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Driving for long stretches out west, we’ll often have the sun scorching us through one side or the other for hours at a time. In the past, we’ve tried several dubious, and likely illegal, things to escape the baking. Things like shoving towels in the windows, baby shades, and sitting nearly sideways with the cruise control on. Yes, we’ve been that desperate to escape the heat.
You’ll remember that one of the finishing touches we put on Das Bus was the addition of some 3M Crystalline Window Film onto the front side windows. This film is touted to reject up to 60% of the heat coming through the windows. It almost sounds too good to be true, but judging from the prices they charge for the film, someone must believe in it. We were just desperate enough to pay to have this rather expensive film installed on our side windows. The two side windows in our Sprinter (4 panels) set us back about $300.
But does it work? Or are we just victims of the Minnesota Marketing Machine? (3M, get it? Ha!) Would some other kind of window film work just as well? That’s what I wanted to try and find out, in a semi-objective kind of way. So I unleashed my inner nerd and set up some tests. Here’s what I did.
The test setup
Here’s a photo of my test rig. It’s not too fancy. The heat source is a 250 watt heat lamp. For window tint, we’re only really worried about radiant heat – not convective or conductive – so this should be sufficient. To measure temperature changes, I used a cheap outdoor thermometer. I wanted to keep the heat lamp a consistent distance from the thermometer, so I built this simple jig. Basically, it holds the lamp 12.5 inches away from, and on-axis with, the thermometer. I also used a second, equally cheap, bulb thermometer to measure any changes in ambient temperature during my test runs. Fortunately, the dial and bulb thermometers gave the same readings when left to settle. Finally, I used my phone as a stopwatch.
Bench testing the rig
I wanted to get a feel for how much the infrared lamp would change the temperature with no glass involved; and I also wanted to make sure that the setup gave reasonable and repeatable results. So to test the rig, I completed two runs just sitting on my bench. The procedure was basically:
- Record the Starting Temperature
- Turn on the lamp
- Wait 10 minutes
- Record any temperature change
- Record any ambient temperature change and use this to get a corrected temperature rise (ΔT)
I then waited for the temperature to drop back to the ambient temperature, and repeated the test. The two runs gave corrected temperature changes (ΔT) of 36 and 38 degrees for a 10 minute exposure. The two runs agreed within two degrees, which I judged to be within the margin of error for such cheap equipment, so it was on to the next test. I followed those same steps for all of the test runs that followed.
Testing clear glass
I really wanted to test the untreated windshield in the Sprinter. Unfortunately, I had some difficulties getting this to work. First, there was not really a good place to get the RV completely into the shade to eliminate sunlight. And second, the curvature of the windshield made it difficult to keep things lined up. As you can see, I was just holding the rig. Testing in this way with the curved windshield wound up deflecting much of the light. As it was, this test gave me a corrected ΔT of only 13°. This didn’t correlate at all with my real-world experience with this windshield, so I decided to throw these results out, and test again with some clear plate glass I had in the shop.
This gave more realistic values. In fact, it gave corrected values that were about the same as not using any glass at all. Now granted, this was not the same as using an automotive windshield, but it was the best I could do. To the best of my knowledge, our 2003 Sprinter does not contain the new IR-filtering glass required as of this year in California.
Testing standard window tint
I have a “regular” window film on the windows of my daily driver. It’s actually a 3M film, but not the Crystalline. On the front window that I tested, it’s also a legal shade for driving here in Utah – the same shade that we had installed in the RV. I decided to test that one next. Here I am testing the window film on the front windows. I ran the tests outside, so the corrected temperature readings became more important. Also, I should note that I did this on a calm day – no wind. This test gave a corrected ΔT of 20°.
Testing another tinted window
The windows in the living area of our RV are CR Laurence Automotive windows. They come tinted, and there is no need to apply any kind of film to these. I decided to test one of these, and here’s the photo of that.
These windows tested out better than the standard window film on my Subaru, and after 10 minutes, gave a corrected ΔT of 17°.
Testing the 3M Crystalline Window Film
Finally, it was time to test the window film that was supposed to perform the best. These were large, flat panels, so I didn’t have any trouble getting things lined up.
After 10 minutes, the test gave a corrected ΔT of 12 degrees. This was the best result of the bunch. The temperature gain was 40% less than the standard window tint on my Subaru, and 30% less than the CR Laurence tinted windows, and a whopping 68% less than the untreated glass.
Here is a summary of the results for all test runs:
|Test Run||T (start)||T (end)||ΔT (raw)||Ending Ambient Temp||ΔT (corrected)|
|CR Laurence Tint||60°||74°||14°||57°||17°|
|3M Crystalline Tint||60°||70°||10°||58°||12°|
But how does it feel?
Numbers in a table are one thing. But it doesn’t matter what my chart says if I’m still uncomfortable in the sun. So subjectively, how did I like it?
Well, on our recent trip to Moab, the weather was great and the sun was out. I made sure to take note as the sun alternated between the windshield and the driver’s side. Now granted, this wasn’t the worst of the summer sun, but it was enough to give me a taste of what’s to come. I can tell you that the sun through the windows with the 3M film was definitely less oppressive than when it came through the un-tinted windshield. The film certainly helped, and the sun felt “muted” through the 3m crystalline film. I could absolutely still tell the sun was there – I could still feel it, but I wasn’t reaching for a towel to shove in the window either.
So would I buy it again?
That depends on what you’ve got to start with, and how much driving you do. If you’re heading west for 14 hours across the Nevada desert in July, you might feel differently about window tint than someone who plans to drive due north for 2 hours in the forests of upstate New York. So consider your own needs when you think about window tint. But if you want my opinion, here’s what I’d do:
- If your windows already have tint on them, don’t rip it off and apply 3M Crystalline film.
- If your windows are already tinted (without any film), it’s also probably not warranted to apply the expensive 3M Crystalline.
- If you have un-tinted windows, then consider this film. But it’s expensive, so apply it where it makes the most sense. In our RV, that meant the driver and passenger side windows, where we needed the best performance with the least amount of tint (to keep things legal).
- If I had a big Class A motorhome, with the gigantic side windows up front, I would apply this film. That’s a lot of glass, and a lot of heat. I’d do everything I could to mitigate it.
- In a car, you can typically tint the rear windows very dark. So for a passenger car, I would go with a cheaper, darker film elsewhere, and save the 3M Crystalline film for the side windows up front, where you’re forced to use a lighter tint.
So there you have it. Hope you found this helpful. If you have any questions, let me know in the comments below and I’ll try to get back to you.