jjunda said:
I'm still a little confused. In regards to using Reflectix on windows, do you still need an air gap? I see so many people putting Reflectix directly against windows, with the shiny side facing out. I am assuming this negates the effect of the radiant barrier, because the heat is conducted directly from window to Reflectix. Correct? If the air gap is needed even for windows, is it sufficient to use polyiso (or similar) as your air gap, with Reflectix mounted right onto the polyiso, or does it quite literally need to be air only? Thanks!
Infrared light is produced when a surface is hot and that surface is not in direct contact with some solid (except glass, see later). It is one of the three ways that heat can be transferred. The other ways are direct
conduction, where one material makes another material, which it is touching, hot. And
convection, which is just a special form of conduction where air was made hot by conduction, and then that air drifts around and makes something else hot, again by conduction. (OK, technically, infrared light is produced even within solid materials, but each of those photons of infrared light are immediately absorbed by the very next molecule in that solid or a solid touching it. So, we can say that, effectively, no infrared light is emitted within solids, or between two solids that are directly touching.)
Now, people are going to ask, "Doesn't the molecules of the shiny surface of the Reflectix bounce those photons of infrared light, even if it is touching the molecule of another surface?" Well, sort of. But, lets say the Reflectix bounced an infrared photon back to the solid surface it is touching. That would just make said solid surface hotter, which would then mean more heat would be transferred into the aluminized surface of the Reflectix by conduction. The effect of any reflection is essentially nil. But that same aluminized surface is busily conducting heat right the heck through it as fast as it can. So, the Reflectix is worthless in that situation.
OK, Reflectix needs an air gap because it is reflecting infrared light. That is it's primary function and its only real benefit. The insulation value of the little bubbles of air in the bubble-wrap insides is almost zilch. The bubble wrap is only there to hold up the aluminized film. And because it looks kinda nifty. A simple plastic film would work almost as well. But then you wouldn't be willing to pay so damn much for it. If it is directly in contact with something, then the aluminized surface (which conducts heat very well) will just conduct the heat right through the aluminum and heat up the air in the little bubbles which heats up the other aluminized layer by convection, which then heats up whatever it is touching. The air bubbles, as small as they are, are still big enough for air to drift around inside them and transfer heat from one side to the other by convection.
So, we constantly harp on the need for an air gap because YouTube is full of videos of people gluing Reflectix to the inside of the metal walls of their vans, then covering that with insulation. If there is no air gap between those surfaces, then heat is transferred by conduction betwixt those directly touching surfaces, and there is simply no opportunity for infrared light to be emitted, let alone reflected back. So they have just wasted a ton of money making the inside of their van shiny and then covering it up. But it looks good in a YouTube video, so morons keep doing it. "For the views, man, for the views."
Contrary to popular opinion, auto window glass can transmit infrared light. And all the other frequencies of light. It just doesn't transmit
ALL of that light. And it transmits infrared light less than it does regular light. That is why cars get so darn hot. The infrared and regular light from the sun come streaming in through the window, the infrared and regular light beaming on the dashboard and seats heats them up. Those surfaces then emit more infrared light. A lot of that gets emitted right back out through the windows. But not all of it. So that infrared light just keeps building up, making the surfaces hotter and hotter. Those hot surfaces then heat the air that is right next to them and that hot air drifts around in the car, making everything else hot too.
Here is an interesting fact about shiny surfaces. They do not emit as much infrared light when they are hot as do dark surfaces. That is why they paint wood stoves black but make the handles chrome. Yes, the handle gets the same temperature as the rest of the stove. But, it doesn't emit as much infrared, so it emits less heat, all total. So, sure, you will burn your hand if you grab that chrome handle, but at least you won't have your skin stick to it and peel off, as it would if you had just laid your hand on the side of the stove. And yes, some of the heat transferred into your hand is by infrared. Our skin can transmit infrared light better than most solids. That's because we aren't really solid. We are really mostly water in lots and lots and lots of really tiny little bags that are all stuck together in the shape of a person. Put a flashlight behind your hand and you will see what I mean.
So... Here is what happens if you put Reflectix inside a window on a sunny day: The sun comes through that window just the same as it always does, with both infrared and regular light. But that Reflectix reflects almost all of that light back towards the window. Almost all of the regular light and most of the infrared light goes back out through the window. This drastically reduces the amount of regular light that is heating up the seats and causing the seats to emit infrared light. This, in turn, drastically reduces the amount of infrared light bouncing around inside your car and heating everything up.
