When it comes to making your home more energy efficient, windows play a major role. Upgrading old windows can make a noticeable difference on your utility bill.
But not all windows are created equal. To make a smart, informed decision, you need to understand how to read and compare energy performance ratings. These ratings go beyond simple labels — they reveal exactly how well a window can insulate, block heat, reduce harmful rays, and more.
In this article, we’ll break down the seven key ratings you need to know when shopping for energy efficient windows:
- U-Factor (insulation performance)
- SHGC (Solar Heat Gain Coefficient) (solar heat control)
- UV Rating (ultraviolet protection)
- VLT (Visible Light Transmittance) (natural light management)
- Air Infiltration Rating (draft prevention)
- Infrared Rating (blocking radiant heat)
- Condensation Resistance
1. U-Factor: Measuring Insulation Performance
The U-Factor measures the rate of heat transfer — or how easily heat passes through the window.
The lower the U-Factor, the better the window insulates. That means less heat loss during the colder months and less heat infiltration in the summer.
Most energy-efficient windows today feature advanced technologies like Low-E glass coatings, multi-pane designs, and insulated frames to achieve lower U-Factors. When shopping, look for windows with a U-Factor that meets or exceeds ENERGY STAR® guidelines for your region.
2. SHGC (Solar Heat Gain Coefficient): Controlling Solar Heat
The Solar Heat Gain Coefficient (SHGC) measures how well your window keeps heat from solar radiation out. SHGC indicates the fraction of solar radiation that passes through the window as heat. The number ranges from 0 to 1 — the lower the number, the less solar heat enters your home.
Lower SHGC = Less heat gain → Lower cooling costs in summer.
In warm and sunny climates, SHGC is especially important. Too much solar heat gain can make rooms uncomfortable and force your air conditioner to work overtime. By selecting windows with a low SHGC, you can block out much of that heat, keeping interiors cooler and reducing your energy bills during hot months.
That said, SHGC isn’t just about blocking heat. It’s also about balance. In colder climates or in homes where passive solar heating is desired, a higher SHGC can help by allowing more sunlight — and its warmth — to enter during winter months. The key is to find the right balance between blocking heat when you don’t want it and letting it in when you do.
3. UV Rating: Protecting Interiors from Sun Damage
Sunlight streaming through your windows feels wonderful — but it’s not always harmless. Along with visible light and heat, the sun emits ultraviolet (UV) rays, which are invisible but powerful. These rays can fade and deteriorate interior furnishings over time, damaging floors, carpets, furniture, artwork, and even fabrics.
That’s where a window’s UV Rating comes into play. This rating tells you how much ultraviolet radiation the window blocks. Unlike U-Factor and SHGC, a higher UV protection rating is better. The higher the percentage of UV blocked, the more protection your home has against sun-related wear and tear.
Many energy-efficient windows today feature Low-E coatings, which not only improve insulation and solar control but also significantly reduce UV transmission — often blocking up to 95–99% of UV rays. This means you can enjoy natural light without worrying as much about your interiors fading or degrading over time.
4. VLT (Visible Light Transmittance): Balancing Light and Comfort
Natural light is one of the biggest perks of having windows — but too much light can bring unwanted glare, faded interiors, and excessive heat. That’s why it’s important to understand a window’s Visible Light Transmittance (VLT) rating.
VLT measures the percentage of visible light that passes through the window glass. The number typically ranges from 0% (no light passes through) to 100% (all visible light passes through), though it’s usually expressed as a percentage.
Higher VLT = Brighter interiors and more natural light.
Lower VLT = Reduced glare and increased privacy.
4. Air Infiltration Rating: Keeping Outdoor Air Where It Belongs
Even the most advanced window glass won’t perform well if air leaks in around it. That’s where the Air Infiltration Rating becomes critical. This rating measures how much outside air can pass through the window assembly and into your home.
Specifically, air infiltration is measured in cubic feet per minute (cfm) per square foot of window area. The lower the number, the tighter the seal — and the better the window is at keeping drafts, dust, and outdoor air out.
Lower Air Infiltration = Better sealing → Greater comfort and efficiency.
Why does this matter?
- Comfort: Drafty windows lead to cold spots in winter and hot spots in summer.
- Energy bills: Leaky windows force your HVAC system to work harder to maintain consistent temperatures.
- Indoor air quality: Preventing unwanted outdoor air helps reduce allergens and pollutants indoors.
ENERGY STAR® certified windows typically must meet specific air infiltration standards, but even among certified products, some perform better than others. If tight sealing and indoor comfort are priorities for your home — especially in windy or extreme weather regions — pay close attention to this rating when comparing options.
In short, lower air infiltration ratings contribute to a quieter, more comfortable, and more energy-efficient home.
5. Infrared Rating: Blocking Radiant Heat
While SHGC covers total solar heat gain, the Infrared Rating (sometimes referred to as Infrared Rejection or IR Blocking) zeroes in on a specific culprit: infrared rays, which carry radiant heat.
Infrared light makes up a significant portion of the sun’s energy, and when it passes through your windows, it heats up your interiors — often leading to that hot, stuffy feeling near sunlit windows. The Infrared Rating measures how well a window blocks this portion of the solar spectrum.
Higher Infrared Rejection = Less radiant heat entering → Cooler, more comfortable interiors.
This rating is especially important in:
- Hot and sunny climates, where controlling heat gain is critical to comfort and energy savings.
- Rooms with large windows or west-facing exposures, which receive strong afternoon sun.
- Spaces where glare reduction and comfort are priorities (living rooms, home offices, bedrooms).
Infrared blocking is typically achieved with Low-E coatings and specialty glass technologies that selectively reflect infrared wavelengths while still allowing plenty of visible light through. This makes it possible to have windows that offer both natural daylight and excellent heat rejection — without the heavy, tinted look of older heat-control glass.
7. Condensation Resistance: Preventing Moisture Buildup
While energy efficiency ratings often focus on heat, light, and air, condensation resistance is all about moisture — and how well a window prevents it from forming on the glass surface.
Condensation happens when warm, humid air meets a cooler surface, like your window glass. This can cause water droplets to form, which not only block your view but can also lead to long-term issues like:
- Mold and mildew growth
- Damage to window frames, sills, and interior finishes
- Reduced indoor air quality
Higher Condensation Resistance = Less moisture buildup → Healthier, more durable interiors.
Condensation resistance is usually measured on a scale from 1 to 100. The higher the number, the better the window is at resisting condensation. Factors that affect this rating include the quality of the window’s insulation, the presence of Low-E coatings, and the use of multi-pane glass with gas fills like argon or krypton.
While climate plays a role — windows in humid or cold environments face more challenges — condensation resistance is important for any home. Not only does it help protect your window investment, but it also contributes to a healthier, more comfortable indoor environment.
Make the Energy Efficient Choice for Your Home
The right windows can reduce energy bills, keep your interiors comfortable year-round, protect your home’s furnishings, and create bright, welcoming living spaces.
Before you make your next window purchase, take the time to review these ratings carefully — or better yet, reach out to a trusted window specialist who can walk you through the options and help you make the smartest decision for your home.
A large part of understanding the energy efficiency of a window is learning about different window glass types.
This article will outline six types of replacement window glass, like Low-E and gas-filled glass, how they work, and what they look like.