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Full Version: Reflectix question -- come in and beat a dead horse.
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Hi CRVL! I have a question that I can not find an answer to and my first thought is that it's a waste of time and money. But, curiosity is getting the best of me. 

I understand the need for an air gap for reflectix to be effective. How about in this situation?

I have the van wall with foam board attached. I did not fill in or cover the ribs and the way I am paneling my walls leaves about 1" between the wall panel and ribs/insulation. Would there be insulation benefit to me attaching reflectix to the back of the wall panels or will that create some awkward situation where I have heat trapped between the insulation and reflectix?
trapping heat is the point of all insulation, you are trying to keep it from moving in or moving out.
Short answer - No!

Since you've already got the foam insulation installed, putting reflectix inside of it won't do much good at all. The insulation will keep most of the heat out anyways and reflectix is a reflective product meaning that it simply reflects the heat back to it's source (the sun). It's not getting through the insulation you've already got so it's not effective.

Layers with reflectix in a van is this: van metal wall, dead air gap, reflectix, insulation, finished wall.
Thanks that is what I thought but I wanted to be sure.
Long answer: It depends.

With an existing 1" gap, adding Reflectix will leave you with a 3\4" gap. Far less than the recommended 2", but better than nothing.

The main factor that people rarely consider is whether they are trying to keep heat out or keep it in. You need the exposed, shiny side facing the source of the heat.

Assuming you want to keep the heat out.
Your proposed structure will be more effective than insulation alone. But then the question becomes: Will that difference in effectiveness be worth the high cost of all that Reflectix? The hotter the environment where you expect to be most of the time, the more "worth it" it will be.

Yes, the Reflectix will "trap" heat in that small, 3\4" gap, but it will be heat that is then more likely to escape back out through the metal wall. The metal will get hot in the sun. It will both radiate heat via infrared AND heat the air in the gap by conduction. A significant portion of the infrared will be reflected back to the metal wall where a lot of it will be reabsorbed, heating the metal wall more. This does produce a bit of a runaway effect, but it still reduces the amount of heat that makes it's way in, through the insulation. No heat is actually generated in any of these processes. It just moved from one place to another. When the metal wall of the van gets hotter, there is more of a temperature difference between it and the outside air. Therefore, it releases more heat to the atmosphere. It also will release more heat as infrared back out to the universe. In the end, it definitely WILL reduce the heat that gets into your van. However, that may be a relatively small amount, compared to insulation alone.

Many people erroneously think that, if the wall of the van is getting hotter, then more heat must be going into the van. However, it is only hotter precisely because more heat is being forced back out through that same metal wall.

In the end, you may be better off investing your money in sun shades to keep that sun from hitting your van in the first place.


If you want to keep heat in:
Nope. Your proposed setup will not work for you. The absolute best way to use Reflectix to keep heat in is to have it completed exposed to the interior of your van, where you can see the shiny side. Then it reflects your body heat right back into you.

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(07-01-2019, 07:12 PM)gwave Wrote: [ -> ]Hi CRVL! I have a question that I can not find an answer to and my first thought is that it's a waste of time and money. But, curiosity is getting the best of me. 

I understand the need for an air gap for reflectix to be effective. How about in this situation?

I have the van wall with foam board attached. I did not fill in or cover the ribs and the way I am paneling my walls leaves about 1" between the wall panel and ribs/insulation. Would there be insulation benefit to me attaching reflectix to the back of the wall panels or will that create some awkward situation where I have heat trapped between the insulation and reflectix?

A better approach will be to apply shiny mylar films (emergency blankets) to the foam board, leave a gap and attach another sheet of these blankets to the back side of the wall panels.  This way, you have made your own true radiative barrier and may save some money in the process.

Radiative barriers like Reflectix, or aluminized Mylar films have low emissivity - a measure of how well a surface absorb or radiate heat.  By adding the reflective mylar film to the back of the wall panel, you're making it a poor radiator of any heat that may soak thru from the inside.  The same argument applies to the reflective film on the foam board.