How to Read a Window’s Energy Label in Daly City (The NFRC Numbers)

Stand in a showroom or flip through a window quote and every unit comes with a small white sticker covered in numbers. That sticker is the NFRC label, and it is the one honest, apples-to-apples way to compare two windows without taking a salesperson's word for it. Learn to read five numbers on it and you can tell a genuinely good window from a nicely marketed one in about ten seconds.

This guide walks through what each rating on the label means, which direction is better, and — just as important for a house on the San Mateo coast — which of them actually matter in Daly City's cool, foggy, wind-driven climate. The numbers that make a window great in Phoenix are not the ones that make it great a few blocks from the Pacific.

What the NFRC Label Is and Why It's on Every Window

The NFRC label is the standardized white sticker the National Fenestration Rating Council requires on certified windows, and it exists so every manufacturer's performance is measured the same way. Rather than trusting brand claims, you get independently verified numbers you can line up side by side across brands. ENERGY STAR certification is built on these same ratings, so the label is also how you confirm a window actually meets the efficiency bar it is being sold under. When you compare quotes, compare the labels, not the brochures.

U-Factor: How Well the Window Holds Heat In

U-factor measures how fast a window loses heat, and it is the single most important number for a Daly City home. The National Fenestration Rating Council puts residential U-factors on a scale from 0.20 to 1.20, where a lower number means the window keeps heat in better. In a cool marine climate where the real job is holding warmth inside against the fog and wind rather than fighting off heat, a low U-factor is what keeps rooms comfortable and the heating bill down. A quality double-pane, low-E, argon-filled unit lands in the lower part of that range; single-pane aluminum sits near the top, which is why the original windows in so many Daly City homes feel cold to sit beside.

SHGC: How Much of the Sun's Heat Gets Through

The solar heat gain coefficient, or SHGC, measures how much of the sun's heat passes through the glass, on a scale from 0 to 1 where a lower number lets in less heat. In hot inland climates this is the number everyone chases, because keeping solar heat out is the whole game. Daly City is the opposite case for most of the house: the fog keeps direct sun off a lot of elevations, so an aggressively low SHGC is rarely the priority and can even leave rooms feeling colder. The exception is the west- and south-facing walls up on the Skyline ridge or above the fog line, where afternoon sun is real and a moderate SHGC earns its place.

Visible Transmittance: How Much Daylight Comes Through

Visible transmittance, or VT, measures how much daylight the window lets in, on a 0 to 1 scale where a higher number means more light. It is an easy number to ignore until you live under the fog. In a city that spends many days under low gray overcast, daylight is a scarce resource, so a higher VT helps rooms feel bright rather than gloomy on the typical foggy afternoon. The trade-off to watch is that some heavily tinted, solar-control coatings cut VT along with SHGC, which is the wrong trade for most Daly City windows where you want to keep the daylight you can get.

Air Leakage: How Much Wind Sneaks Through

Air leakage measures how much air passes through the window assembly, and the NFRC caps a certified rating at 0.3, with a lower number meaning a tighter window. This rating punches above its weight in Daly City. The city is one of the windiest corners of the Bay Area, and steady wind turns every small gap into a felt draft and a heating loss, so a tight air-leakage figure matters more here than in a sheltered inland town. When two windows have similar U-factors, the one with the lower air-leakage number is usually the better choice for a windy coastal lot.

Condensation Resistance: How Well It Fights Fogging

Condensation resistance rates how well a window resists moisture forming on the inside of the glass, on a scale where a higher number resists better. In the damp fog belt this is more than a comfort nicety — persistent interior condensation feeds mold and rots sills over time. A window earns a better condensation-resistance number partly through a warm-edge spacer and an insulating gas fill, which the Department of Energy notes keep the inner pane closer to room temperature. Not every label shows this rating, but when it does, it is worth reading on a coast this humid.

The Low-E Coating Behind the Numbers

Most of what makes those label numbers good is a coating you cannot see. A low-emissivity, or low-E, coating is a microscopically thin metallic layer on the glass that reflects heat while letting light through, and the Department of Energy reports that low-E coatings cut energy loss by 30 to 50 percent for only about 10 to 15 percent more than plain glass. The coating is what lets a window post a low U-factor and a tuned SHGC at the same time, and it can be formulated for high, moderate or low solar gain. On the coast you generally want a low-E that holds the U-factor down without crushing the VT, so the foggy-day light still comes through.

ENERGY STAR and What It Means for a Daly City Home

ENERGY STAR sets target U-factor and SHGC values by climate zone, and a certified window is simply one that meets the bar for your region. The Department of Energy advises homeowners to look for the ENERGY STAR label when buying new windows as a reliable shortcut. For Daly City, the practical reading of all of this is a clear priority order: a low U-factor and a tight air-leakage number come first, a healthy VT keeps the foggy-day light, condensation resistance guards against the damp, and SHGC is a secondary concern reserved for the genuinely sunny elevations. That order is what a proper energy-efficient window installation is specified around, opening by opening, and it is also why a failed seal is often just a double-pane glass replacement rather than a whole new window.

Frequently Asked Questions

What is the most important number on a window label?

For a Daly City home, the U-factor and the air-leakage rating matter most, because the climate's real demands are holding heat in and keeping the wind out. In hot inland regions the SHGC would top the list instead. There is no single answer for everywhere — the right priority depends on your climate.

Is a lower U-factor always better?

Lower is better for keeping heat in, yes, and in a cool coastal climate a low U-factor is almost always worth prioritizing. The point of diminishing returns comes when you pay a large premium to push it slightly lower on a sheltered wall, when that same money might do more on air-tightness or on more windows.

Why do my new windows still get condensation?

A little interior condensation on a cold, damp morning is normal and is about indoor humidity meeting cold glass, not a window defect. Persistent fogging sealed between the panes is different — that means the insulated glass unit's seal has failed and the unit, though often not the whole window, needs replacing.

Does ENERGY STAR certification guarantee the right window for Daly City?

It guarantees the window meets the efficiency bar for the region, which is a solid floor, but it does not tailor the choice to your specific exposures. Reading the individual NFRC numbers and matching them to each elevation — sun, wind and fog — is how you get from a generically good window to the right one for your house. If you want that done for you, Daly City Window Replacement can walk the openings and spec each one.