U-factor, SHGC, and why your room still cooks.
U-factor is the number windows lead with. SHGC is the one that decides what a Texas room feels like in August — and they're both on the same label.
There's a room in a lot of Dallas houses that runs ten degrees hotter than the rest of the place at six in the evening, with the AC going the whole time. It almost always faces west. The instinct is to blame the glass for conducting heat — hot pane, hot room. But in a Texas summer that usually isn't the main event: the dominant path is solar radiation passing straight through the window, heating the floor, the furniture, and you directly. Two different physical mechanisms, measured by two different numbers, fixed by two different things.
This is the thermal version of the argument we make about noise ratings. One number gets quoted constantly; the other governs the problem you actually have — and both sit on the same NFRC label. Before any of it, the same admission we make about acoustics: we have no lab-measured U-factor, SHGC, or R-value for Insulatx panels. Every figure on this page is a published range for a product or assembly category, or a rating body's own definition — not a test of our product. Where you see a number, it's cited as such.
U-factor is a
conduction number.
U-factor is one of the ratings the National Fenestration Rating Council (NFRC) certifies, and it measures thermal transmittance — how readily heat conducts and convects through the whole window assembly (glass, gaps, frame) when there's a temperature difference between inside and out. It's reported in Btu/h·ft²·°F, typically somewhere between about 0.25 and 1.25 for residential windows, and lower is better. A single pane sits near the top of that range; a double-pane low-E unit sits far lower.
Here's the catch: U-factor is a conduction-and-convection number. It describes heat leaking through the window driven by a temperature gap — the exact thing that matters on a cold night, when you're trying to keep warmth from escaping. That's why it was the priority metric in heating-dominated northern climates. What it does not capture is the sun. It says nothing about radiant solar energy coming straight through the glass, which in Texas is the bigger half of the problem. A sealed air gap and a thermal break improve U-factor — that's the mechanism a panel adds to your window — but a great U-factor and a hot room can absolutely coexist.
SHGC is a
sunlight number.
The Solar Heat Gain Coefficient is the other number on the same NFRC label, and it measures a completely different thing: the fraction of the sun's energy striking the window that actually gets through, as heat, into the room. It runs on a scale of 0 to 1 — typically between about 0.25 and 0.80 — and in a cooling climate, lower is better. An SHGC of 0.30 means roughly a third of the incident solar energy comes through; 0.70 means most of it does.
For a DFW summer, this is the governing number, and it's usually the one quoted less. A window can post an excellent U-factor and still bake a west-facing room, because U-factor never looks at the sun pouring through — and SHGC is exactly that. The rating programs know it: ENERGY STAR's Southern-zone criteria, updated in October 2023, are cooling-driven — they cap SHGC at 0.23 and U-factor at 0.32, and it's the tighter SHGC line that does the work in a hot climate. Here's how the two compare:
| U-factor | SHGC | |
|---|---|---|
| What it measures | conductive + convective heat flow through the assembly | fraction of the sun's energy that passes through |
| Scale / units | Btu/h·ft²·°F (~0.25–1.25) | 0 to 1 (~0.25–0.80) |
| Better is | lower | lower (in a cooling climate) |
| Matters most in | cold, heating-dominated climates | hot, cooling-dominated climates — Texas |
| Rated by | NFRC | NFRC |
If that pattern feels familiar, it should: it's the same shape as STC versus OITC for noise. One number is easy to quote and gets led with; the other actually governs the problem you're trying to solve, and it's on the same label, from the same body, quoted less. There, it was STC over OITC. Here, it's U-factor over SHGC. Same trick, different physics.
A west window is
the worst case.
SHGC only tells you how much solar energy a window lets through per unit of sun striking it. How much sun strikes it depends on which way it faces — and in Texas that difference is enormous. West and southwest glass takes the late-afternoon sun at a low angle, driving straight through the glazing at the hottest part of the day. That's why the west-facing room is the one that's still climbing at six in the evening while the AC loses ground. East-facing glass gets the same treatment in the morning; south-facing glass is more manageable in summer because the sun is high and an overhang can shade it.
