Window Terms Glossary:
A Homeowner’s Guide to Understanding Windows
Shopping for new windows can come with a lot of unfamiliar terms. U-factor. SHGC. Low-E glass. Argon gas. If you’ve ever looked at a window specification sheet and wondered what any of it actually means, you’re not alone.
This glossary breaks down some of the most common window terms in plain language, so you can better understand what you’re comparing, what the numbers mean, and what features can make a difference in your home.
Whether you're replacing a few windows or updating your entire home, use this guide to get familiar with the terminology before you talk with a dealer.
Energy Performance Terms
These are the numbers you'll see most often on a window's energy label, and the ones that actually affect your utility bills.
U-Factor
A measure of how much heat a window lets escape. The lower the U-factor, the better the window is at keeping conditioned air inside. Unlike some other ratings, lower is always better here, regardless of your climate.
SHGC
(Solar Heat Gain Coefficient) Measures how much solar heat passes through the glass. A lower SHGC blocks more heat, which matters more in warmer climates or south- and west-facing windows. A higher SHGC can help with passive heating in colder regions.
R-Value
The inverse of U-factor, measuring resistance to heat flow rather than heat loss. Higher R-value means better insulation. You'll see this more often in wall and insulation specs than on window labels, but it's the same underlying concept.
Low-E Glass
Short for "low emissivity." A microscopically thin, virtually invisible coating on the glass that reflects heat while still letting visible light through. It's one of the simplest ways a window improves its U-factor and SHGC without changing how the glass looks.
Argon and Krypton Gas Fill
The space between panes in a dual- or triple-pane window is often filled with argon or krypton gas instead of plain air. Both are denser than air and slow down heat transfer between panes, improving overall energy performance. Krypton performs better in tighter gaps, which is why it shows up more often in triple-pane and narrow-cavity designs.
Endur Spacer Technology
The spacer is the component that separates and seals the panes of glass at the edge. Older aluminum spacers conduct heat and cold, creating a weak point around the perimeter of the glass. Warm-edge spacer systems like Endur Spacer reduce that heat transfer, which improves comfort near the window and helps prevent condensation. It's a small detail that makes a measurable difference over the life of the window.
Materials and Construction Terms
What the window is actually made of, and why it holds up.
uPVC
(Unplasticized Polyvinyl Chloride)
A rigid, weather-resistant vinyl material used in European-engineered window frames. Unlike standard vinyl, uPVC doesn't rely on plasticizers that can break down over time, which helps the frame resist warping, fading, and temperature swings for decades.
Multi-Chamber Frame Construction
Rather than a single hollow frame cavity, a multi-chamber frame is divided into several internal sections. Each chamber traps air and interrupts heat transfer through the frame, similar to how double-pane glass works, just applied to the frame material itself.
REHAU Profile
A European frame engineering standard, developed by REHAU, widely used by German window fabricators. It's a specific uPVC formulation and multi-chamber design that's become a benchmark for performance in the European market, and one that's increasingly available through domestic dealer networks in the US, including our Euro Series.
Full-Perimeter Seal
Rather than sealing only at a single lock point or relying on friction along a track, some window designs compress the sash against the frame on all four sides when closed. This creates a consistent seal around the entire opening, which tends to hold up better over time than seals that depend on one mechanism.
Window Style and Operation Terms
How a window actually opens, and what that means day to day.
Tilt and Turn Window
A European-style window with two distinct opening positions from a single handle: tilt the sash for controlled ventilation from the top, or turn it further for the sash to swing fully inward like a door. Read the full breakdown in Why More US Homeowners Are Choosing European Window Styles.
Casement Window
A window hinged on one side that cranks outward to open, giving a wide, unobstructed opening. It's one of the most common window styles in US homes. See how it stacks up against tilt and turn windows in our side-by-side comparison.
Double-Hung Window
A traditional US window style with two sashes that slide vertically, one up and one down, within the same frame. Familiar and widely available, though it offers only one method of ventilation compared to a tilt and turn window's two.
Egress
A code term referring to a window's ability to serve as an emergency exit, typically in bedrooms. Egress requirements set minimums for clear opening size, sill height, and how the window operates. Both casement and tilt and turn windows can qualify for egress depending on the unit size and local code.
Tilt and Slide Door
A door style that combines a large glass opening with a tilt position for ventilation, designed for spaces where a swinging door isn't practical, like balconies or tight rooms. Part of our Euro door lineup.
Why This Matters When You're Comparing Windows
Two windows can look nearly identical and perform very differently once you get into the specs. Knowing what U-factor, SHGC, and spacer technology actually measure means you can compare apples to apples instead of relying on a sales pitch, and it means you'll know exactly what to ask a dealer when you're ready to move forward.
If a term you were looking for isn't listed here, or you want to see how these features come together in an actual window, explore the 4500 Euro Series, browse design options, or find a dealer near you to see the difference in person.