Materials and ratings
Energy efficient windows for Reno: how to read the NFRC label, from U-factor to air leakage
The temporary sticker on a new window is the only document in the whole transaction that was produced by somebody with nothing to sell. It carries five numbers. Here is what each one measures, which direction is good, and which of them matters most on a Northern Nevada wall.
What the label is, and what it deliberately is not
The National Fenestration Rating Council operates the certification programme that produces the label. Three properties matter when comparing quotes:
- It rates the whole product (glass, frame, spacer and sash), not the glass alone, so a brochure figure quoted for "the glass" is not comparable.
- It rates a specific configuration: a stated size, operating type, glass option and grid pattern. Change any of those and the correct label is a different label.
- It comes from an independent programme rather than a marketing department, which is the whole reason it is worth reading.
What it is not is a description of your wall. Every number was measured on a correctly installed product in a laboratory. Nothing on the label knows whether the rough opening was flashed, the unit shimmed square, or the gap around it insulated and air-sealed. Those are the installer's numbers, and nobody certifies them.
U-factor: the number that matters most in a heating climate
U-factor is the rate at which heat flows through the assembly. It is reported on the label on a scale running from about 0.10 to 1.20, and lower is better: the inverse of the R-value scale used for insulation, which is the most common source of confusion when a window quote is set beside an attic quote.
Northern Nevada spends far more of the year heating than cooling, so on most houses this is the line to optimise first. What moves it:
- Cavities and the gas in them. The obvious one, and not the largest one on a small window.
- The low-emissivity coating, which reflects long-wave heat back to the side it came from.
- The spacer at the edge of the sealed unit. A conductive spacer is a heat path straight around the glass; warm-edge spacer technology is one of the best-value lines on any quote.
- The frame. On a small unit the frame is a large fraction of the area, so frame material and chamber design carry real weight.
SHGC and VT: the two numbers that should be read together
Solar heat gain coefficient
SHGC is the fraction of incident solar energy that ends up inside, between 0 and 1. Lower means less solar heat admitted. Unlike U-factor there is no universally correct direction: it depends on the wall.
- West and south-west elevations. The strongest sun at the hottest part of the day, so a lower SHGC does useful work every summer afternoon.
- South elevations. A genuine trade. The winter sun is low enough to come deep into a room and that gain is free heat, while the summer sun is high enough that a modest overhang shades much of the same glass. With a decent eave, a higher SHGC is often right.
- North elevations. Little direct gain to manage; optimise U-factor and daylight instead.
Visible transmittance
VT is the fraction of visible light that gets through, also between 0 and 1, and higher means a brighter room. It is the number that stops a house being specified into gloom.
Reading VT and SHGC together exposes the quality of the coating. A spectrally selective low-emissivity coating blocks a lot of solar heat while passing a lot of visible light (a high VT relative to its SHGC). A cheap tint reaches a low SHGC by making everything darker, which shows up immediately as a low VT. Two windows with the same SHGC can be very different products, and this is where you see it.
One trap: grids, screens and applied film change the numbers, and the label describes the configuration as tested.
Air leakage and condensation resistance: the two most ignored lines
Air leakage
AL is reported in cubic feet per minute per square foot of window area, and lower is tighter. It is an optional line on the label, and its absence is itself informative.
This is the number we would most like homeowners to start asking about, because it describes the draught. A window with an excellent U-factor and a mediocre air leakage figure still feels wrong to sit next to on a windy evening. Operating type is most of the story: a casement or awning pulls its sash into a compression gasket as it latches, while a hung or sliding unit has to seal against something moving past it. That is a design difference rather than a quality difference, and it is why one line shows different AL figures across its operating types.
Condensation resistance
CR is an optional rating on a 1 to 100 scale where higher resists condensation better. It is a comparative index between products, not a humidity prediction.
In a dry climate this line gets dismissed, and it should not be. Winter interior humidity is generated indoors, and when it meets a cold interior glass or frame surface it condenses regardless of the outdoor air. Because CR is driven by the same things as U-factor, it is a good cross-check that a low headline U-factor came from improving the whole product rather than one clever component.
Three reasons a brochure number and a quoted number disagree
| Line | Scale | Direction | What moves it most |
|---|---|---|---|
| U-factor | about 0.10 to 1.20 | Lower is better | Cavities and fill, coating, spacer, frame |
| Solar heat gain coefficient | 0 to 1 | Depends on the elevation | Coating type and number of coated surfaces |
| Visible transmittance | 0 to 1 | Higher is brighter | Coating selectivity, tint, grids |
| Air leakage | cfm per square foot | Lower is tighter | Operating type, weatherstrip, hardware |
| Condensation resistance | 1 to 100 | Higher is better | Frame material, spacer, coating |
With that in hand, the three traps:
- A brochure figure is the best configuration in the line. A "from" U-factor is usually the largest fixed unit with the most expensive glass package.
- Fixed and operable units in one line rate differently. A picture window has no sash seal to leak through and less frame per square foot, so its numbers flatter the line.
- ENERGY STAR certification is per climate zone. Whether a configuration qualifies for the zone we are in is stated on the label, and it is not one national threshold. A product can be certified somewhere and not here.
What we hand over, and why a plan reviewer wants it
Every estimate we write names the line, the series, the operating type and the glass package per opening, and the certified rating documents for those configurations go with it. That is the paperwork a plan reviewer asks for where a permit is involved, and the record that lets a warranty claim be assessed years later against what was actually supplied.
Two closing pieces of advice. Do not peel the temporary label off until the inspection is done and you have photographed it. It is the proof of what was installed, and the permanent identifier etched into the unit is far harder to read. And when comparing two quotes, compare labels for matching configurations rather than headline figures.
Frame materials, glass packages and the lines we order from are on our replacement window page and the brands page.
Reading the label
Questions about the numbers on the sticker
U-factor, for most houses. We spend far more of the year heating than cooling, and U-factor is the line that describes heat leaving the building through the assembly. Lower is better, which is the opposite direction from the R-value scale used for insulation. Solar heat gain coefficient comes next, and it is decided per elevation rather than per house.
Because a brochure figure is almost always the best-performing configuration in the line (typically the largest fixed unit with the most expensive glass package), and a certified label describes one specific size, operating type, glass option and grid pattern. An operable unit in the size you need has more frame per square foot and a sash seal, so it rates differently. Compare labels for matching configurations, not headline numbers.
Not until the job is signed off and the label has been photographed. It is the proof of what was actually supplied and installed, a plan reviewer may ask to see it where a permit is involved, and it is far easier to read than the permanent identifier etched into the unit. Once the inspection is done and you have a photograph on file, the sticker can come off.
Next step
Send us the openings and we send back labels, not adjectives
The useful version of a window quote names the line, the series, the operating type and the glass package for every opening, and comes with the certified rating documents for those exact configurations attached. That is the paperwork a plan reviewer asks for where a permit is involved.
Prefer to talk it through? The shop line is (775) 391-8487, Monday to Friday, 9:00 AM to 4:00 PM.
- Every opening measured, with orientation recorded
- Glass package chosen per elevation rather than per house
- Certified rating documents handed over with the written scope
Request a free estimate
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We will call you back on the number you gave us to set a measure visit. If you would rather talk now, the shop line is (775) 391-8487, Monday to Friday, 9:00 AM to 4:00 PM.