Do my windows already block UV, or do I need something else?

Do windows block UV on their own, and what still gets through?

Sunlight arriving at your glass is a spectrum, and the parts behave differently.

• UVB is the shorter, higher-energy ultraviolet band most associated with sunburn. Ordinary glass blocks the bulk of it. That is why you can sit in a sunny window seat all afternoon without burning while doing exactly that outdoors would leave a mark.
• UVA is longer-wavelength ultraviolet, and a substantial portion of it passes through ordinary glass. It is present all day, all year, and it is a well-established contributor to fading and to skin damage.
• Visible light is what you see by, and it carries plenty of energy to break down dye molecules on its own. Fabric fades in a north-facing room with no direct sun for exactly this reason.
• Near-infrared is not visible and is not UV. It is the part you feel as heat on your arm through the glass, and heat accelerates most of the chemistry involved in fading and drying.

The single most useful correction here: nobody should be told that UV is the whole fading story. Whatever share you assign to each part of the spectrum, all four bands arrive together, and a product that removes only one of them leaves the rest of the job undone. Anyone giving you a precise percentage split between UV, visible light and heat should be asked where the number came from, because the version that circulates in this industry has no traceable source.

Does Low-E glass block UV?

Some, incidentally. But that is not the job it was designed for, and this is where most of the confusion starts.

A Low-E coating is a microscopically thin layer, usually metal oxide, deposited on a glass surface inside a sealed unit. Its purpose is to control emissivity, meaning how readily a surface radiates heat energy, and to reflect long-wave infrared. That is how it keeps summer heat out and winter heat in. The engineering targets are thermal: U-factor, solar heat gain coefficient, radiant comfort near the glass.

UV performance is a side effect of that coating chemistry rather than its objective, and it varies substantially between products. A Low-E unit from one manufacturer and a Low-E unit from another can perform quite differently on ultraviolet while sharing a similar thermal rating.

The right move is to stop generalizing and look up your actual windows.

• Find the NFRC label if the windows are recent, or the maker's stamp etched into a corner of the glass.
• Call the manufacturer with that code and ask for the performance data for that specific glazing package.
• Ask for the ultraviolet number separately from the solar heat gain coefficient. They are different measurements and a good SHGC does not imply a good UV figure.

If you cannot get the data, you do not know. That is a legitimate answer, and it is better than assuming either way.

How do I tell what glass I already have?

You can learn most of this yourself in ten minutes, without buying anything.

Count the panes. Look at the edge of the glass where it meets the frame. A single sheet is single pane. Two sheets with a metal or composite spacer between them is a sealed insulating unit.

Do the flame test after dark. Hold a small flame near the glass and look at the reflections. Each glass surface gives you one, so a double-pane unit shows four. If one reflection is a noticeably different color from the others, a coating is present on that surface. This tells you that a coating exists. It does not tell you what the coating does, so it is a starting point rather than an answer.

Look for laminated glass. Laminated units are two sheets bonded to a plastic interlayer, and you can sometimes see that layer as a fine line at an exposed edge. Tapped, they sound duller than a single sheet. They are common in specific applications, and the Skin Cancer Foundation's Seal of Recommendation program covers laminated glass among the product categories it evaluates, so laminated products can be rated for high UV blocking.

Find the etched stamp. Most manufacturers etch a small code in a corner. Photograph it and call the maker. That single phone call replaces every guess on this page.

Check the age. If the windows came with a house built decades ago and have never been replaced, single-pane annealed glass with no coating is the likely answer.

If a film blocks 99.9% of UV, why do things still fade?

Because UV was never the whole mechanism.

The International Window Film Association's FAQ puts it about as plainly as an industry body can. Film can reduce the impact of the sun's UV rays as a cause of fading by up to 99%, and no window film can eliminate fading. Both halves of that sentence come from the same source, and the second half is the one that gets left out of sales conversations.

What keeps working after the UV is gone:

• Visible light. Dye molecules absorb visible wavelengths and break down. This is why a watercolor fades in a gallery with UV-filtered lighting, and why museums dim rooms rather than relying on UV filtration alone.
• Heat. Elevated surface temperature speeds chemical reactions, dries leather and wood, and stresses finishes.
• Time. Every one of these effects is cumulative and none of them reverse.

So the right way to read a 99.9% UV rejection figure is that it removes one major driver completely and leaves the others reduced but present. That is genuinely valuable. It is not a fade-proof room, and a product sold to you as one is being oversold.

It also means the specification that matters for fading is not UV alone. Visible light transmission and total solar energy rejection tell you how much of the rest you are removing, and those numbers are where films actually differ from each other.

What do the ratings mean — the Skin Cancer Foundation Seal, VLT, TSER?

Three numbers do most of the work, plus one third-party mark.

The Skin Cancer Foundation's Seal of Recommendation, a program the Foundation has run since 1981, covers architectural window film along with automotive window tint, laminated glass and sunglass lenses. To earn it, a window film or tint product must block 99% or more of both UVA and UVB radiation, and submissions are reviewed by the Foundation's volunteer Photobiology Committee of dermatologists. It is an independent, checkable mark rather than a manufacturer's own claim, which is what makes it worth looking for.

Visible light transmission (VLT) is the percentage of visible light that gets through. Higher means a brighter room and a more natural view. It also means more of the visible energy that fades dye.

Total solar energy rejected (TSER) is the share of total solar energy, across the whole spectrum, that the glazing plus film keeps out. This is the number to compare between films for heat.

For reference, 3M's published Prestige Series technical data sheet lists minimum UV rejection of 99.9%, TSER of 50 to 62% at normal incidence and up to 66% at a 60-degree angle, infrared rejection up to 97%, and VLT options from 21% to 69%. 3M's Scotchshield Ultra S800 safety film data sheet separately lists UV transmission of less than 1% in the 300 to 380 nanometer range. Ask for the data sheet for the exact product being quoted, not a brochure for the family.

Should I just replace the windows instead?

Sometimes yes, and it is worth being straight about when.

Replacement makes sense when the windows have failed at their actual job: broken seals with condensation between the panes, rotted or racked frames, sashes that will not operate, single-pane glass in a house you intend to keep for decades. In those cases you are buying a working window and the glazing performance comes along with it. The U.S. Department of Energy's guidance holds that heat gain and loss through windows accounts for roughly 25 to 30% of residential heating and cooling energy use, so the thermal upgrade is real.

The cost is the constraint. A Texas-focused 2026 cost guide puts typical installed replacement at $600 to $1,000 per window, with a whole-home job running roughly $6,000 to $10,000 for ten windows and $12,000 to $22,000 or more above twenty. Film applied to sound existing glass is a different order of expense entirely.

Two technical notes if you go the film route on older glass. Thermal stress tolerance varies by glass type: annealed float glass has a much lower safe temperature differential, in the region of 105°F, while rough-cast and wired glass are more sensitive still, and tempered or heat-strengthened glass is highly resistant. That is why a competent installer assesses glass type, thickness, pane size and frame before recommending a film, especially a darker or higher-absorption one. And 3M backs its own assessment: the August 2024 warranty bulletin covers film-caused glass breakage for up to five years, up to $1,500 per window including labor.