A window label carries up to four ratings. Georgia is a mixed climate, so none of them get ignored. But U-factor is close to one decision for the whole house, while the right SHGC changes wall by wall. That is where the attention belongs, and it is why the glass on a north wall does not have to match the glass on a west wall.
A window label carries up to four ratings. They get read like a report card, where higher marks across the board mean a better window.
A window does not have one score.
Two of those numbers pull against each other. The coating that keeps heat out also changes the light coming in. Push one hard enough and you pay for it somewhere you can see.
And one of them has a different right answer on every wall of the house.
Here is what each rating means, which one deserves the most thought in Georgia, and what it costs to chase it.
What are the ratings on a window label?
Four, and each one answers a different question.
U-factor. How fast heat moves through the window. Lower is better. This is the number that does the work when you are trying to keep heat in.
SHGC, or solar heat gain coefficient. How much of the sun’s heat comes through. It is a fraction, so lower means less heat getting in. This is the number that does the work when you are trying to keep heat out.
VT, or visible transmittance. How much visible light comes through. Higher means a brighter room. This is the one you actually see.
AL, or air leakage. How much air gets past the assembly. Lower is better. It is optional on the label, so it will not always be there.
One thing worth knowing before you compare anything. These ratings cover the whole window, frame included, not just the glass.
Which rating deserves the most attention?
Not the one the market’s reputation would suggest.
Georgia is a mixed climate. Both loads are real here, and on an annual basis the heating side is the larger of the two. That surprises people who think of Atlanta as a hot-weather market, but the winters are long enough and cool enough to add up.
So neither number gets ignored.
What separates them is what you can still do about them.
U-factor is close to a single decision for the whole house. The same unit performs about the same on every wall. Pick a line and a package and that number is largely settled.
SHGC does not behave that way. The right answer changes wall by wall, because the sun does not treat every wall the same. A west wall in August and a north wall in August are not in the same climate.
That is why SHGC earns more of your attention. Not because it outranks U-factor. Because it is the one still open on every elevation.
One thing changes the wall-by-wall answer. Shade.
A window under a deep covered porch is not taking the same solar load as one on a bare wall. A west window with a real overhang above it behaves closer to a north window than a west one. Look at the elevation before you settle the glass, because the roof is part of the glass package whether anybody put it on the spec or not.
What does a low SHGC coating cost you?
Light and color.
Coatings that block solar heat do it by reflecting part of the spectrum. Push that hard and two things follow. VT drops, so less light comes in. And the light that does come through can carry a tint, usually toward blue or green depending on the coating.
On a small window in a hallway, nobody notices.
On a two-story wall of glass in the room you actually live in, people notice.
That is the tradeoff the label does not mention. Every number is printed on the sticker. How the room feels is not.
Which is why seeing a coating at full size, in daylight, settles this faster than any spec sheet. Ask to see the options you are choosing between under the same light.
Should every window in the house have the same glass?
No, and that is where the value is.
A north wall and a west wall are doing different jobs. The west wall takes the afternoon sun, so keeping heat out is the priority. The north wall gets very little of it, so you can accept a higher SHGC there and pick up more light and truer color for nothing.
Same house, two glass packages, a better result on both walls.
One rule goes with it. Do not mix coatings where both are visible at once. Two packages meeting at a corner, or two windows in the same room, can read as slightly different colors of glass. That looks like a mistake even when the numbers are right.
Split by elevation, not by room. Then check the corners.
What is not on the energy label?
Structure.
The energy label says nothing about how much wind and water a unit can take. That is a separate rating, usually called a performance grade or a DP number, and it comes from a different standard entirely.
On a standard window in a protected wall it rarely comes up. On a large unit, a tall unit, or anything on an exposed elevation, it is the number that matters most, and it is not on the sticker you are looking at.
Ask for it separately.
Where do these numbers get read wrong?
Four places.
Whole window against center of glass. The label rates the whole assembly. Center-of-glass numbers look better and sometimes turn up in sales material. If two numbers for the same product look far apart, check which one you are reading.
Comparing across different units. The same glass in a different frame, size, or configuration produces a different whole-window number. A rating belongs to a specific tested unit, not to a brand.
Chasing a number past the point it does anything. There is a point on every line where a better rating stops changing how the room feels and only changes the price. Ask where that point is on the line in front of you.
Reading the label without reading the wall. The rating describes one unit. It knows nothing about what is above it, what faces it, or how much of that wall is glass. A very good number on a wall that is mostly window still adds up to a lot of heat.
How does Lummus Supply help with this?
Lummus Supply carries a wide range of window lines, including Andersen, Pella, Quaker, Sierra Pacific, Windsor, Western Window Systems, and Lincoln. Glass packages vary across all of them, and so does what each one costs to move up a step.
Your local building department sets the minimum for your county. The team can tell you what each line actually offers against it, and where the useful upgrades stop.
There is also showroom space and full-size display homes in the Atlanta area. Seeing full-size product and real finish options in person settles a glass decision faster than a spec sheet does. Visits are by appointment, so book ahead.
What should you do next?
Send the plans with the elevations marked for direction. North, south, east, west. That one detail changes the whole conversation, because it turns a single glass decision into four smaller ones.
Then ask for options by elevation rather than one package for the house.
If there is a large wall of glass anywhere in the plan, ask about the coating and the performance grade in the same conversation. Those two numbers together decide more about that wall than the brand does.
Request a Consultation, or send the plans and let someone read them.
What is the difference between U-factor and SHGC on a window?
U-factor measures how fast heat moves through the whole window, and SHGC measures how much of the sun’s heat comes through the glass. Both are rated so that lower numbers mean less heat transfer, but they answer opposite questions. U-factor matters most when the goal is keeping heat inside, and SHGC matters most when the goal is keeping heat out.
Which window rating matters most in Georgia?
Georgia is a mixed climate, so U-factor and SHGC both carry real weight and neither should be ignored. On an annual basis the heating load here is larger than the market’s warm-weather reputation suggests. SHGC still deserves the most attention, because U-factor performs about the same on every wall while the right SHGC depends on which direction the wall faces and how much shade sits above it.
Can different windows in the same house have different glass?
Yes, and splitting the glass package by elevation is usually the better decision. A west wall taking afternoon sun benefits from a lower SHGC, while a north wall can accept a higher one and gain more light and truer color. The one caution is not to mix coatings where both are visible at once, since two packages meeting at a corner can read as different colors of glass.
