The NFRC label — the only spec sheet that matters
Every ENERGY STAR-certified window carries an NFRC (National Fenestration Rating Council) label with four numbers:
- U-factor: heat transmission rate. Lower is better. 0.30 is ENERGY STAR baseline; 0.20 is high-performance; 0.15 approaches Passive House levels.
- SHGC (Solar Heat Gain Coefficient): fraction of solar radiation passing through. 0.40 for mixed climates, 0.25 for hot climates (block heat gain), 0.50+ for cold climates with passive solar.
- VT (Visible Transmittance): how much visible light passes through. 0.50+ is bright; 0.40 is moderate.
- AL (Air Leakage): cfm/sqft at 75 Pa. Lower is better. 0.30 is acceptable; 0.10 is excellent.
ENERGY STAR criteria (2024 update) vary by climate zone: Northern zones require U ≤ 0.27; Southern zones require SHGC ≤ 0.25.
Frame materials
- Vinyl. PVC frame, no maintenance, 20–30 year life. Cheapest. Slight thermal expansion in extreme climates can cause seal failure on large units. Dominant residential choice in 2026. Brands: Pella, Andersen 100 Series, Milgard, Jeld-Wen.
- Fiberglass. Pultruded fiberglass frame, minimal thermal expansion, paint-able, 40+ year life. Premium price. Best dimensional stability for picture/large units. Brands: Marvin Essential, Pella Impervia, Milgard Ultra.
- Wood-clad. Wood interior, aluminum or vinyl exterior cladding. Premium aesthetic, decades of life with interior maintenance. Most expensive. Brands: Andersen 400/A-Series, Marvin Ultimate, Pella Architect Series.
- Aluminum. Hot-climate / commercial. High thermal conductivity (poor insulator) unless thermal-break design is used. Best fit: Sunbelt new construction or modernist aesthetics.
Double-pane vs triple-pane
Double-pane Low-E with argon fill is the 2026 standard residential window. U-factor 0.28–0.30, SHGC tuned to climate zone. Reasonably priced, available everywhere.
Triple-pane Low-E with argon or krypton fill drops U-factor to 0.18–0.22 (35–40% better) and helps significantly in IECC zones 5+. Trade-offs: $200–$400 more per window, 25% heavier (matters for very large operable units), and slightly lower VT (less light). The right call in Massachusetts, Minnesota, Maine, Vermont, and similar climates. Overkill in zones 1–3.
The honest payback math
ENERGY STAR’s own marketing materials estimate $126–$465/year savings replacing a 1,800 sqft home’s single-pane windows with ENERGY STAR doubles. Divide by 14 windows = $9–$33/window/year. Average $/window installed = $650–$1,300 (vinyl).
Simple payback on energy alone: 25–40 years. Most windows reach end-of-life before that. If your only goal is reducing utility bills, window replacement has the worst $/return of any envelope upgrade.
That said, there are legitimate reasons to replace windows:
- Rotting frames, seized hardware, or broken seals (visible fog between panes)
- Persistent drafts you can’t seal even with weatherstrip + caulk
- Single-pane historic windows leaking visible air
- Street/neighbor noise — triple-pane windows reduce sound transmission 30+ dB vs single-pane
- Aesthetic / curb appeal in resale context
- Concurrent re-side or major remodel
If energy savings is the primary motivation: attic insulation + air-sealing returns 5–10× better $/year. See the insulation calculator.
Storm windows — the alternative
Exterior or interior storm windows added over existing single-pane achieve 60–80% of the energy benefit of full replacement at 20–30% of the cost. $200–$500/window installed. Best fit: historic homes where window replacement isn’t permitted, or budget-constrained efficiency upgrades on older single-pane windows.
Brands: Larson, Harvey, Provia (interior); Indow inserts (compression-fit interior, removable). Quaker Energy Star storm windows are tested by DOE for 40% air infiltration reduction.
Install quality red flags
- No house wrap or pan flashing under the sill. Water management around the rough opening is the #1 cause of premature failure (rot, leaks).
- Spray foam only insulation between window and rough opening. Code requires non-expanding window/door foam (different from regular gun foam) plus backer rod and sealant.
- No interior or exterior sealing detail. Air leakage around window perimeter often exceeds leakage through the glass itself.
- Pocket installs (insert into existing frame) sold as full replacement. Cheaper to install but leaves old jamb/frame air leaks unfixed. Verify "full frame" vs "pocket" with the contractor.
- Aluminum spacer between panes. 2026 standard is warm-edge spacer (Super Spacer, Intercept, Swiggle Strip). Aluminum spacers form cold edges that condense and crack seals.