Paint roof white ?

[AlanfromBigEasy]

Does anyone have any experience in painting their i3 roof white ? Snow white should reflect about 85% of the heat from sunlight (much better than black). I would chose the white that matches the factory white though. There appears to be a clear coat over the carbon fiber roof. Is this a normal clear coat or did BMW use some exotic chemical paint ?

I see a white roof as doing at least as much good during the summer (I live in New Orleans) as the 3M Crystalline window tint I had installed.

I bought a white 2015 i3 specifically because white is cooler (and white paint is usually the most durable paint).

 

[Fisher99]

I don't have an answer to your question, but although I understand the benefits of a more reflective roof, I have to say that I really love the carbon fiber look of the black roof.

I am, however considering painting the white part of the rear end black (mine is a white/black car). I think that white section beneath the black hatch looks really strange...

 

[vreihen]

How about a vinyl wrap, so you can easily remove it if you change your mind?????

 

[AlanfromBigEasy]

Vinyl wraps cost more and last only 5 or so years on horizontal surfaces.

I will NOT change my mind - I think the black carbon fiber roof is both quite stupid and ugly,

 

[alohart]

In Honolulu's very high average sun angles, I feel more heat radiating down from the roof than I do through the side or rear windows. Sun shining through the windshield is another major contributor to interior heat. A good IR-rejecting tint could reduce heat entering through the windshield. However, no tint installers seem to have any suggestions of a film that could be installed on the roof exterior, that would reject almost all IR radiation, and that would last more than a couple of years, depending on how much the roof is exposed to sunlight.

Painting the roof of our Arravani Gray i3 white would make it a 3-tone car which I think would look bad. If ours was a white i3, I would certainly be willing to paint its roof the same white.

 

[joeguzman10]

I'm curious to see if a solar panel array can be installed on the roof.
The array can charge the batteries and reflect the IR radiation.

 

[robthebold]

I'm curious to see if a solar panel array can be installed on the roof.
The array can charge the batteries and reflect the IR radiation.

TL;DR doesn't look promising.

This is all totally back-of-the-envelope stuff with nice round numbers . . .

Even using rather . . . uh . . . sunny assumptions about the usable area of your roof (1-2 m^2?) and solar irradiance given your latitude, altitude and cloud cover and really good solar cell efficiency (~20% is pretty high) it's hard to imagine generating more than 100-200W during the brightest part of the day.

Sanity check: this off-the-shelf kit https://www.harborfreight.com/100-Watt-Solar-Panel-Kit-63585.html looks about the size of the roof and has a peak output of 100W. So my estimates are a little high but not too much: maybe we're using a better grade of cell in a really sunny place.

However, even if the charging circuit could set to pull only 1A (it can't can it?), that would really put the "trickle" in "trickle charge."

What's a good miles/kWh, maybe 5, right? So a nice long 12h charge, forgetting efficiency loss since I'm an optimist, is maybe 1200 - 2400Wh. Let's pick the bigger number, so 2.4kWh. 12 extra miles for a full day charge on a really sunny day using really optimistic guesstimates?

Back in the real world, though, you'd get a disappointing fraction of that. Probably better to invest in solar cells in a fixed location and cut your electric bill on that end.

 

[jadnashuanh]

FWIW, for an IR barrier to work best, it needs an air gap otherwise, it will conduct some of that heat to its back side.

When parked, the BMW cover does help keep the interior cooler. If you aren't a neat freak and must fold it back into its pouch when finished, it doesn't take much time to install or remove. It takes care of a good portion of the roof and all of the front windows (windshield and front doors).

 

[robthebold]

When parked, the BMW cover does help keep the interior cooler. If you aren't a neat freak and must fold it back into its pouch when finished, it doesn't take much time to install or remove. It takes care of a good portion of the roof and all of the front windows (windshield and front doors).

What about the sunroof (sunroofs? T-top?) Even with the shade pulled, it lets in a lot of heat.

