8 short lessons, one behind every question. Each opens when you answer a question about it, and they all open when you finish.
01 · RAT = 100%Where the sun's energy goes
Sunlight hits your windows as three kinds of energy. By the IWFA's figures, roughly 53% is infrared, 44% is visible light and just 3% is ultraviolet. Other references shift those numbers a point or so, because standards bodies draw the band edges in slightly different places.
When that energy reaches glass, exactly three things can happen. It bounces off, it soaks into the glass, or it passes through. Reflected, absorbed, transmitted: RAT. The three always total 100%. One point off is rounding. More than that and I call the manufacturer before I trust the sheet.
Plain clear 1/8-inch glass reflects about 8%, absorbs about 9% and transmits about 83%. A bare window is close to an open door for solar energy.
Reflected energy never gets inside. Transmitted energy heats whatever it lands on. Absorbed energy is the tricky one. The glass warms up and sheds that heat inward, outward or both, by convection and by re-radiating it as far infrared, and some conducts into the frame. Remember that part, because absorbed heat is also what cracks glass.
Reflected plus absorbed plus transmitted always equals 100%, and a spec sheet that doesn't add up isn't worth trusting.
Sources: IWFA Flat Glass Solar Control Education Guide (PDF) · IWFA Advanced Solar Control Education Guide (PDF)
02 · SHGCSHGC, TSER and the shading coefficient
SHGC, the solar heat gain coefficient, is the number the glass and film business runs on. It's the share of the sun's energy that ends up inside: what passes straight through, plus absorbed heat the glass later releases into the room. It runs from 0 to 1. Lower means less solar heat.
Customers hate a scale where smaller is better, so the film industry flips it. TSER, total solar energy rejected, is 1 minus SHGC as a percent. SHGC 0.35 means 65% rejected. Notice what that rules out: you can't get rejection from transmittance alone, because some absorbed heat still finds its way in.
The older shading coefficient compares a glazing with plain clear glass and ignores the frame. SHGC took over because it can describe the whole window. That's also why a spec sheet's center-of-glass SHGC won't match the whole-window values in the NFRC applied film listings. Two honest numbers, measured two different ways.
One more honest note. Lower SHGC cuts cooling load, but it also cuts free winter sun. In a heating-dominated climate the yearly energy bill can even creep up a little, which is why big buildings deserve a real energy analysis.
Lower SHGC means less solar heat in, TSER is just 1 minus SHGC, and center-of-glass and whole-window numbers are not the same thing.
Sources: IWFA Flat Glass Solar Control Education Guide (PDF) · IWFA Advanced Solar Control Education Guide (PDF) · National Fenestration Rating Council (NFRC)
03 · U-valueU-value, emissivity and low-E
SHGC is about sunshine. U-value is about temperature difference: how much heat moves through a square foot of glass, per hour, for each degree between inside and outside. Lower is better. R-value is the flip side, 1 divided by U, where higher is better.
What drives U-value down is low emissivity. Emissivity runs from 0 to 1 and says how readily a surface gives off heat as radiation. Polished aluminum is about 0.05. Plain glass is 0.84, close to a perfect emitter, which is why windows leak so much heat. That chill you feel by a window in winter is often your own body heat radiating to the cold glass, though most people blame the air.
A low-E film adds a low-emissivity surface. Heat flows from warm to cold, outward in winter and inward in summer, so a lower U-value slows both. The bigger payoff is on heating.
Low-E glass comes in two kinds: standard for heating climates, and high-performance, with solar reflection, for cooling climates. Where the coating sits in a dual-pane unit barely moves U-value but can swing SHGC. And never film over an exposed low-E coating. You'd erase the very property you paid for.
U-value tracks heat moved by temperature difference, lower is better, and low emissivity is what pushes it down.
Sources: IWFA Flat Glass Solar Control Education Guide (PDF) · IWFA Advanced Solar Control Education Guide (PDF)
04 · VLT / VLRLight, glare and the mirror effect
VLT is the percent of visible light that gets through the glass. VLR is the percent that bounces back. Glare control is almost entirely a VLT story, though the color of the light plays a part, with blue light the bigger troublemaker.
Darker isn't automatically cooler. Films with the same VLT can carry very different heat numbers. LSG sorts that out: VLT divided by SHGC. A 60% VLT film with SHGC 0.40 scores 1.5, and anything over 1.00 counts as spectrally selective, meaning more daylight per unit of solar heat.
Reflective films are strong heat stoppers. On those, lower VLT generally means more heat rejected and a shinier face. The daytime mirror also buys privacy, but only while it's brighter outside than in. Turn the lights on at night and that privacy all but disappears. Some reflective films are built with less room-side reflection for buildings used after dark.
Most homeowners don't want a dark house, so for glare I steer them to a medium VLT in a neutral shade with low reflectivity. On a commercial facade, judge the look with full-size samples at different times of day and in different weather.
Glare follows VLT, solar heat follows SHGC, and LSG tells you how much daylight you keep for the heat you let in.
Sources: IWFA Flat Glass Solar Control Education Guide (PDF) · IWFA Advanced Solar Control Education Guide (PDF)
05 · FadeFading: what film can promise
Here's the honest version. No film or glazing product stops fading. Good film slows it down, and that's still worth plenty.
