Energy-efficient windows — what the ratings mean

Staff

17 May 2025 — 4 min read

This article is for educational purposes only and is not a substitute for professional advice. Local codes, regulations, and best practices vary by region.

Windows account for a significant portion of heat loss and gain in a home. Energy-efficient windows reduce heating and cooling costs. However, window energy efficiency is measured using several different ratings, and understanding what these ratings mean helps you choose windows that actually perform well in your climate.

U-Value

U-value measures how much heat passes through the window. A lower U-value means better insulation. U-values typically range from 0.15 (excellent) to 0.60 (poor).

A single-pane window has a U-value around 1.0 (terrible insulation). A double-pane window has a U-value around 0.50. A quality insulated double-pane window might have a U-value of 0.25 or lower.

When comparing windows, look for lower U-values. A window with a U-value of 0.20 is significantly more efficient than a window with a U-value of 0.40.

U-value is the most important measure of window insulation. It’s regulated for new construction in most jurisdictions.

Solar Heat Gain Coefficient (SHGC)

SHGC measures how much solar heat passes through the window. A lower SHGC means less solar heat transmission. SHGC values range from 0 to 1.

In cold climates, a higher SHGC (around 0.60 to 0.75) is beneficial because solar heat contributes to warmth in winter.

In hot climates, a lower SHGC (around 0.20 to 0.40) is beneficial because it reduces unwanted heat gain in summer.

SHGC is most relevant in extreme climates. In moderate climates, SHGC matters less.

Visible Transmittance (VT)

VT measures how much light passes through the window. A higher VT means more light transmission. VT values range from 0 to 1.

Standard clear glass has a VT around 0.85 (transmits 85 percent of light). Tinted or reflective glass has lower VT values (perhaps 0.60 or lower).

VT is relevant if you want maximum light from windows. If privacy or glare control is important, you might prefer lower VT.

ENERGY STAR Ratings

ENERGY STAR is a government program that rates product efficiency. ENERGY STAR windows meet efficiency standards for your climate zone.

The United States is divided into climate zones. ENERGY STAR has different efficiency requirements for different zones. A window that’s ENERGY STAR-rated for a cold climate might not be rated for a hot climate.

ENERGY STAR labels indicate that windows meet efficiency standards for your region. This is a useful starting point, though ENERGY STAR windows aren’t necessarily the most efficient available.

National Fenestration Rating Council (NFRC) Labels

NFRC labels are found on new windows and provide standardized ratings for U-value, SHGC, VT, and other factors.

NFRC ratings are based on standard testing procedures, so you can compare windows from different manufacturers using the same baseline.

When shopping for windows, look for NFRC labels to compare efficiency ratings across brands.

Air Leakage

Air leakage is measured as cubic feet of air passing through a window per minute. Lower numbers are better. Quality windows have air leakage ratings of 0.3 cubic feet per minute or lower. Poor-quality windows might have ratings of 0.6 or higher.

Air leakage becomes noticeable when windows aren’t sealed properly. Drafts around window frames indicate excessive air leakage.

Double-Pane vs. Triple-Pane

Double-pane windows have two layers of glass with an insulating air space between them. Double-pane is standard for residential windows.

Triple-pane windows have three layers of glass with insulating spaces between them. Triple-pane windows are more expensive but provide better insulation than double-pane.

In cold climates, triple-pane windows provide noticeable efficiency improvements. In moderate climates, the cost difference might not be justified by energy savings.

Low-E Glass

Low-E (low-emissivity) glass has a special coating that reflects heat while transmitting light. Low-E coatings reduce heat transmission significantly.

There are different types of Low-E coatings. Some are optimized for cold climates (high-gain Low-E), others for hot climates (low-gain Low-E).

Most quality modern windows include Low-E glass. Low-E glass is worth the small additional cost because energy savings pay for it over time.

Insulating Gas Fills

Window panes are sometimes filled with insulating gases like argon or krypton instead of air. These gases slow heat transfer better than air.

Argon is common in quality windows. Krypton is better insulating than argon but more expensive.

Gas-filled windows perform better than air-filled windows, but the improvement is modest compared to other factors like Low-E coating and frame insulation.

Frame Insulation

Window frames conduct heat. A thermally broken frame (with insulation inside) reduces heat transfer compared to a standard frame.

Aluminum frames without thermal breaks are poor insulators. Vinyl and fiberglass frames are better insulators. Thermally broken aluminum frames are reasonable insulators.

For energy efficiency, the frame matters. A poor frame can undermine good glass performance.

Climate-Specific Recommendations

For cold climates: Look for low U-values (0.20 or lower), higher SHGC to capture solar gain, and triple-pane or insulated double-pane with Low-E coating.

For hot climates: Look for low U-values, lower SHGC to reduce solar gain, and Low-E coating designed for solar control.

For moderate climates: Standard efficiency double-pane with Low-E coating is usually adequate.

Energy Savings Reality

Replacing old single-pane windows with efficient double-pane windows can reduce heating and cooling costs by 10 to 20 percent, depending on climate and how much window area your home has.

The cost of replacing windows is $20,000 to $50,000 for a typical home. The annual energy savings might be $500 to $1500. It takes 15 to 30 years to recover the cost through energy savings.

However, windows also improve comfort, reduce drafts, and reduce outside noise. These benefits have value beyond energy savings.

If your existing windows are single-pane or in poor condition, energy-efficient replacement windows are justified. If your windows are modern double-pane, the upgrade cost might not be justified by energy savings alone.

The Reality

Energy-efficient windows reduce heating and cooling costs. The most important factors are U-value, SHGC (depending on climate), and Low-E coating. When comparing windows, focus on NFRC ratings appropriate for your climate.

Upgrades pay for themselves through energy savings, but the timeframe is long. Window replacement decisions should also consider comfort, aesthetics, and noise reduction, which have value beyond energy savings.