Now, this goes to the crux of your question: Why can that shiny surface reflect that light if it is actually touching the surface of the glass? Because, if you haven't noticed, glass is not like other solids. Photons can go right through it, almost as if it wasn't there. So, if a hot object is pressed against some glass, and the hot object emits a photon of infrared light, then the molecules of the glass won't absorb those photons. At least, not most of them. Most just pass right on through. A photon of light can go through the glass, bounce off the Reflectix, and go right back. If the Reflectix itself is hot and emits one of those rare infrared photons that shiny surfaces sometimes emit, then that, too, will likely go right through the glass, and out of the car. Another thing you may not have noticed is that the shiny side of a mirror is on the back of the mirror. All the light, infrared or not, has to go through the glass, bounce off the shiny surface, then go right through the glass again.
Of course, this can cause the small space between the Reflectix and the glass to get really hot. I had one of the chips in my windshield expand into a real crack because of this. But this is still better than having the whole inside of my car reach 180 degrees F.
So, the shiny inside of the Reflectix,
only when it is not directly touching some solid other than glass, will transfer less heat all total. This means, if you glue it to the inside of the metal wall of your van, and then build your interior wall such that there is some gap between the Reflectix and the wall, then that Reflectix will not transfer as much heat to said interior wall as simple insulation
IF THAT INSULATION WAS THE SAME TEMPERATURE AS THE REFLECTIX. However, A) This is the exact same effect as if you had just glued cheap aluminum foil to the inside of the metal walls of your van, and saved several hundred dollars. And B) The inside of any normal insulation will be cooler than the inside side of a sheet of Reflectix that has been glued directly to the metal surface of your van. The inside surface of the Reflectix, in this situation, will be almost exactly the same temperature as the metal wall of the van was, without the Reflectix. Again, that Reflectix was just a massive waste of money.
So, when you build an interior wall, it is far better and less expensive to simply use regular insulation between the metal wall of the van and the interior wall. If you want, use insulation that has a reflective film on it. This costs almost nothing more than the insulation alone. Because only one side is going to be reflective, you want to put it facing where you will most likely have the most infrared light that you want to bounce back. Therefore, it should face the metal wall of your van unless you plan to only live in really, really cold areas. Then face it towards the inside, to bounce your own inside infrared heat back in. Again, that reflective surface would need some air gap. But, because you paid almost nothing extra for that reflective surface (on your insulation), it is not such a big deal if your don't make the most effective use of it.
And this brings us to the last point about using Reflectix on windows: If you use Reflectix on the inside of a window, such that you can literally see the shiny surface, then it will reflect the infrared light emitted by your own body back at you. This is huge. If you have a window facing a clear night sky and you are sitting next to it, your body on that side will feel cold. It is not just because of cold air drifting off the window. It is because you are used to the objects around you beaming about the same amount of infrared light at you as you beam at them. This is because they are all close to the same temperature as you, in the grand scheme of things. But, sitting by that uncovered window, there is almost no infrared light beaming back at you. So your body emits heat energy in the form of infrared light, but it ain't getting any back, in return. So it feels cold.
And now to my final recommendation for window coverings: Make one side black and one side reflective. If its too hot out, put the reflective side out. If it is cold out, put the reflective side in. Most vehicles have symmetrically designed windows, so most of the time you can just swap the right side window coverings for the left side ones and flip the front and back window coverings over. You can use Reflectix or you can use aluminum foil glued to cardboard. It doesn't really matter. The only reason I used Reflectix is because it is rather durable. You can fold it, roll it, shove it into the crannies around the window a thousand times and it will still, mostly, hold its shape. Cardboard will wear out in a week. Corrugated plastic sheeting is super durable, but it is extra stiff. More stiff than I like.
My ideal window covering would be a sheet of Reflectix (or maybe that aluminized blanket you can get at WalMart) with a black cloth sewn to one side with quilt batting sewn in between, and a durable bias tape sewn around the edges. Depending on the design of your vehicle windows, you may be able to just tuck that between the window and the frame, like I can. Or you will need to make it press fit, it you don't have a little gap like I do. And if even that won't work, think about using Velcro or magnets.
Note: If you use Velcro, remember that the inside of your glass is likely covered with a tint film that is only barely adhered to the glass. The Velcro may pull that film off the glass. Plus, the adhesive on sticky-back Velcro does not hold up in the heat. So you are better to use non-sticky-back Velcro and use contact cement to glue it in place. That is, IF you wouldn't be gluing it to the window tinting instead of the glass.
And just to be clear: No, polyiso can not be used as "the air gap." because it is a solid. Just because it is a solid with lots of little air bubbles trapped in it, does not mean the actual solid surface of the polyiso won't act just like any other solid, when it comes to emitting or absorbing infrared light. The reason polyiso is a good insulator is that all the little air bubbles are too small for the air in them to drift around. So, even if one side of the insulation gets hot, that heat doesn't get transferred to the other side very easily. There are a lot more physics involved here, but I have spend over four hours composing this e-mail already. And I'm sure you have read quite enough already.
Annnnnd, as I have been going over and over this message for almost five hours now, and can no longer keep all the parts separate and clear in my mind, I am just going to post it as is. If any part is unclear, please let me know.
