, Virginia, and Maryland area (IECC Climate Zone 4), ENERGY STAR requires a maximum U-factor of 0.30. Standard double-pane windows with Low-E and argon achieve 0.25-0.30. Triple-pane windows achieve 0.15-0.20. A U-factor of 0.25 or lower is considered excellent for the DMV climate. U-factor accounts for the entire window assembly including frame, glass, and spacer -- not just the glass center.
Solar Heat Gain Coefficient (SHGC) measures the fraction of solar radiation that passes through a window as heat, on a scale of 0 to 1. Lower SHGC means less solar heat enters the building. ENERGY STAR requires SHGC of 0.40 or lower for Zone 4 (DMV area). In the mid-Atlantic climate with both heating and cooling seasons, a moderate SHGC of 0.25-0.35 is ideal -- low enough to reduce summer cooling loads but high enough to allow beneficial solar heat gain in winter. South-facing windows can benefit from slightly higher SHGC (0.30-0.40) for passive solar heating.
Low-emissivity (Low-E) coatings are microscopically thin layers of metallic oxide (typically silver-based) deposited on glass surfaces. They are transparent to visible light but reflect 85-95% of infrared (heat) radiation. There are two types: hard-coat (pyrolytic) Low-E is applied during glass manufacturing and is more durable but less effective. Soft-coat (sputtered) Low-E is applied in a vacuum chamber after manufacturing and provides superior thermal performance. Soft-coat Low-E must be placed on an interior glass surface (surface 2 or 3 in an IGU) because it is delicate and cannot be exposed to weather.
ENERGY STAR certification means a window meets or exceeds the EPA energy efficiency criteria for its climate zone. For Zone 4 (DC, Virginia, Maryland), the current requirements are U-factor of 0.30 or lower and SHGC of 0.40 or lower. ENERGY STAR windows must be tested and certified by the National Fenestration Rating Council (NFRC). The ENERGY STAR label does not indicate the best windows available -- it is a minimum threshold. Many high-performance windows significantly exceed ENERGY STAR requirements. Federal and state tax credits may be available for ENERGY STAR certified windows.
Visible Transmittance (VT) measures the fraction of visible light that passes through a window, on a scale of 0 to 1. Higher VT means more natural daylight enters the room. Standard clear double-pane windows have VT of 0.55-0.70. Low-E coated windows typically have VT of 0.40-0.55. Triple-pane windows may have VT of 0.35-0.50. A good energy efficient window balances low U-factor and SHGC with acceptable VT -- the ratio of VT to SHGC (called Light-to-Solar Gain ratio or LSG) should be 1.0 or higher for optimal daylighting with minimal heat gain.
Replacing single-pane windows with ENERGY STAR double-pane (Low-E + argon) windows in the DMV area typically saves $200-750 per year on heating and cooling costs, depending on the number of windows, home size, and HVAC efficiency. Replacing older double-pane windows (no Low-E) with current ENERGY STAR windows saves approximately $100-300 per year. The payback period for window replacement in the DMV is typically 8-15 years for energy savings alone. Federal tax credits of up to $600 per year for qualifying ENERGY STAR windows can reduce the effective payback period significantly.
Fiberglass frames offer the best thermal performance with U-factors as low as 0.10-0.15 for the frame portion, because fiberglass has very low thermal conductivity and the hollow profiles can be insulated with foam. Vinyl frames are the most popular choice, with U-factors of 0.15-0.25, offering good thermal performance at lower cost. Wood frames provide U-factors of 0.15-0.20 but require more maintenance. Aluminum frames have the poorest thermal performance (U-factor 0.40-0.60) unless they include a thermal break. Composite frames (wood-polymer blends) offer U-factors of 0.15-0.20 with low maintenance.
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