So what did the tests indicate?

Where's the data?

-AC

I have a window to test.
I will do a small test of the plain glass (with black tape to test off of) compared to some packing material white Styrofoam sheet( black tape again) , 1 layer tested then a 2nd layer.
I suspect I will see a notable improvement.
Also the white Styrofoam let the sunlight threw , it was still effective as a window for light in that regard but not so easy to see out..not that I cared , it's a side window looking over the neighbors house.

I'm going off the test data done by various laboratories that do R value testing and research. I'm sure you've read the same stuff.

What might be more interesting is foil bubble wrap testing, although i'm not sure you could measure the actual results with an infrared gun as it would provide more benefit than measured off just the backside.
Horizontal heat reflectiveness isn't near that of the vertical.

The Rvalue testing measures the change in the environments on each side of the insulator, not the backside temp change. The back side temp is useful in window testing for feel it provides when you are next to it as the difference is dramatic.

I think that if you look into this, you will find that radiant energy travels equally in all directions, and the ability to reflect radiant energy is equal in all directions as well.

However, if the convective effects of air movement are considered, then the "R-value" will be different with different orientations.

-AC

1/2 inch white Stryfoam vs single pane
 

Styrofoam results

On one side of the window's half section the Glass was left exposed.
The other window's half was covered with 1/2" thick Styrofoam sheet 2'x3ft. top to bottom.

Test #1
Bare single pane glass - 54*

(1 sheet ) Styrofoam - 63.2*

~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~

Test #2

I added one more sheet of Styrofoam, for a total of 1 inch thickness.

Bare single pane glass - 53.2*

(2 sheet) Styrofoam - 63.3*

Outdoor Temp was 31* in both tests

About a 10 dgree improvement.
Strange there was no gain from the 2nd sheet of Styrofoam, I double checked the Data.