What the U Factor on the Sticker Means, and Why It Is Not the Number That Matters Most in Riverside

Published August 27, 2026

There is a white sticker on every new window in California. Most people peel it off within a week of installation, which is a small problem, and most people never read it, which is a larger one. Almost everything worth knowing about the window you are buying is printed on that label, and the state has written a building code around two of the numbers on it.

This is a plain reading of that label for a Riverside homeowner.

What the NFRC Label Actually Is

The National Fenestration Rating Council is an independent body that certifies window performance. Manufacturers submit units for testing under a standard procedure and the certified results get printed on a temporary label that ships attached to the glass.

The point of it is comparability. Before it existed, every manufacturer measured performance their own way and every window was the best window. Now a vinyl slider from one company and a fiberglass casement from another are tested identically, so the numbers on the two labels mean the same thing.

Four values appear on essentially every label. Two of them decide whether the window is legal in your wall.

U Factor

U factor is a measure of how fast heat conducts through the whole window assembly. Not the glass on its own. The frame, the glass, the spacer between the panes, the gas fill, all of it, averaged together and expressed as a single number.

Lower is better. A lower U factor means heat moves through more slowly in either direction.

Some rough anchors, so the numbers mean something. An old single pane aluminum window, the kind still sitting in a lot of Riverside houses built before the eighties, sits somewhere around 1.2. Aluminum is an excellent conductor, which is exactly what you do not want in a window frame, and a single sheet of glass insulates almost not at all. A basic dual pane vinyl window lands around 0.45 to 0.50. A modern code compliant unit with a warm edge spacer and argon fill comes in at 0.30 or below.

That is a very large improvement, and it is worth understanding where it comes from. Most of it is not the glass coating. It is the switch from aluminum to vinyl or fiberglass in the frame, the second pane of glass, the argon between the panes and the non metallic spacer holding them apart. The coating on the glass does something else entirely.

SHGC

The solar heat gain coefficient is the fraction of solar energy landing on the window that ends up inside the house as heat. It runs from zero to one, and lower means less heat comes through.

This is not conduction. This is direct radiation from the sun, arriving as light, hitting your floor and your furniture and turning into heat in the room. It is a completely different mechanism from U factor, and it is controlled by a completely different part of the window, which is the low emissivity coating baked onto the glass.

An uncoated dual pane window sits around 0.70. A window with a standard low-E coating lands near 0.35 to 0.40. A solar control coating aimed at hot climates gets down to 0.20 or lower.

The Two Other Numbers

Visible transmittance is the fraction of visible light that gets through, again from zero to one. Higher means a brighter room. It is worth watching, because the coatings that drive SHGC down also cut visible light, and a whole house done with the most aggressive solar glass can end up feeling dim and slightly grey inside.

Air leakage measures how much air passes through the assembly under a standard pressure. Lower is better. In practice almost every modern window passes this comfortably and it is rarely the thing that decides a purchase.

What Title 24 Requires

The California Energy Code sets limits on both of the first two numbers.

On the prescriptive path, which is the straightforward route almost every Riverside replacement job takes, an altered fenestration product currently needs a U factor no higher than 0.30 and an SHGC no higher than 0.23. Meet both on every window and there is no energy modelling to do.

There is also a performance path, where an energy model of the whole house can trade one measure off against another. That is how a project with an unusual amount of glass, or one where the owner insists on a particular unit that misses a limit slightly, sometimes gets approved anyway. It costs money and takes time, so on a normal replacement job it is not worth doing.

Two honest caveats. The standards are revised on a cycle, so a limit that was correct two years ago is not proof about today, and any contractor quoting these numbers from memory rather than confirming them is guessing. And your local building department is the authority on how the code is applied to your permit, not the contractor and not a website.

Why SHGC Does More Work Here Than U Factor

Now the part that changes what you should actually buy.

U factor is the number the marketing leans on. It is on the front of the brochure, it improves every year, and it is genuinely the right number to obsess over if you live somewhere with real winters, where the job of the window is to hold expensive heat inside a house against a cold outdoors for months at a time.

Riverside is not that place. Our problem is the other direction and it is concentrated into an afternoon.

On a July day here the outdoor air might be well above body temperature, but that temperature difference is doing far less to your comfort than the sun coming through the glass on the west side of the house. Heat conducting slowly through a frame is a trickle. Direct sunlight through a large west facing slider is a flood, and it arrives between roughly three in the afternoon and sunset, which is exactly when nobody wants it.

That is what SHGC controls, and nothing else on the window does.

The practical test is one you have already run without thinking about it. Walk into a west facing room at five in the afternoon in August and it is noticeably hotter than the rest of the house, even with the air conditioning running. That difference is solar gain. Cutting the SHGC on that window from around 0.40 down to near 0.20 halves the energy arriving. You feel it. Improving the U factor on the same window by a similar proportion is much harder to notice.

So when someone hands you a quote, do not just check that the U factor passes. Check what the SHGC is, and then ask which way that wall faces.

Orientation Should Change the Order

This is the recommendation that follows from everything above, and it is one very few homeowners are offered.

The right glass is not the same on every side of the same house.

West and southwest facing openings take the full afternoon load and should get the most aggressive solar control coating available inside your budget. Low SHGC, and accept the reduction in visible light, because those rooms are getting plenty of light regardless.

South facing glass takes sun for a longer part of the day but at a higher angle, and a decent overhang or eave already shades a good deal of it. It wants solar control, though not necessarily the most extreme option.

East glass gets the morning, which is cooler and shorter, and is rarely the room anyone complains about.

North facing glass gets essentially no direct sun at all. Putting your most aggressive solar coating there buys you nothing and costs you daylight. Those openings can take a package with higher visible transmittance and still sit comfortably inside the code limits.

Ordering two glass packages instead of one adds less to the total than most people assume. Ask for it.

Leave the Labels On Until the Final

One practical thing to close on, because it is the most common way an otherwise clean job fails.

The inspector at the final walks the house and reads the temporary NFRC label on each installed window. They compare the printed U factor and SHGC against the values listed on the energy compliance form filed with your permit. That comparison is the verification.

If the labels have been peeled off, there is nothing to read. What follows is a paperwork exercise involving the manufacturer, a delay, and possibly a second inspection fee, all over a sticker.

They are unattractive. They are on there for a matter of weeks. Peel them the morning after sign off, and not one day sooner.

If You Want a Second Opinion on a Quote

Take a photo of the spec line on whatever quote you are holding, the one listing U factor and SHGC, and call us. We will tell you whether it passes on the prescriptive path and whether the glass suits the direction that wall faces. If it is a good quote we will say so and you can go ahead with it.

Bookjs, 1130 Magnolia Ave, Riverside. Call (951) 556-6455.