Energy Performance
Energy efficiency is product- and climate-specific. Use ENERGY STAR climate guidance and the NFRC label to compare verified performance values.
Heat transfer through the window. Lower is better for limiting heat loss.
Solar heat admitted through the product. The preferred value depends on climate and orientation.
Visible transmittance: how much visible light passes through the product.
Air passing through the assembly. Lower values mean less infiltration.
Whole-window performance
Frame construction, glazing, spacers and sealing all contribute to measured window performance. Compare the certified values of the complete product rather than judging efficiency from frame material alone.
Energy-efficient window cross section. Solrman, CC0 1.0, via Wikimedia Commons.
Glazing anatomy
Insulated glazing units use multiple panes separated by a sealed space. Glass coatings, gas fills and spacer design can further change the finished product's thermal and solar performance.
Double-glazed fixed-window diagram. NcLean, CC BY-SA 3.0, via Wikimedia Commons.
ENERGY STAR states that certified residential windows are independently tested, certified and verified through NFRC procedures and must meet EPA energy-efficiency criteria for the applicable climate zone.
ENERGY STAR says replacing single-pane windows with certified products can lower household energy bills by an average of up to 13% nationwide in its stated comparison context. Actual savings depend on the home, climate, existing windows and installation quality.
Use the numbers
Change window count, frame material, style and location in the estimator. It returns a planning range and shows the assumptions instead of hiding them.
Open the cost calculator →Continue your research
Quick answers
Concise answers first; source details stay visible elsewhere on the page.
ENERGY STAR reports average savings of up to 13% on household energy bills in the stated single-pane replacement comparison, but real savings vary by climate, home and installation.
U-Factor and SHGC are two core ratings, but the right target depends on climate and orientation. Use the complete NFRC label and ENERGY STAR climate guidance.