...Given enough time however the body will heat up to the same temperature regardless of the colour. (almost the same...
I'd have to take exception to that. What you say is true if the body is in a vacuum, but not if surrounded by an atmosphere, because different characteristics (surface finish, etc.) affect the rate at which heat is dissipated to the atmosphere, a variable that doesn't exist in a vacuum.
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For those interested in more info about using reflectix & other materials for insulating a van, here's some more info...
Here's why gluing a single layer of Reflectix to the interior surface of the metal skin of the vehicle is actually not a bad idea, but there are better alternatives.
First, a quick look at the physics involved:
The thermal energy in sunlight is approximately 52% in the narrow infrared & near-infrared part of the spectrum, and 48% in every other part (visible light). When sunlight hits a surface, the visible light that is not reflected is converted to heat energy, and some of that heat is absorbed and some is rejected, depending on the color and other factors. As the energy from the sunlight is absorbed by the sheet metal skin of a van, all of the energy that is not reflected or dissipated to the atmosphere is transmitted to the interior in the form of heat energy.
That incoming heat energy from sunlight can be conveyed to the interior of the van by one of three methods (conduction, convection or radiation) as has already been discussed. "Radiation" is just another term for infrared energy, more of which is emitted by an object as its temperature rises (all else being equal).
Radiant barriers work because they have low "emissivity" ("Low-E") which means they do not readily emit infrared energy. Reflectix and other metallic barriers (anf even household aluminum foil) typically emit less than 5% of the heat energy as infrared radiation. In comparison, most plastics and most paints emit over 80%, and a van upfit can be designed to take advantage of that.
In stock form, the inside of a van's exterior skin has infrared emissivity that is almost as high as the paint on the outside.
That means that
in stock form, almost as much radiant heat energy is emitted inwardly as outwardly.
But if a radiant barrier is applied to the inside of a van's exterior skin, the interior of that barrier will emit (radiate) heat inwardly at a much lower rate than the exterior surface. This will cause relatively more heat to be rejected outwardly (where it will be dissipated to the air)
rather than inwardly (relative to the temperature differential),
which is desirable in this application (at least on a hot summer day).
However, it does not have to be Reflectix or a similar metalized bubble wrap product.
A foil layer will work almost as well, *AS LONG AS A SEALED AIR GAP IS MAINTAINED ON THE INSIDE OF THAT MATERIAL,* and assuming that bulk thermal insulation (the kind that has "real" R-value, like rigid foam) is installed on the inner side of that air gap. And
two foil layers with an air gap between them will work slightly better than a single layer of "double-bubble" Reflectix.
Here are the layers for optimal insulation, from outside to inside:
1. OEM Sheet Metal skin
2. Radiant Barrier Foil, glued to skin
3. AIR GAP, 1/4" minimum (preferably 1/2"-3/4") with all gaps, top, bottom, sides, etc. closed off.
4. Radiant Barrier Foil...
5. ...on bulk ("real" R-Value rating) foam insulation (Polyiso if primary concern is summer heat. JohnsManville AP Foil is acceptable, but it's not quite as good as Dow Tuff-R or Dow Thermax.)
Avoid using the "Rmax R-Matte" product sold at Home Depot if possible, because it is inferior to Tuff-R or AP Foil.
Of course, don't forget about the glass area. Everybody knows that, but what they may not know is that exterior sunshades are about twice as effective at keeping heat out as anything installed inside the glass. Reflectix works OK for that but there are a few better products available (but not at local Lowes or HD).
Beyond that, the only other way to do any better for reducing heat gain on a sunny summer day is to paint the exterior with a high performance roof coating, with a "
Solar Reflective Index" of 110 or better, such as Gardner S-100.
The only advantage Reflectix has over single layer of foil (like Attic foil) in this application is that Reflectix is technically a double layer radiant barrier separated by a pseudo-air-gap. But unfortunately there is a lot of plastic bridging those two surfaces, so
Reflectix is actually not as effective as two layers of foil separated by a true air gap. Sometimes it's hard to maintain a true sealed air gap everywhere in a vehicle installation, and that's why one layer of Reflectix is not a terrible choice, especially since it doesn't take a huge quantity just to do a van, and local Lowes/Home Depot carry it (but generally do not stock radiant barrier foil like Attic Foil, etc.).
The bottom line is: Reflectix will reduce heat transfer to the interior of a van if applied directly to the inside of a van's exterior skin,
but any radiant barrier product needs to be part of a more extensive insulation system using conventional bulk thermal insulation, and frankly, there are better options than Reflectix for this application.
There are generic equivalents of Reflectix available online, but here is what I recommend using instead of any "bubble-foil" product, for the inside of a van's skin:
1. Single layer non-perforated (solid) foil, e.g.
www.atticfoil.com/index.php/products/
2. Silent Silver Underlayment (or equivalent), radiant barrier + acoustic dampening
https://www.rubberflooringinc.com/underlayment/silent-silver-underlayment.html
Either one of these can be applied directly to the inside of the sheet metal but needs 1/4" air gap at the very minimum between it and additional insulation, and preferably at least 1/2" (more than 3/4" adds nothing, according to specs from Dow, and JohnsManville). The bulk insulation added on the inside of that air gap should also have a foil (both Tuff-R & AP Foil do) facing the air gap.
As for the spray-applied expanding foams (e.g.: "Great Stuff): These one-component spray foams sold at Lowes & Home Depot do not have nearly as high closed cell percentage as two-part foams, and that's the main reason they can trap water, either from leaks or condensation. The R-value of these foams is not as high as 2-part foams.
Manufactured polyiso foam sheathing is always preferable to field-sprayed foams because the manufactured foam is the only type that actually achieves its rated R-value; even professional spray application is not as consistent as the manufactured boards. The generally accepted "real-world" R-value for field-sprayed polyiso foam is R 5.6 per inch, but only in mild temps, because the R-Value of all Polyiso foam (sheet & sprayed) drops substantially in very cold temperatures.
For filling voids, two part foams are greatly preferable, and cheaper than 2-part spray foam:
http://www.uscomposites.com/foam.html
and they do not corrode sheet metal or trap water unless water was present when they are poured.
In some cases, non-rigid foam is needed:
https://www.smooth-on.com/product-line/flexfoam-it/
Two-part foams must be mixed correctly and applied at room temperature to expand fully. There are also DIY two-part spray foam units available, but they aren't cheap.
It is preferable to use foam sheathing in any area where that is possible, because it is a more consistent product that will maintain its R-Value longer than any field-applied foam.
I hope this helps.
I posted this here after getting frustrated by all the misinformation that seems to be so prevalent on youtube.
Seems like everybody's got a video up of their van-to-RV upfit, and based on the comments I saw, a lot of people getting misled too.
FWIW, I design RVs for a local upfitter, and am up to date with the latest ways to squeeze out every last wasted BTU or watt-hour.
Here are two of my favorite go-too components:
http://www.hotspotenergy.com/DC-air-conditioner/
http://www.lensunsolar.com/Flexible-solar-panel/Aluminum-flexible-solar-panel
Important note: if you glue PV panels down to the exterior skin of an RV, you loose one of the other advantages that an aluminum-backed PV panel can provide:
shade... which you will have if there's an air gap under it.
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