The short answer: U-factor measures how fast heat flows through the window, and SHGC measures how much of the sun’s heat the glass lets in. Both are better when lower. In North Texas, where you spend far more of the year cooling than heating, SHGC is the number that decides whether a room is comfortable in August.
Where these numbers come from
Every window sold with an NFRC label has been tested to the same standard, which is what makes the numbers comparable across brands. If a window has no NFRC label, you have no basis for comparison and should treat any performance claim about it as marketing.
U-factor
The rate heat is conducted through the whole window assembly: glass, frame and spacer together. Typical residential values run roughly from 0.20 to 0.50, and lower is better.
U-factor is the number that dominates in heating climates, where the task is keeping warmth in. It still matters here, for January and for comfort near the glass, but it is not the one that fixes a hot west facing room.
SHGC, and why it is the one to watch
Solar heat gain coefficient is the fraction of the sun’s energy that gets through the window, from 0 to 1. An SHGC of 0.25 means a quarter of the solar heat hitting that glass ends up in your house.
In a cooling dominated climate this is the lever. A window can have an excellent U-factor and still make a room miserable in summer if its SHGC is high, because it is letting solar energy straight in and your air conditioner is then paying to remove it.
ENERGY STAR sets different criteria by climate zone for exactly this reason, and the southern criteria are built around a low SHGC rather than a low U-factor.
What gets you a low SHGC
Mostly the low-E coating, a microscopically thin metallic layer on the glass that reflects infrared while letting visible light through. Low-E coatings are tuned differently for different climates, which is why a window specified for Minnesota is the wrong window for Texas even at the same price.
The number of panes, the gas fill and the spacer affect U-factor more than SHGC. That is the common confusion: people add a third pane expecting the summer problem to improve, and it mostly improves the winter one. We wrote separately about whether triple pane is worth it here.
Reading a label in thirty seconds
- Find the NFRC label. If there is not one, stop.
- Read SHGC first. For sun facing elevations in North Texas, lower is better and this is your priority.
- Read U-factor second. Lower is better.
- Ignore visible transmittance unless daylight matters to you; it tells you how much visible light gets through, not how much heat.
- Compare like for like across quotes. Two bids are only comparable if you are reading the same two numbers on both.
A caution about orientation
You do not necessarily want the lowest SHGC on every window in the house. North facing glass gets little direct sun, and on those openings a very low SHGC buys you less while costing the same. The sensible approach is to prioritize the west and south elevations, which is where the afternoon load actually is.
Anyone quoting a single glass package for an entire house without asking which way it faces is not optimizing for your home. More on how we think about this on our energy efficient windows page.
We will walk the house with you, note the orientation of each opening, and spec the glass accordingly. Going Green Concepts is family owned and has been replacing windows and doors across DFW for more than twenty years. Estimates are free and there is no obligation. Call (682) 209-1449 or see our project cost calculator for a rough range first.