The two numbers
U-factor measures how much heat passes through the assembly by conduction. Lower is better. It is the number that matters in a cold climate, where the problem is heat escaping.
SHGC — solar heat gain coefficient — measures how much of the sun's energy comes through the glass. It runs from 0 to 1, and lower means less solar heat admitted. It is the number that matters in a cooling climate, where the problem is heat arriving.
Both appear on any legitimate specification. If a supplier will quote you one and not the other, that tells you something.
In Texas, SHGC is the one doing the work
All four metros we cover — Houston, Dallas–Fort Worth, Austin and San Antonio — sit in Climate Zone 2A. The prescriptive caps are a maximum U-factor of 0.40 and a maximum SHGC of 0.25, with Austin tightening the U-factor to 0.35 on new construction since it moved to the 2024 IECC in July 2025.
Here is what that means in practice. A thermally broken aluminum system clears U-0.40 comfortably — ours is specified at 0.23, well inside it. So the U-factor is not the constraint. The 0.25 SHGC is, and it is a hard cap on the prescriptive path with no relief. It means a solar-control low-E coating is not an upgrade you can decline; it is the thing that makes the opening legal.
The practical consequence is that the glass make-up, not the frame, decides whether a large glazed opening passes. Two suppliers quoting the same frame can be quoting very different doors.
Orientation changes the answer
A west-facing wall of glass on a Texas afternoon is a completely different load case from a shaded north elevation, and specifying one glass make-up for a whole house ignores that.
Austin has written this into its code. Glazing within 45° of east or west is capped at 0.25 SHGC unless it is shaded, north-facing glass is excluded from the area-weighted calculation altogether, and there is a projection-factor credit — a deep enough overhang above the opening relaxes the SHGC it has to meet. On a west-facing Hill Country elevation that credit is often what makes the opening you actually want buildable.
We specify to the orientation of the actual opening rather than quoting one build-up for the whole project.
What about the frame?
Aluminum conducts heat well, which is exactly what you do not want in a window frame. The answer is a thermal break — a polyamide barrier running the full length of the profile, separating the inside face from the outside so the two are not in direct metal-to-metal contact.
This is the single biggest difference between a modern aluminum system and the 1970s slider it usually replaces. Older frames were often not thermally broken at all, which is why they sweat in winter and feel cold to the touch. If you are comparing an aluminum door against a vinyl one on thermal grounds alone, make sure the aluminum one is actually broken — an unbroken aluminum frame deserves the reputation.
Does a bigger door mean a worse building?
Not necessarily, and this is worth saying because it is a persistent myth: there is no prescriptive cap on glazing area in the residential energy code. Large glazed areas are handled through the total-UA trade-off or the performance path, which is what we work to.
The constraint is the performance of the glass, not the size of the hole in the wall. A well-specified forty-foot opening can comply; a badly specified eight-foot one might not.
How to read a quote
Ask for the U-factor and SHGC of the glass make-up being quoted for your project, not the system's best-case figures from a brochure. Those are frequently different numbers, and the gap between them is where disappointment lives.