The IWFA guides name at least five contributors: ultraviolet, visible light from the sun and from lamps, humidity, dye fastness, and chemical vapors in the air. UV is considered the harshest, but its share changes from one material to the next. Wood is extremely vulnerable, and paper, inks, natural dyes and natural fibers fade faster than synthetics.
So fade protection isn't one number. It comes from blocking UV, from cutting visible light, and to some extent from less near-infrared coming straight through. Maximum protection means a dimmer room, and most people don't want to live in a museum. That trade-off gets discussed, not hidden.
On a spec sheet, UVT is the actual measurement: the percent of UV that gets through. "UV rejected" is the same fact flipped for customers. A fade index that weighs UV and visible light can compare two glazings protecting the same material, but it can't tell you how fast or how far something will fade. Anyone promising "fade-proof" is selling a word.
Film slows fading by cutting UV, visible light and some near-infrared, but nothing stops it, so never trust a fade-proof promise.
Sources: IWFA Flat Glass Solar Control Education Guide (PDF) · IWFA Advanced Solar Control Education Guide (PDF)
06 · Thermal stressGlass first: will this pane crack?
Before I talk film, I identify the glass. Annealed is the most fragile and breaks into sharp shards. Heat-strengthened is about twice as strong but breaks much the same way. Tempered is about four times as strong, and it's highly unlikely a tempered system will crack from heat. Annealed laminated is touchier than plain annealed. Dual-pane units, or IGUs, carry extra risk too, and their surfaces are numbered 1 to 4 from outside in.
Thermal cracks come from a hot center and a cold edge under the frame. They start at the edge, run at a right angle for the first half inch to an inch, then wander. Absorption is often the biggest film factor, and risk stacks with thickness, size, tint, coatings, east exposure, altitude, partial shading and tight blinds.
Without a manufacturer's chart, the general rules say no solar film on clear annealed glass over 3/8 inch, tinted annealed over 1/4 inch, annealed laminated of any thickness, single-pane annealed over 100 square feet, or annealed IGUs over 40 square feet. With a chart, follow it and get the film approved before you quote it.
Fogged IGUs? There's little objective evidence film causes seal failure, though film can make an existing one obvious. Ask the film maker about warranty terms.
Know the glass and run the manufacturer's film-to-glass check before you sell, because absorbed heat plus a cold edge is how panes crack.
Sources: IWFA Advanced Solar Control Education Guide (PDF) · IWFA Flat Glass Solar Control Education Guide (PDF)
07 · Cure daysJudging the install and the cure
Fresh film isn't finished film. Right after install you'll often see water bubbles, water haze and slight distortion. That's curing, not a defect, and it clears as the water leaves. Sputtered films dry slower, and their haze can take several days.
Typical cure time grows with thickness: about 30 days up to 4 mil, 60 days from 4 to 8 mil, and 140 days from just over 12 up to 17 mil. Some high-performance coatings run longer, so ask the manufacturer.
Here's how a job gets judged. Look from the room side, at right angles to the glass, from at least 6 feet (2 m), in natural daylight but not direct sun. The main check covers the normal vision area, not the 2-inch band at the edge. Specks, hairs, small bubbles or light scratches pass if you can't pick them out that way.
Edges get a deliberate gap, normally 1/32 to 1/16 inch, so water squeegees out and the film doesn't touch the frame and peel. Darker films do best at 1/16 inch or less. Big panes may need splices, which run straight and parallel to a frame edge. And go easy on the squeegee there: large lites can break under the pressure.
Cure-time water marks are normal, the standard is judged at 6 feet in daylight, and a small even edge gap is a feature, not a flaw.
Sources: IWFA Architectural Visual Inspection Guideline for Applied Window Film (PDF) · IWFA Flat Glass Solar Control Education Guide (PDF)
08 · InstallBefore the squeegee
Every flat glass install starts the same way. Peel the liner, spray soapy water or the maker's slip solution on both the glass and the film's adhesive, position the film, cut it to size and squeegee the water out. Most films use a pressure sensitive adhesive that bonds by pressure. Some use a water-activated, or dry, adhesive that bonds chemically: very strong, very clear and long-lived, but harder to get off if the film is ever damaged.
Then come the windows to slow down for. Wired glass: no solar film on the inside of an exterior-facing wired pane, because the wire soaks up the heat the film reflects back, expands faster and can break the glass. Patterned or textured glass: adhesive can't bond to the texture, and since the smooth side usually faces out, exterior film is the usual answer. Louvered windows leave the film edge out in the weather, so exterior films are recommended there too.
Plastic is its own problem. Don't put film on acrylic or polycarbonate unless the film is made for it. Plastic can out-gas and blister the film, it expands and contracts differently than film over a day and a night, which can delaminate it, and it's soft enough that stripping failed film scratches it. Skylights mean lifts, overhead work and, outside, rain and snow that sit on the film, and they're often annealed laminated glass. Ask the film maker first.
Big glass takes care, too. On a very large pane, the squeegee pressure alone can break it. When a pane is both longer and wider than the roll, splice it: a straight seam parallel to one edge of the frame, with butted pieces no more than 1/64 inch (1 mm) apart, or overlapped. A clean splice line isn't a defect.
Check for wired, textured, plastic and oversized glass before you quote, and splice a pane that outgrows the roll straight, with no more than a 1/64 inch gap.
Sources: IWFA Flat Glass Solar Control Education Guide (PDF) · IWFA Architectural Visual Inspection Guideline for Applied Window Film (PDF)