# Window energy performance: how to compare glass and ratings

Summary: Use U-factor, Solar Heat Gain Coefficient, visible transmittance, air leakage, and installation scope to compare windows without treating one label as a universal answer.  
Publisher: HomeFAQ  
Published: 2026-09-29  
Canonical: https://homefaq.com/windows/window-energy-performance-guide/  
Tags: window energy ratings, U-factor, SHGC

Window labels can help you compare products, but a low number is not a complete recommendation. The right package depends on climate, orientation, comfort complaints, daylight, and the condition of the opening. Start with the room problem—winter drafts, afternoon heat, glare, noise, or condensation—then ask how the proposed glass and installation address it.

## Read the common ratings in homeowner terms

**U-factor** describes how readily a window transfers heat; lower generally means less heat flow. **Solar Heat Gain Coefficient (SHGC)** describes how much solar heat enters; a lower number can reduce unwanted sun heat, while a higher number can be useful in some heating-focused situations. **Visible transmittance** indicates how much visible light passes through. **Air leakage** is a separate rating and does not replace a good installation. Ask the seller to show the label for the exact product and glass package proposed.

## Compare the room, not a marketing claim

For a hot west-facing room, solar control and glare may matter more than for a shaded room. For a cold drafty room, U-factor, air sealing, and the opening condition may all be relevant. If condensation is the concern, humidity, ventilation, and existing surface temperature can matter as much as the glass. Do not promise a utility-bill result from a label alone.

## Make installation part of the performance discussion

A high-performing product cannot compensate for water entry, poor flashing, missing air sealing, or concealed frame damage. Require the proposal to name the installation method, insulation and air-sealing work, exterior water management, trim restoration, and who is responsible for correcting an opening problem. Read [insert versus full-frame replacement](https://homefaq.com/windows/insert-vs-full-frame-window-replacement/) if the frame condition is uncertain.

## Questions to put in every quote

- What are the U-factor, SHGC, visible-transmittance, and air-leakage ratings for this exact configuration?
- Which room problem does this glass package address, and what tradeoff does it create?
- What installation, flashing, air sealing, and finish work are included?

Use [the window-estimate comparison guide](https://homefaq.com/windows/how-to-compare-window-estimates/) to make the product label and installation scope comparable.

## Sources

- [National Fenestration Rating Council: Understanding Window Ratings](https://www.nfrc.org/)
- [ENERGY STAR: Windows, Doors & Skylights](https://www.energystar.gov/products/windows_doors_skylights)