U-Factor, SHGC and ENERGY STAR, Explained

Window performance labels are written in a dialect nobody speaks at home. Here's the translation. Four ratings show up on every window, and each one answers a plain question: how much heat leaks out, how much sun gets in, how bright the room is, and whether the glass fogs up.

The numbers below are Alside's published ratings for its East Coast Mezzo line, which are the replacement windows we sell. They vary by model, glass package and options, so treat the ranges as a map rather than a promise about any one window.

U-factor: how much heat gets through

U-factor measures how easily heat moves through the whole window: glass, frame and spacer together. Lower is better. A lower number means the window holds heat in during the winter and keeps it out during the summer. Mezzo ratings run from 0.25 to 0.47.

SHGC: how much of the sun's heat gets in

Solar heat gain coefficient (SHGC) is the fraction of the sun's heat that passes through the window. Unlike U-factor, lower isn't always better. A lower SHGC keeps a hot house cooler in summer. A higher one lets winter sun help warm a house in a cold climate. Which you want depends on where you live and which way the window faces. Mezzo ratings run from 0.23 to 0.58.

VT: how bright the room is

Visible transmittance (VT) is how much visible light comes through the glass. Higher means a brighter room. Mezzo ratings run from 0.47 to 0.62.

CR: whether the glass fogs up

Condensation resistance (CR) rates how well the window resists moisture forming on its inside surface. Higher is better. Mezzo ratings run from 43 to 62.

"Triple Strength" is not triple-pane

You may see glass listed as Double Strength or Triple Strength. Reasonable people read that as two panes or three. It's actually about the thickness of the glass, not the number of panes. Triple Strength is a durability and size option, not an energy upgrade.

The ratings show it. For one Mezzo double-hung configuration (the 3001 with composite reinforcement), Alside rates Double Strength glass at a U-factor of 0.28 and Triple Strength at 0.30. Slightly worse, not better. So don't choose "triple" expecting a warmer window.

ENERGY STAR depends on where you live

ENERGY STAR isn't a label a window carries everywhere. Qualification is based on climate zone, so whether a window qualifies depends on the exact model, the glass package, the options and your zip code. Alside notes that some models need cavity foam to reach their U-factor.

Alside publishes a lookup for this: ENERGY STAR 7.0 requirements for Alside windows. Treat any ENERGY STAR claim, from anyone, as true for a specific configuration in a specific climate zone, not for a window in general.

How to use the numbers

1. Compare like with like. The same model, size and glass package, with one thing changed at a time.
2. For U-factor, lower wins. It describes the whole window, not just the glass.
3. For SHGC, pick by climate. More heat to keep out, go lower. More winter to heat through, a higher number can help.
4. Don't choose thicker glass expecting more insulation. Check the ratings for the actual glass you're choosing.

Can't find the rating for the configuration you're looking at? Ask us. And if you're weighing performance against price, Window Replacement Cost: What Moves the Price covers what each choice does to the price.