 

[jadnashuanh]

The BMW cover is a silver material that does seem to reflect at least a fair amount of the heat. It covers the windshield and front windows up to the doorpost including the roof. Blocking the direct sun into the front three windows and reflecting a lot of the heat from the front, probably 60% of the roof. I find it makes a significant difference when parked in the sun when I use it. It's anchored in place partly by pockets around the mirrors and a tape that closes into the right-hand side door's window from the top to make it harder for someone to just rip it off. There's a velcro tab that locks it to each door handle. the cover shows up on their website sometimes. I ordered it from an on-line vendor for a discount.

 

[Fisher99]

Where I live our fall/winter is longer than our spring/summer. Wonder which is better: A black roof absorbing heat from the sun in the fall/winter or a white roof reflecting heat from the sun in the spring/summer...

Actually, it doesn't make any difference since I'm keeping the black carbon fiber look anyway. Of course if I lived in Hawaii or some similar sunny place, I might feel differently. I know that my window shade in the front window makes a huge difference. I'm sure that the BMW cover is even better, but the cost and dealing with storage are more than I want to deal with.

 

[EVMan]

Many non i3 cars with regular roof ,have also started to come in black roof
The question i keep asking is , why are they not white ??????


e.g Camry , nissan kicks etc...

 

[alohart]

How about a vinyl wrap, so you can easily remove it if you change your mind?????

Having just returned to Honolulu after more than 3 months in Europe, I have been shocked by high temperatures that are setting records almost daily. This has energized me to find a solution to heat entering the roof.

While painting or wrapping the roof white would create an ugly 3-tone car, wrapping with silver mirror vinyl might be slightly less ugly because silver mirror isn't exactly a color and the top of the roof isn't very visible and is usually visible at a shallow angle. In addition, it is best at reflecting visible and probably infrared radiation. The roof of our i3 isn't exposed to direct sunlight as much as most cars because I don't drive much and our parking space is inside a parking garage, so its durability should be decent. I'll be visiting vinyl wrapping companies soon to learn more.

 

[watersb]

In general, a very white color is about the best you can do for quick IR rejection.



One of my to-do projects is to play with some pigments that are tuned to provide radiative cooling in addition to IR reflection.

 

[tongsli]

https://www.mybmwi3.com/threads/wrapping-roof-with-vinyl-film.17881/

 

[BMWCCA#41]

My husband's always been a fan of white paint for cooling. (We life in Florida, where such stuff really matters!)

But new research has shown that most white paints don't work. The researchers found one that does, but whether it's available for car roofs, I don't know.

https://www.purdue.edu/newsroom/rel...-is-here-and-its-the-coolest.-literally..html

I do know that Consumer Reports did a test of light vs dark interiors in hot conditions and the light one wasn't significantly cooler than the dark.

 

[accidentalyEV]

Honestly not sure painting the roof white would make much of a difference with all the solar heat gain through the windows

 

[alohart]

Honestly not sure painting the roof white would make much of a difference with all the solar heat gain through the windows

Might depend on the average sun angle where one lives. Here in the tropics, the average sun angle is very high. More heat almost certainly enters through the roof than through the side windows. Probably most of the heat enters through the windshield, but tinting it to block IR radiation isn't easy due to its large size or is impossible due to illegality in some jurisdictions.

Without light-colored vinyl film on the roof, I could feel heat radiating down on my head while the rest of my body was cooled by air conditioning. That was almost eliminated after the roof was wrapped.

 

[Mondomensch]

Might depend on the average sun angle where one lives. Here in the tropics, the average sun angle is very high. More heat almost certainly enters through the roof than through the side windows. Probably most of the heat enters through the windshield, but tinting it to block IR radiation isn't easy due to its large size or is impossible due to illegality in some jurisdictions.

Without light-colored vinyl film on the roof, I could feel heat radiating down on my head while the rest of my body was cooled by air conditioning. That was almost eliminated after the roof was wrapped.

Do you have a link to info about your vinyl roof wrap?

 

[AlanfromBigEasy]

A month ago I painted the roof of my 2021 i3 REx white, just as I did my 2015 i3. A body shop with carbon fiber experience (Vargas in New Orleans) did it.
My windows on both cars are tinted. The white vs black roof makes a significant difference in heat gain.

Carbon fiber is porous and needs a special coat to seal it. I have to wait a month before washing, waxing etc for this sealing coat to evaporate.

I would recommend a body shop with experience with painting carbon fiber.