North-facing glass barely matters for solar gain at all — it sees mostly diffuse sky light, almost no direct sun. Which means the same window, with the same SHGC on the same label, produces a very different room depending on orientation. A mediocre SHGC on a north window is a non-issue; a mediocre SHGC on a west window is the hot room. Before you compare any two window products on their thermal numbers, you have to know which direction the glass faces — the rating alone doesn't tell you what you'll feel.
What a panel does —
and doesn't — for heat.
This is the part that has to be said plainly, because the whole value of this page is saying it before you find it yourself. An interior acrylic panel adds a sealed pocket of air over your existing window. That is a conduction-and-convection fix: the trapped air resists heat flow, and it genuinely improves the U-factor of the assembly — the same reason double glazing was built for heat in the first place.
What it is not is a solar-radiation fix. Clear acrylic is optically clear across the visible band and passes most of the near-infrared, and that's where the bulk of the sun's heat lives. A clear panel lets the great majority of that solar energy pass straight through, just as your clear glass already does. For actually lowering SHGC — cutting the solar heat at the window — an exterior shade, a low-E coating, or a tinted or reflective film does considerably more, because those either intercept the sun before it reaches the glass or reject it at the glass surface.
So the honest version, for the west-facing window in July: a clear panel reduces the problem — it cuts the conductive share and the air gap helps — but it does not solve it the way an exterior solution would. If solar gain is your whole problem, we are not the strongest tool for it, and we'd rather tell you that.
The counterweight is real, though, and it's a tradeoff rather than a save. Exterior shades and tinted films cost you something — the view, the daylight, sometimes both — and most of them aren't options for renters, condo owners, or anyone under an HOA that governs the exterior or the glass. A panel keeps the glass fully clear, changes nothing you can see from the street, and lifts off without a trace. Different tools for different constraints: if you can shade or film the window, that's the bigger solar win; if you can't touch the outside or the glass, a panel is the reversible one, with clear-eyed expectations about which half of the problem it's fixing.
Blocking UV isn't
blocking heat.
Our own homepage says the panels filter up to 99% of UV, and it's worth being precise about what that does and doesn't mean, because it's easy to hear it as a heat claim. Ultraviolet is a small fraction of the sun's energy — published breakdowns put it at roughly 3 to 5% of the solar spectrum, with the rest split between visible light and infrared. Blocking UV is genuinely useful: UV is what fades hardwood floors, upholstery, fabric, and artwork over time, so cutting it protects your interior. But it is not the same as blocking heat. The heat is carried overwhelmingly by the visible and infrared — the other 95% or so — which a clear panel largely passes. Treat the UV number as a fade-protection benefit, not a cooling one. Conflating the two is exactly the move this page exists to not make.
Ranked honestly
for a Texas summer.
For solar heat specifically, here's roughly where each tool lands — strongest first, with honest caveats where they overlap, and where we actually sit in it:
- Exterior shading — awnings, overhangs, exterior shades, and trees. The most effective, because it stops the sun before it ever reaches the glass — and it matters most on exactly the orientation this page is about. The DOE reports window awnings cutting solar heat gain by up to 65% on south-facing windows and up to 77% on west-facing ones; separate Building America (PNNL) field research on exterior shades during an extreme-heat event found about a 55% reduction. The catch is that it's permanent, visible, and an exterior change — often not yours to make.
- Glazing and film — low-E glass or a tinted / reflective window film. Reduces SHGC right at the glass. Very effective, but it changes the glass or the view, film can affect a manufacturer's glass warranty, and it's often not permitted in rentals or condos.
- An interior air-gap panel — what we do. Its real strengths: it keeps the glass fully clear, preserves the view and the daylight, works year-round without being opened or closed, does the U-factor half of the job, and comes off without a trace. On solar gain specifically it's modest — the sealed air gap helps, but a clear panel was never primarily a solar tool. Honest placement: choose it for those advantages, or when the exterior and the glass are off-limits — not as the strongest solar fix.
