Will adding solar film to my windows help with temperature control?
Q: I need help deciding whether to put solar film on my west-facing windows in the Seattle area, and, if so, whether the film should go on the inside or the outside.
We have no air conditioning and haven’t needed it until recently. Now, though, it can reach 90 degrees in the house during the summer. At the same time, the sun coming through the windows in the winter brings warmth that would be blocked if I have UV light-blocking film on the windows. I’m told the film would help keep in the heat in the house in winter. What should I do?
A: Window films come in a wide range of types, varying in how much visible light they let through and how much they block other wavelengths. Some are designed to go on the inside of windows; others go on the outside. These differences, as well as the various trade-offs, can make it all confusing.
It helps to understand a bit about the wavelengths that window films affect. The air is full of invisible electromagnetic waves, ranging from radio waves at the longest to gamma rays and X-rays at the shortest. In the middle are the wavelengths targeted by window films: infrared, visible and ultraviolet. Among these three categories, infrared light has the longest wavelengths; it’s the warmth you feel when the sun shines in during the winter and is what you want to keep out in the summer. UV rays are shortest; they bleach and degrade furnishings. Visible light is in between, with violet having the shortest wavelengths and red having the longest.
All window films generally do a good job of blocking UV rays, which can ruin artwork, make leather upholstery brittle and change the color of flooring that’s hit by a patch of sunshine every day. Window films also make it harder to break glass, which protects against intruders. And if glass does break, most films grip the shards so they don’t fly across the room. But there’s a lot of variation in how the films deal with infrared and visible light.
In areas where air conditioners run for many months of the year, cooling costs and summer comfort are the biggest issues. Window film should block as much infrared light as possible. But films that are best at that also reduce daylight the most. So people need to weigh the trade-offs.
To find one that fits your priorities, look up the technical details for different films to see how they would perform on the type of windows you have — clear or tinted, single- or double-pane. Compare two key metrics. One is the solar heat gain coefficient, a measure of how well the film blocks heat from the sun. The lower the number, the better. The other key metric is the visible transmittance, a measure of how much visible light gets through. For this, the higher the number, the better.
As an example, someone in a hot climate might consider 3M’s Prestige Sun Control films, which come in both interior and exterior lines, with film types labeled by the approximate percentage of visible light they let through clear, single-pane glass. PR20X, the darkest-tinted film in the Prestige Exterior line, has a solar heat gain coefficient of .38 and a visible light transmittance of 21 percent. For the lightest-tinted film, PR90X, the corresponding numbers are .64 and 88%, which means this film blocks a lot less heat but lets through almost all of the natural light. If you wanted more protection from the sun’s heat and were willing to give up a little more daylight, you might choose the PR40X or PR70X films, which have metrics in between.
“It’s always a trade-off,” said Bret Seeger, a Seattle-area estimator for Solar Art, which installs many types of window films in locations across the country.
In areas where heating costs dominate, many homeowners appreciate window films that block some of the sun’s heat in the summer. But they also want a film that reflects heat back into the interior, so wintertime heat stays inside, rather than streaming out through the glass. Window films with low-e coatings do that. Low-e is shorthand for low emissivity; it refers to the fact that the film reflects heat wavelengths in both directions and so does not itself heat up enough to emit much energy. Low-e coatings let through a lot of visible light, but just like the films for hot-weather areas, they do reduce it.
When advising potential customers in the Seattle area, Seeger said he usually recommends films that block only 30 to 50% of visible light, even though they won’t keep rooms as cool in the summer or as warm in the winter. “We have so many dark days,” he said. “Do you want to live in a dungeon?”
Low-e window films, also known as insulating window films, come in different types. Besides comparing how much visible light they let through and how much solar heat gain they prevent, you can also compare how well they insulate. That’s listed in the technical details as U value, the same term used to compare insulation for walls or ceilings. The lower the number, the better. Choosing a low-e film that makes a room darker will keep a room cooler in the summer but won’t significantly change how well it insulates.
All of the data is measured at the glass surface, so effects in actual houses vary based on insulation and other factors. Seeger said that in a typical home in the Seattle area without air conditioning, low-e window film that lets in about 70% of visible light might lower room temperature in the summer by 3 to 4 degrees. Opting for low-e film that blocks more visible light might lower the summertime indoor temperature by up to 10 degrees. But if there is a long heat spell and temperatures don’t drop significantly at night, heat will build up and no film can overcome that. In the winter, you’d lose less heat through the glass if you add low-e film. Seeger said that in his home, which has film that blocks about half the visible light, he still feels warmth from the winter sun when he’s near a window on a sunny day. The film seems to block heat from the sun most when it is really hot and the heat is unwelcome, he said.
If you do opt for window film, should it go on the interior or exterior? Exterior film is much better at keeping out heat from the sun on double-pane windows, but there is no significant difference on single-pane windows, Seeger said. So except for that situation, he recommends interior film. Exterior films typically carry a warranty of about 10 years, while interior films often have a lifetime warranty. Plus, installation is easier when the work is on the interior side, which means interior films usually have a lower installed cost. The low-e film his company installs, from 3M and Solar Gard, is available only for interior applications.
Seeger said his company has a $500 minimum fee, which is usually enough to cover film on several windows. The installed cost for 3M’s Prestige sun-blocking film usually works out to about $19 or $20 per square foot on the interior and $22 on the exterior. Low-e film is about $22. Other brands can be less expensive, and specific costs vary by details such as how hard a window is to access.