- Interior blinds, shades, and curtains. These vary enormously by type and fit, so a single number for the category would be dishonest. Building America's extreme-heat field research put interior roller shades around 18% and insulating cellular shades around 23%; the DOE puts highly reflective blinds, fully closed, around 45%. They're the cheapest option, and because the sun has already passed the glass by the time an interior covering sees it, even the best interior product trails an exterior one. The honest part that matters for us: a tightly fitted cellular or reflective shade is a real solar tool, and on solar gain specifically it can match or beat a clear interior panel — because a clear panel passes most of the sun's energy. What a shade costs you is the view, the daylight, and having to operate it.
There's no single right answer — the best tool is the one that fits your constraints. If you can shade the outside, that's the biggest solar win. If you rent, or the HOA governs the exterior, or you don't want to alter the glass, an interior panel is the reversible option — and it does the U-factor half of the job well, even if it isn't the solar half's strongest player.
The questions
worth asking.
- Ask for SHGC, not just U-factor. Both are on the NFRC label. In a Texas summer, the one that governs your room is usually the one quoted less.
- Look at the actual NFRC label. The real sticker with the certified numbers, not a rounded figure in a brochure or a sales line.
- Know the window's orientation before you compare anything. A given SHGC on a north window and the same SHGC on a west window are not the same problem. Direction changes everything.
- Be sceptical of any thermal claim with no test conditions. A number with no NFRC rating, no assembly described, and no lab behind it is a claim, not a measurement — the same standard we hold ourselves to here.
That's the whole thermal picture, told the same way we tell the acoustic one: name the number that governs the problem, be honest about which half we actually fix, and point you at the tool that fits. If it's noise rather than heat you're fighting, the soundproofing guides are the other half of this site — and if you want us to come look at your specific windows, a free on-site assessment is where that starts.
Common questions,
answered.
Still wondering something? Call (945) 395-9424 — we'll tell you straight what we can do for your home.
What's the difference between U-factor and SHGC?
U-factor measures heat conducting and convecting through the window — it matters most on a cold night when you're keeping warmth in. SHGC measures the fraction of the sun's energy that passes straight through the glass — it matters most on a hot afternoon. In Texas, SHGC usually governs how the room feels.
My windows are energy-efficient but the room still cooks. Why?
Efficiency ratings usually lead with U-factor. If the window has a high SHGC and faces west, the sun pours straight through in the late afternoon regardless of how well the window insulates. Orientation plus SHGC, not U-factor, explain the hot room.
Does a clear acrylic panel block the sun's heat?
Partly. It adds a sealed air gap that cuts the conductive and convective path and improves the assembly's U-factor. But clear acrylic passes most visible and near-infrared solar energy, so it isn't primarily a solar (SHGC) fix. For raw solar blocking, an exterior shade or a tinted/reflective film does more.
Doesn't 99% UV blocking mean it blocks heat?
No. UV is a small slice of the sun's energy — published figures put it around 3–5%. Blocking UV protects floors, fabric, and artwork from fading, which is real, but most solar heat rides in the visible and infrared, which a clear panel largely passes. UV protection and heat rejection are not the same thing.
Will this lower my electric bill?
It can, but honestly the size depends. A panel reduces one heat path — conduction and convection through the glass — and adds an insulating air gap. How much that shows up on the bill depends on the window's orientation, how much glass you have, and what the rest of the envelope (insulation, roof, ducts) is doing. We won't put a percentage on it: there's no lab figure for our panels, and it varies too much house to house.
What actually stops west-facing summer heat best?
Roughly in order: exterior shading, then low-E glass or tinted film, then an interior air-gap panel, then interior blinds. A clear interior panel keeps the view and comes off clean, but it isn't the strongest solar fix — pick the tool that fits your constraints.
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